LINEAR TECHNOLOGY LT1976, LT1976B Technical data

FEATURES
LT1976/LT1976B
High Voltage 1.5A, 200kHz
Step-Down Switching Regulator
with 100μA Quiescent Current
U
DESCRIPTIO
Wide Input Range: 3.3V to 60V
1.5A Peak Switch Current (LT1976)
100μA Quiescent Current (LT1976)**
1.6mA Quiescent Current (LT1976B)
Low Shutdown Current: IQ < 1μA
Power Good Flag with Programmable Threshold
Load Dump Protection to 60V
200kHz Switching Frequency
Saturating Switch Design: 0.2Ω On-Resistance
Peak Switch Current Maintained Over Full Duty Cycle Range*
1.25V Feedback Reference Voltage
Easily Synchronizable
Soft-Start Capability
Small 16-Pin Thermally Enhanced TSSOP Package
U
APPLICATIO S
High Voltage Power Conversion
14V and 42V Automotive Systems
Industrial Power Systems
Distributed Power Systems
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
*Protected by U.S. Patents, including 6498466 **See Burst Mode Operation section for conditions
The LT®1976/LT1976B are 200kHz monolithic step-down switching regulators that accept input voltages up to 60V. A high efficiency 1.5A, 0.2Ω switch is included on the die along with all the necessary oscillator, control and logic circuitry. Current mode topology is used for fast transient response and good loop stability.
Innovative design techniques along with a new high volt­age process achieve high efficiency over a wide input range. Efficiency is maintained over a wide output current range by employing Burst Mode operation at low currents, utilizing the output to bias the internal circuitry, and by using a supply boost capacitor to fully saturate the power switch. The LT1976B does not shift into Burst Mode operation at low currents, eliminating low frequency out­put ripple at the expense of efficiency. Patented circuitry maintains peak switch current over the full duty cycle range.* Shutdown reduces input supply current to less than 1μA. External synchronization can be implemented by driving the SYNC pin with logic-level inputs. A single capacitor from the C
pin to the output provides a
SS
controlled output voltage ramp (soft-start). The devices also have a power good flag with a programmable thresh­old and time-out and thermal shutdown protection.
The LT1976/LT1976B are available in a 16-pin TSSOP package with Exposed Pad leadframe for low thermal resistance.
TYPICAL APPLICATIO
14V to 3.3V Step-Down Converter with
100μA No Load Quiescent Current
V
3.3V TO 60V
1500pF
10k
IN
1μF
4.7μF 100V CER
330pF
V
IN
SHDN
V
C
C
T
SYNC GND
LT1976
BOOST
C
V
BIAS
PGFB
0.33μF
SW
0.1μF
SS
47pF
FB
PG
33μH
10MQ60N
U
4148
165k 1%
100k 1%
V
3.3V 1A
100μF
6.3V TANT
1976 TA01
OUT
LT1976 Supply Current vs Input Voltage
150
125
100
75
50
SUPPLY CURRENT (μA)
25
0
0
10
30 40
20
INPUT VOLTAGE (V)
LT1976 Efficiency and Power Loss vs Load Current
V
= 3.3V
OUT
= 25°C
T
A
50
60
1976 F05
100
75
5V
50
EFFICIENCY (%)
25
0
0.1
EFFICIENCY
3.3V
1
10
LOAD CURRENT (mA)
TYPICAL POWER LOSS
100
1000
1976 TA02
10000
1976bfg
10
1
POWER LOSS (W)
0.1
0.01
0.001
1
LT1976/LT1976B
FE PACKAGE
16-LEAD PLASTIC TSSOP
1
2
3
4
5
6
7
8
TOP VIEW
16
15
14
13
12
11
10
9
NC
SW
NC
V
IN
NC
BOOST
C
T
GND
PG
SHDN
SYNC
PGFB
FB
V
C
BIAS
C
SS
17
WW
W
U
ABSOLUTE AXI U RATI GS
(Note 1)
VIN, SHDN, PG, BIAS .............................................. 60V
BOOST Pin Above SW ............................................ 35V
BOOST Pin Voltage ................................................. 68V
SYNC, CSS, PGFB, FB................................................ 6V
Operating JunctionTemperature Range
LT1976EFE/LT1976BEFE (Note 2) ... – 40°C to 125°C
LT1976IFE/LT1976BIFE (Note 2) ..... –40°C to 125°C
LT1976HFE (Note 2) ........................ –40°C to 140°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
UUW
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
LT1976EFE LT1976IFE LT1976HFE LT1976BEFE LT1976BIFE
FE PART MARKING
1976EFE 1976IFE
θJA = 45°C/W, θ
EXPOSED PAD IS GND (PIN 17)
MUST BE SOLDERED TO GND (PIN 8)
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
JC(PAD)
= 10°C/W
1976HFE 1976BEFE 1976BIFE
ELECTRICAL CHARACTERISTICS
The denotes the specifications which apply over the full –40°C to 125°C
operating temperature range, otherwise specifications are at TJ = 25°C. VIN = 12V, SHDN = 12V, BOOST = 15.3V, BIAS = 5V, FB/PGFB = 1.25V, CSS/SYNC = 0V unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
SHDN
I
SHDN
I
VINS
I
VIN
I
BIASS
I
BIAS
V
REF
I
FB
2
SHDN Threshold 1.2 1.3 1.4 V SHDN Input Current SHDN = 12V 520 μA Minimum Input Voltage (Note 3) 2.4 3 V Supply Shutdown Current SHDN = 0V, BOOST = 0V, FB/PGFB = 0V 0.1 2 μA Supply Sleep Current (Note 4) (LT1976) BIAS = 0V, FB = 1.35V 170 230 μA
Supply Quiescent Current BIAS = 0V, FB = 1.15V, VC = 0.8V (VC = 0V LT1976B) 3.2 4.10 mA
Minimum BIAS Voltage (Note 5) 2.7 3 V BIAS Sleep Current (Note 4) (LT1976) 110 180 μA BIAS Quiescent Current SYNC = 3.3V 700 800 μA Minimum Boost Voltage (Note 6) ISW = 1.5A 1.8 2.5 V Input Boost Current (Note 7) ISW = 1.5A 40 50 mA Reference Voltage (V FB Input Bias Current 75 200 nA EA Voltage Gain (Note 8) 900 V/V EA Voltage g EA Source Current FB = 1.15V 20 40 55 μA EA Sink Current FB = 1.35V 15 30 40 μA VC to SW g VC High Clamp 2.1 2.2 2.4 V VC Switching Threshold (LT1976B) 0.1 0.4 0.8 V
m
m
) 3.3V < V
REF
FB = 1.35V
BIAS = 5V, FB = 1.15V, VC = 0.8V (VC = 0V LT1976B) 2.6 3.25 mA
< 60V 1.225 1.25 1.275 V
VIN
dI(VC)= ±10μA 400 650 800 μMho
45 75 μA
3A/V
1976bfg
LT1976/LT1976B
ELECTRICAL CHARACTERISTICS
The denotes the specifications which apply over the full –40°C to 125°C
operating temperature range, otherwise specifications are at TJ = 25°C. VIN = 12V, SHDN = 12V, BOOST = 15.3V, BIAS = 5V, FB/PGFB = 1.25V, C
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
PK
I
CSS
I
PGFB
V
I
CT
V
PGFB
CT
SW Current Limit LT1976 1.5 2.4 3.5 A
Switch On Resistance (Note 9) 0.2 0.4 Ω
Switching Frequency BOOST = OPEN 180 200 230 kHz
Maximum Duty Cycle 90 92 %
Minimum SYNC Amplitude 1.5 2.0 V
SYNC Frequency Range 230 600 kHz SYNC Input Impedance SYNC = 0.5V 100 kΩ
CSS Current Threshold (Note 10) FB = 0V 7 13 20 μA
PGFB Input Current 25 100 nA
PGFB Voltage Threshold (Note 11) 88 90 92 %
CT Source Current (Note 11) 2 3.6 5.5 μA
CT Sink Current (Note 11) 1 2 mA
CT Voltage Threshold (Note 11) 1.16 1.2 1.26 V PG Leakage (Note 11) PG = 12V 0.1 1 μA PG Sink Current (Note 11) PGFB = 1V, PG = 400mV 120 200 μA
/SYNC = 0V unless otherwise noted.
SS
LT1976B 1.2 2.5 4 A
The denotes the specifications which apply over the full –40°C to 140°C operating temperature range, otherwise specifications are at TJ = 25°C. VIN = 12V, SHDN = 12V, BOOST = 15.3V, BIAS = 5V, FB/PGFB = 1.25V, CSS/SYNC = 0V unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
SHDN
I
SHDN
I
VINS
I
VIN
I
BIASS
I
BIAS
V
REF
I
FB
SHDN Threshold 1.2 1.3 1.4 V SHDN Input Current SHDN = 12V 520 μA
Minimum Input Voltage (Note 3) 2.4 3 V Supply Shutdown Current SHDN = 0V, BOOST = 0V, FB/PGFB = 0V 0.1 2 μA Supply Sleep Current (Note 4) (LT1976) BIAS = 0V, FB = 1.35V 170 300 μA
FB = 1.35V
Supply Quiescent Current BIAS = 0V, FB = 1.15V, VC = 0.8V 3.2 4.10 mA
BIAS = 5V, FB = 1.15V, V
Minimum BIAS Voltage (Note 5) 2.7 3 V BIAS Sleep Current (Note 4) 110 180 μA BIAS Quiescent Current SYNC = 3.3V 700 800 μA
Minimum Boost Voltage (Note 6) ISW = 1.5A 1.8 2.5 V
Input Boost Current (Note 7) ISW = 1.5A 40 50 mA
Reference Voltage (V
FB Input Bias Current 75 200 nA
EA Voltage Gain (Note 8) 900 V/V
EA Voltage g EA Source Current FB = 1.15V 20 40 55 μA EA Sink Current FB = 1.35V 15 30 40 μA
VC to SW g
VC High Clamp 2.1 2.2 2.4 V
m
m
) 3.3V < V
REF
dI(VC)= ±10μA 400 650 800 μMho
< 60V 1.212 1.25 1.288 V
VIN
= 0.8V 2.6 3.25 mA
C
45 100 μA
3A/V
1976bfg
3
LT1976/LT1976B
ELECTRICAL CHARACTERISTICS
The denotes the specifications which apply over the full –40°C to 140°C
operating temperature range, otherwise specifications are at TJ = 25°C. VIN = 12V, SHDN = 12V, BOOST = 15.3V, BIAS = 5V, FB/PGFB = 1.25V, C
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
PK
I
CSS
I
PGFB
V
PGFB
I
CT
V
CT
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.
Note 2: The LT1976EFE/LT1976BEFE are guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT1976IFE/LT1976BIFE/LT1976HFE are guaranteed and tested over the full –40°C to 125°C operating junction temperature range. The LT1976HFE is also tested to the LT1976HFE electrical characteristics table at 140°C operating junction temperature. High junction temperatures degrade operating lifetimes.
Note 3: Minimum input voltage is defined as the voltage where switching starts. Actual minimum input voltage to maintain a regulated output will depend upon output voltage and load current. See Applications Information.
Note 4: Supply input current is the quiescent current drawn by the input pin. Its typical value depends on the voltage on the BIAS pin and operating state of the LT1976. With the BIAS pin at 0V, all of the quiescent current required to operate the LT1976 will be provided by the V BIAS voltage above its minimum input voltage, a portion of the total quiescent current will be supplied by the BIAS pin. Supply sleep current for the LT1976 is defined as the quiescent current during the “sleep” portion of Burst Mode operation. See Applications Information for determining application supply currents.
SW Current Limit 2.4 A Switch On Resistance (Note 9) 0.2 0.6 Ω
Switching Frequency BOOST = OPEN 150 200 260 kHz
Maximum Duty Cycle 90 92 %
Minimum SYNC Amplitude 1.5 2.0 V
SYNC Frequency Range 230 600 kHz SYNC Input Impedance SYNC = 0.5V 85 kΩ
CSS Current Threshold (Note 10) 7 13 20 μA
PGFB Input Current 25 100 nA
PGFB Voltage Threshold (Note 11) 87 90 93 %
CT Source Current (Note 11) 1.5 3.6 5.5 μA
CT Sink Current (Note 11) 1 2 mA
CT Voltage Threshold (Note 11) 1.16 1.2 1.26 V PG Leakage (Note 11) PG = 12V 0.1 1 μA PG Sink Current (Note 11) PGFB = 1V, PG = 400mV 120 200 μA
/SYNC = 0V unless otherwise noted.
SS
pin. With the
IN
Note 5: Minimum BIAS voltage is the voltage on the BIAS pin when I sourced into the pin.
Note 6: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the internal power switch.
Note 7: Boost current is the current flowing into the BOOST pin with the pin held 3.3V above input voltage. It flows only during switch on time.
Note 8: Gain is measured with a V Note 9: Switch on resistance is calculated by dividing VIN to SW voltage by
the forced current (1.5A LT1976, 1.2A LT1976B). See Typical Performance Characteristics for the graph of switch voltage at other currents.
Note 10: The C the C
pin which results in an increase in sink current from the VC pin.
SS
See the Soft-Start section in Applications Information. Note 11: The PGFB threshold is defined as the percentage of V
which causes the current source output of the C sinking (below threshold) to sourcing current (above threshold). When sourcing current, the voltage on the CT pin rises until it is clamped internally. When the clamp is activated, the output of the PG pin will be set to a high impedance state. When the C be set active low with a current sink capability of 200μA.
threshold is defined as the value of current sourced into
SS
swing from 1.15V to 750mV.
C
pin to change from
T
clamp is inactive the PG pin will
T
REF
BIAS
voltage
is
4
1976bfg
UW
1.20
VOLTAGE (V)
1.21
1.23
1.24
1.25
1.30
1.27
1.22
1.28
1.29
1.26
TEMPERATURE (°C)
–50 0
50
75
1976 G01
–25
25
100
150125
TYPICAL PERFOR A CE CHARACTERISTICS
LT1976/LT1976B
LT1976 Efficiency and Power Loss vs Load Current
100
EFFICIENCY
75
5V
3.3V
50
10
TYPICAL POWER LOSS
100
EFFICIENCY (%)
25
0
0.1
1
LOAD CURRENT (mA)
1000
10000
1976 TA02
10
1
0.1
0.01
0.001
LT1976B Efficiency and Power Loss vs Load Current
100
75
POWER LOSS (W)
50
EFFICIENCY (%)
25
0
0.1
1
Oscillator Frequency SHDN Threshold
250
240
230
220
210
200
190
FREQUENCY (kHz)
180
170
160
150
–50
–25
50
25
0
TEMPERATURE (°C)
75
100
150125
1976 G02
1.40
1.35
1.30
1.25
1.20
VOLTAGE (V)
0.15
1.10
1.05
1.00 –50 0 50 75–25 25 100 150125
LT1976 Sleep Mode Supply
Shutdown Supply Current
25
20
15
10
CURRENT (μA)
5
0
–50 0
VIN = 42V VIN = 12V
–25
TEMPERATURE (°C)
25
50
VIN = 60V
75
100
150125
1976 G05
Current
240
220
200
180
160
140
120
100
CURRENT (μA)
80
60
40
20
0
–50 0
–25
EFFICIENCY
5V
3.3V
TYPICAL POWER LOSS
100
10
LOAD CURRENT (mA)
TEMPERATURE (°C)
V
= 0V
BIAS
V
= 5V
BIAS
50
25
TEMPERATURE (°C)
75
100
1000
10000
1976 G25
1976 G03
1976 G06
10
1
0.1
0.01
0.001
150125
FB Voltage
POWER LOSS (W)
SHDN Pin Current
5.5 TJ = 25°C
5.0
4.5
4.0
3.5
3.0
2.5
CURRENT (μA)
2.0
1.5
1.0
0
0
LT1976 Bias Sleep Current
200
180
160
140
120
100
80
CURRENT (μA)
60
40
20
0
–50 0
10
–25
30 40
20
SHDN VOLTAGE (V)
50
25
TEMPERATURE (°C)
75
100
50
1976 G04
1976 G07
1976bfg
5
60
150125
LT1976/LT1976B
LOAD CURRENT (A)
0
VOLTAGE (mV)
50
150
200
250
500
350
0.7
0.9
1976 G13
100
400
450
300
0.5
1.1
1.3
1.5
TJ = 125°C
TJ = 25°C
TJ = –50°C
UW
TYPICAL PERFOR A CE CHARACTERISTICS
PGFB Threshold
1.20
1.18
1.16
1.14
1.12
1.10
1.08
VOLTAGE (V)
1.06
1.04
1.02
1.00 –50 0
–25
50
25
TEMPERATURE (°C)
Soft-Start Current Threshold vs FB Voltage
50
TJ = 25°C
45
40
35
30
25
20
CURRENT (μA)
15
10
5
0
0
0.2
0.6 0.8
0.4 FB VOLTAGE (V)
75
100
SOFT-START DEFEATED
1.0
1976 G08
1976 G11
PG Sink Current
250
200
150
100
CURRENT (μA)
50
150125
0
–50 0
–25
50
25
TEMPERATURE (°C)
75
100
150125
1976 G08
Switch Peak Current Limit
3.5
3.0
2.5
2.0
PEAK SWITCH CURRENT (A)
1.5 –50
–25 –0 25 50
TEMPERATURE (°C)
75 100 150125
1976 G10
Oscillator Frequency
1.2
vs FB Voltage
250
TJ = 25°C
200
150
100
FREQUENCY (kHz)
50
0
0.2
0
0.6 0.8
0.4 FB VOLTAGE (V)
1.0
1.2
1976 G12
Switch On Voltage (V
CESAT
)
LT1976 Supply Current vs Input Voltage
150
125
100
75
50
SUPPLY CURRENT (μA)
25
0
0
10
6
30 40
20
INPUT VOLTAGE (V)
LT1976 Burst Mode Threshold
Minimum Input Voltage
V
= 3.3V
OUT
= 25°C
T
A
50
60
1976 F05
7.5
7.0
6.5 5V START
6.0
5.5
5.0
3.3V START
4.5
INPUT VOLTAGE (V)
4.0
3.5
3.0
200
0
400
5V RUNNING
3.3V RUNNING
800
600
LOAD CURRENT (mA)
1000
1200
1400
1976 G19
1600
vs Input Voltage
200
V
= 3.3V
OUT
180
L = 33μH
= 100μF
C
OUT
160
140
120
100
80
60
LOAD CURRENT (mA)
40
20
0
97
5
17 19 23
1311
15
INPUT VOLTAGE (V)
21
25
1975 G20
1976bfg
UW
TYPICAL PERFOR A CE CHARACTERISTICS
LT1976B VC Switching Threshold vs Temperature
700
600
500
400
300
VOLTAGE (V)
C
V
200
100
0
–50
–10 3010–30 50
TEMPERATURE (˚C)
70 90 110
1976 G26
Minimum On Time Boost Current vs Load Current
500
450
400
350
300
LOAD CURRENT = 0.5A
250
200
ON TIME (ns)
150
100
50
0
–50
–30 10
LOAD CURRENT = 1A
–10
30
TEMPERATURE (°C)
50
90
70
110
1976 G21
LT1976/LT1976B
50
45
40
35
30
25
20
15
BOOST CURRENT (mA)
10
5
0
200 600
0
400
LOAD CURRENT (mA)
800
1000
1200
1400
1976 G22
Dropout Operation
4.0 V
= 3.3V
OUT
BOOST DIODE = DIODES INC. B1100
3.5
3.0
2.5
2.0
1.5
OUTPUT VOLTAGE (V)
1.0
0.5
0
2
LOAD CURRENT = 1.25A
LOAD CURRENT = 250mA
2.5 3 4 INPUT VOLTAGE (V)
LT1976 Burst Mode Operation
V
OUT
50mV/
DIV
I
SW
100mA/
DIV
0A
= 12V TIME (10μs/DIV) 1976 G15
V
IN
V
= 3.3V
OUT
= 100μA
I
Q
3.5
1976 G23
4.5
Dropout Operation
6
V BOOST DIODE = DIODES INC. B1100
5
4
3
2
OUTPUT VOLTAGE (V)
1
0
2
LT1976B No Load Operation (Pulse-Skipping Mode)
V
OUT
50mV/DIV
AC
COUPLED
I
SW
100mA/DIV
0A
V
IN
V
OUT
I
Q
= 5V
OUT
LOAD CURRENT = 250mA
LOAD CURRENT = 1.25A
5
34
2.5 3.5 INPUT VOLTAGE (V)
= 12V TIME (10μs/DIV) 1976 G27
= 3.3V
= 1.6mA
4.5
5.5
1976 G24
100mA/DIV
6
100mV/DIV
500mA/DIV
LT1976 Burst Mode Operation
V
OUT
50mV/DIV
I
SW
0A
= 12V TIME (5ms/DIV) 1976 G14
V
IN
V
= 3.3V
OUT
= 100μA
I
Q
LT1976 No Load 1A Step Response
V
OUT
1A
I
OUT
0A
= 12V TIME (1ms/DIV) 1976 G17
V
IN
V
= 3.3V
OUT
= 47μF
C
OUT
1976bfg
7
LT1976/LT1976B
UW
TYPICAL PERFOR A CE CHARACTERISTICS
LT1976 Step Response
V
OUT
100mV/DIV
1A
I
OUT
500mA/DIV
0A
= 12V TIME (1ms/DIV) 1976 G18
V
IN
V
= 3.3V
OUT
C
= 47μF
OUT
= 250mA
I
DC
U
UU
PI FU CTIO S
NC (Pins 1, 3, 5): No Connection. Pins 1, 3, 5 are electrically isolated from the LT1976. They may be con­nected to PCB traces to aid in PCB layout.
SW (Pin 2): The SW pin is the emitter of the on-chip power NPN switch. This pin is driven up to the input pin voltage during switch on time. Inductor current drives the SW pin negative during switch off time. Negative voltage is clamped with the external Schottky catch diode to prevent exces­sive negative voltages.
VIN (Pin 4): This is the collector of the on-chip power NPN switch. VIN powers the internal control circuitry when a voltage on the BIAS pin is not present. High di/dt edges occur on this pin during switch turn on and off. Keep the path short from the VIN pin through the input bypass capacitor, through the catch diode back to SW. All trace inductance on this path will create a voltage spike at switch off, adding to the VCE voltage across the internal NPN.
BOOST (Pin 6): The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. Without this added voltage, the typical switch voltage loss would be about 1.5V. The additional BOOST voltage allows the switch to saturate and its voltage loss approximates that of a 0.2Ω FET structure, but with much smaller die area.
CT (Pin 7): A capacitor on the CT pin determines the amount of delay time between the PGFB pin exceeding its thresh­old (V When the PGFB pin rises above V
) and the PG pin set to a high impedance state.
PGFB
, current is sourced
PGFB
from the CT pin into the external capacitor. When the volt­age on the external capacitor reaches an internal clamp (VCT), the PG pin becomes a high impedance node. The resultant PG delay time is given by t = C voltage on the PGFB pin drops below V
• VCT/ICT. If the
CT
, CCT will be
PGFB
discharged rapidly to 0V and PG will be active low with a 200μA sink capability. If the CT pin is clamped (Power Good condition) during normal operation and SHDN is taken low, the CT pin will be discharged and a delay period will occur when SHDN is returned high. See the Power Good section in Applications Information for details.
GND (Pins 8, 17): The GND pin connection acts as the reference for the regulated output, so load regulation will suffer if the “ground” end of the load is not at the same voltage as the GND pin of the IC. This condition will occur when load current or other currents flow through metal paths between the GND pin and the load ground. Keep the path between the GND pin and the load ground short and use a ground plane when possible. The GND pin also acts as a heat sink and should be soldered (along with the exposed leadframe) to the copper ground plane to reduce thermal resistance (see Applications Information).
8
1976bfg
LT1976/LT1976B
U
UU
PI FU CTIO S
CSS (Pin 9): A capacitor from the CSS pin to the regulated output voltage determines the output voltage ramp rate during start-up. When the current through the CSS capaci­tor exceeds the CSS threshold (I the output is limited. The C the FB voltage (see Typical Performance Characteristics) and is defeated for FB voltage greater than 0.9V (typical). See Soft-Start section in Applications Information for details.
BIAS (Pin 10): The BIAS pin is used to improve efficiency when operating at higher input voltages and light load current. Connecting this pin to the regulated output volt­age forces most of the internal circuitry to draw its operating current from the output voltage rather than the input supply. This architecture increases efficiency espe­cially when the input voltage is much higher than the output. Minimum output voltage setting for this mode of operation is 3V.
V
(Pin 11): The VC pin is the output of the error amplifier
C
and the input of the peak switch current comparator. It is normally used for frequency compensation, but can also serve as a current clamp or control loop override. The V pin sits about 0.45V for light loads and 2.2V at current limit. The LT1976 clamps the VC pin slightly below the burst threshold during sleep periods for better transient response. Driving the VC pin to ground will disable switch­ing and also place the LT1976 into sleep mode.
FB (Pin 12): The feedback pin is used to determine the output voltage using an external voltage divider from the output that generates 1.25V at the FB pin . When the FB pin drops below 0.9V, switching frequency is reduced, the SYNC function is disabled and output ramp rate control is enabled via the CSS pin. See the Feedback section in Applications Information for details.
SS
), the voltage ramp of
CSS
threshold is proportional to
C
PGFB (PIN 13): The PGFB pin is the positive input to a comparator whose negative input is set at V PGFB is taken above V the C
pin starting the PG delay period. When the voltage
T
on the PGFB pin drops below V discharged resetting the PG delay period. The PGFB volt­age is typically generated by a resistive divider from the regulated output or input supply. See Power Good section in Applications Information for details.
SYNC (Pin 14): The SYNC pin is used to synchronize the internal oscillator to an external signal. It is directly logic compatible and can be driven with any signal between 20% and 80% duty cycle. The synchronizing range is equal to maximum initial operating frequency up to 700kHz. When the voltage on the FB pin is below 0.9V the SYNC function is disabled. See the Synchronizing section in Applications Information for details.
SHDN (Pin 15): The SHDN pin is used to turn off the regulator and to reduce input current to less than 1μA. The SHDN pin requires a voltage above 1.3V with a typical source current of 5μA to take the IC out of the shutdown state.
PG (Pin 16): The PG pin is functional only when the SHDN pin is above its threshold, and is active low when the internal clamp on the C high impedance when the clamp is active. The PG pin has a typical sink capability of 200μA. See the Power Good section in Applications Information for details.
, current (I
PGFB
PGFB
pin is below its clamp level and
T
) is sourced into
CSS
, the CT pin is rapidly
PGFB
. When
1976bfg
9
LT1976/LT1976B
BLOCK DIAGRA
V
IN
4
BIAS
10
SYNC
14
SHDN
15
+
SHDN COMP
1.3V
C
SS
9
UNDERVOLTAGE
W
INTERNAL REF
LOCKOUT
THERMAL
SHUTDOWN
SOFT-START
2.4V
SLOPE COMP
200kHz
OSCILLATOR
ANTISLOPE
COMP
BURST MODE
DETECT
LT1976 ONLY
Σ
CURRENT
COMP
R
S
+
SWITCH
LATCH
Q
DRIVER
CIRCUITRY
BOOST
SW
6
2
FOLDBACK
DETECT
FB
12
1.25V
V
C
11
PGFB
13
1.12V
C
T
7
ERROR
AMP
+
+
PG
COMP
V
C
CLAMP
1.2V C CLAMP
Figure 1. LT1976/LT1976B Block Diagram
The LT1976 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscilla­tor pulse which sets the RS latch to turn the switch on. When switch current reaches a level set by the current compara­tor the latch is reset and the switch turns off. Output volt­age control is obtained by using the output of the error amplifier to set the switch current trip point. This technique means that the error amplifier commands current to be
PG
16
T
GND
17
PGND
8
1976 BD
delivered to the output rather than voltage. A voltage fed system will have low phase shift up to the resonant fre­quency of the inductor and output capacitor, then an abrupt 180° shift will occur. The current fed system will have 90° phase shift at a much lower frequency, but will not have the additional 90° shift until well beyond the LC resonant fre­quency. This makes it much easier to frequency compen­sate the feedback loop and also gives much quicker tran­sient response.
Most of the circuitry of the LT1976 operates from an internal 2.4V bias line. The bias regulator normally draws
10
1976bfg
BLOCK DIAGRA
LT1976/LT1976B
W
power from the VIN pin, but if the BIAS pin is connected to an external voltage higher than 3V bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency.
High switch efficiency is attained by using the BOOST pin to provide a voltage to the switch driver which is higher than the input voltage, allowing switch to be saturated. This boosted voltage is generated with an external capaci­tor and diode.
To further optimize efficiency, the LT1976 automatically switches to Burst Mode operation in light load situations.
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APPLICATIO S I FOR ATIO
CHOOSING THE LT1976 OR LT1977
The LT1976/LT1976B and LT1977 are high voltage 1.5A step-down switching regulators. The LT1976 and LT1977 contain circuitry which shifts into burst mode at light loads reducing quiescent current to typically 100μA. The LT1976B pulse skips in light load situations, eliminating low fre­quency burst mode output ripple at the expense of light load efficiency. The difference between the LT1976/ LT1976B and LT1977 is that the fixed switching frequency of the LT1976/LT1976B is 200kHz versus 500kHz for the LT1977. The switching frequency affects: inductor size, input voltage range in continuous mode operation, effi­ciency, thermal loss and EMI.
In Burst Mode operation, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 45μA.
The only difference between the LT1976 and the LT1976B is that the LT1976B does not shift into burst mode in light load situations, eliminating low frequency output ripple at the expense of light load efficiency.
The LT1976 contains a power good flag with a program­mable threshold and delay time. A logic-level low on the SHDN pin disables the IC and reduces input suppy current to less than 1μA.
INPUT VOLTAGE RANGE
The minimum on and off times for all versions of the ic are equivalent. This results in a narrower range of continuous mode operation for the LT1977. Typical minimum and maximum duty cycles are 6% to 92% for the LT1976/ LT1976B and 15% to 90% for the LT1977. Both parts will regulate up to an input voltage of 60V but the LT1977 will transistion into pulse-skipping/Burst Mode operation when the input voltage is above 30V for a 5V output. At outputs above 10V the LT1977’s input range will be similar to the LT1976/LT1976B. Lowering the input voltage below the maximum duty cycle limitation will cause a dropout in regulation.
OUTPUT RIPPLE AND INDUCTOR SIZE
Output ripple current is determined by the input to output voltage ratio, inductor value and switch frequency. Since the switch frequency of the LT1977 is 2.5 times greater than that of the LT1976/LT1976B, the inductance used in the LT1977 application can be 2.5 times lower than the LT1976/LT1976B while maintaining the same output ripple current. The lower value used in the LT1977 application allows the use of a physically smaller inductor.
Table 1. LT1976/LT1976B/LT1977 Comparison
PARAMETER ADVANTAGE
Minimum Duty Cycle LT1976/LT1976B
Maximum Duty Cycle LT1976/LT1976B
Inductor Size LT1977
Output Capacitor Size LT1977
Efficiency LT1976
EMI LT1976B
Input Range LT1976/LT1976B
Output Ripple LT1977
Light Load Output Ripple LT1976B
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11
LT1976/LT1976B
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APPLICATIO S I FOR ATIO
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1976 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage and the remaining part talks about frequency foldback and soft-start features. Please read both parts before committing to a final design.
Referring to Figure 2, the output voltage is determined by a voltage divider from V
to ground which generates
OUT
1.25V at the FB pin. Since the output divider is a load on the output care must be taken when choosing the resistor divider values. For light load applications the resistor values should be as large as possible to achieve peak efficiency in Burst Mode operation. Extremely large values for resistor R1 will cause an output voltage error due to the 50nA FB pin input current. The suggested value for the output divider resistor (see Figure 2) from FB to ground (R2) is 100k or less. A formula for R1 is shown below. A table of standard 1% values is shown in Table 2 for common output voltages.
RR
12
=+•
V
125 2 50
.•
125
–.
OUT
RnA
For LT1976B aplications, the suggested value for R2 is 10k or less, eliminating output voltage errors due to feedback pin current and reducing noise susceptibility.
More Than Just Voltage Feedback
The FB pin is used for more than just output voltage sensing. It also reduces switching frequency and con­trols the soft-start voltage ramp rate when output voltage is below the regulated level (see the Frequency Foldback and Soft-Start Current graphs in Typical Performance Characteristics).
Frequency foldback is done to control power dissipation in both the IC and in the external diode and inductor during short-circuit conditions. A shorted output requires the switching regulator to operate at very low duty cycles. As a result the average current through the diode and induc­tor is equal to the short-circuit current limit of the switch (typically 2.4A for the LT1976). Minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 200kHz, so frequency is reduced by about 4:1 when the FB pin voltage drops below 0.4V (see Frequency Foldback graph). In addition, if the current in the switch exceeds 1.5 times the current limitations speci­fied by the V
pin, due to minimum switch on time, the
C
LT1976 will skip the next switch cycle. As the feedback voltage rises, the switching frequency increases to 200kHz with 0.95V on the FB pin. During frequency foldback, external syncronization is disabled to prevent interference with foldback operation. Frequency foldback does not affect operation during normal load conditions.
LT1976
OSCILLATOR
12
200kHz
SW
SOFT-START
FOLDBACK
DETECT
ERROR
AMP
Figure 2. Feedback Network
C
+
1.25V
V
2
C1
SS
9
FB
12
V
C
11
1976 F02
OUT
R1
R2
In addition to lowering switching frequency the soft-start ramp rate is also affected by the feedback voltage. Large
Table 2
OUTPUT R1 OUTPUT
VOLTAGE R2 NEAREST (1%) ERROR
(V) (kΩ, 1%) (kΩ)(%)
2.5 100 100 0
3 100 140 0
3.3 100 165 0.38
5 100 300 0
6 100 383 0.63
8 100 536 – 0.63
10 100 698 – 0.25
12 100 866 0.63
1976bfg
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APPLICATIO S I FOR ATIO
LT1976/LT1976B
capacitive loads or high input voltages can cause a high input current surge during start-up. The soft-start func­tion reduces input current surge by regulating switch current via the VC pin to maintain a constant voltage ramp rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from the CSS pin to the output determines the maximum output dV/dt. When the feedback voltage is below 0.4V, the VC pin will rise, resulting in an increase in switch current and output voltage. If the dV/dt of the output causes the current through the CSS capacitor to exceed I reduced resulting in a constant dV/dt at the output. As the feedback voltage increases I increased dV/dt until the soft-start function is defeated with 0.9V present at the FB pin. The soft-start function does not affect operation during normal load conditions. However, if a momentary short (brown out condition) is present at the output which causes the FB voltage to drop below 0.9V, the soft-start circuitry will become active.
INPUT CAPACITOR
Step-down regulators draw current from the input supply in pulses. The rise and fall times of these pulses are very fast. The input capacitor is required to reduce the voltage ripple this causes at the input of LT1976 and force the switching current into a tight local loop, thereby minimiz­ing EMI. The RMS ripple current can be calculated from:
I
I
RIPPLE RMS
Ceramic capacitors are ideal for input bypassing. At 200kHz switching frequency input capacitor values in the range of
4.7μF to 20μF are suitable for most applications. If opera- tion is required close to the minimum input required by the LT1976 a larger value may be required. This is to prevent excessive ripple causing dips below the minimum operat­ing voltage resulting in erratic operation.
Input voltage transients caused by input voltage steps or by hot plugging the LT1976 to a pre-powered source such as a wall adapter can exceed maximum VIN ratings. The sudden application of input voltage will cause a large surge of current in the input leads that will store energy in
OUT
V
IN
increases, resulting in an
CSS
VVV
()
OUT IN OUT()
the VC voltage is
CSS
=
the parasitic inductance of the leads. This energy will cause the input voltage to swing above the DC level of input power source and it may exceed the maximum voltage rating of the input capacitor and LT1976. All input voltage transient sequences should be observed at the V the LT1976 to ensure that absolute maximum voltage ratings are not violated.
The easiest way to suppress input voltage transients is to add a small aluminum electrolytic capacitor in parallel with the low ESR input capacitor. The selected capacitor needs to have the right amount of ESR to critically damp the resonant circuit formed by the input lead inductance and the input capacitor. The typical values of ESR will fall in the range of 0.5Ω to 2Ω and capacitance will fall in the range of 5μF to 50μF.
If tantalum capacitors are used, values in the 22μF to 470μF range are generally needed to minimize ESR and meet ripple current and surge ratings. Care should be taken to ensure the ripple and surge ratings are not exceeded. The AVX TPS and Kemet T495 series are surge rated AVX recommends derating capacitor operating volt­age by 2:1 for high surge applications.
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective series resistance (ESR) because this is what determines output ripple voltage. To get low ESR takes volume, so physically smaller capacitors have higher ESR. The ESR range for typical LT1976 applications is 0.05Ω to 0.2Ω. A typical output capacitor is an AVX type TPS, 100μF at 10V, with a guaranteed ESR less than 0.1Ω. This is a “D” size surface mount solid tantalum capacitor. TPS capacitors are specially constructed and tested for low ESR, so they give the lowest ESR for a given volume. The value in microfarads is not particularly critical and values from 22μF to greater than 500μF work well, but you cannot cheat Mother Nature on ESR. If you find a tiny 22μF solid tantalum capacitor, it will have high ESR and output ripple voltage could be unacceptable. Table 3 shows some typical solid tantalum surface mount capacitors.
pin of
IN
1976bfg
13
LT1976/LT1976B
I
VVV
VLf
OUT IN OUT
IN
P-P
=
()
()()()
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APPLICATIO S I FOR ATIO
Table 3. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current
E CASE SIZE ESR MAX (Ω) RIPPLE CURRENT (A)
AVX TPS 0.1 to 0.3 0.7 to 1.1
D CASE SIZE
AVX TPS 0.1 to 0.3 0.7 to 1.1
C CASE SIZE
AVX TPS 0.2 0.5
Many engineers have heard that solid tantalum capacitors are prone to failure if they undergo high surge currents. This is historically true and type TPS capacitors are specially tested for surge capability but surge ruggedness is not a critical issue with the output capacitor. Solid tantalum capacitors fail during very high turn-on surges which do not occur at the output of regulators. High discharge surges, such as when the regulator output is dead shorted, do not harm the capacitors.
Unlike the input capacitor RMS, ripple current in the output capacitor is normally low enough that ripple cur­rent rating is not an issue. The current waveform is triangular with a typical value of 200mA
. The formula
RMS
to calculate this is:
Output capacitor ripple current (RMS)
.–
VVV
029
I
RIPPLE RMS
()
()( )
=
OUT IN OUT
LfV
()()( )
IN
I
P-P
=
12
filter capacitor CF in parallel with the RC/CC network, along with a small feedforward capacitor C
, is suggested to
FB
control possible ripple at the VC pin. The LT1976 can be stabilized using a 47μF ceramic output capacitor and V
C
component values of CC = 0.047μF, RC = 12.5k, CF = 100pF and CFB = 27pF.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform for the LT1976. Ripple voltage is determined by the impedance of the output capacitor and ripple current through the inductor. Peak-to-peak ripple current through the inductor into the output capacitor is:
For high frequency switchers the ripple current slew rate is also relevant and can be calculated from:
didtV
IN
=
L
Peak-to-peak output ripple voltage is the sum of a triwave created by peak-to-peak ripple current times ESR and a square wave created by parasitic inductance (ESL) and ripple current slew rate. Capacitive reactance is assumed to be small compared to ESR or ESL.
CERAMIC CAPACITORS
Higher value, lower cost ceramic capacitors are now becoming available. They are generally chosen for their good high frequency operation, small size and very low ESR (effective series resistance). Low ESR reduces output ripple voltage but also removes a useful zero in the loop frequency response, common to tantalum capacitors. To compensate for this a resistor RC can be placed in series with the VC compensation capacitor CC (Figure 10). Care must be taken however since this resistor sets the high frequency gain of the error amplifier including the gain at the switching frequency. If the gain of the error amplifier is high enough at the switching frequency output ripple voltage (although smaller for a ceramic output capacitor) may still affect the proper operation of the regulator. A
14
V I ESR ESL
RIPPLE
47μF TANTALUM
ESR 100mΩ
47μF CERAMIC
=
()( )+()
P-P
V
OUT
20mV/DIV
V
OUT
20mV/DIV
V
SW
5V/DIV
V
= 12V 1μs/DIV 1976 F03
IN
V
= 3.3V
OUT
I
= 1A
LOAD
L = 33μH
Figure 3. LT1976 Ripple Voltage Waveform
di dt
1976bfg
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APPLICATIO S I FOR ATIO
LT1976/LT1976B
Example: with V
0.08Ω, ESL = 10nH:
I
=
P-P
di dt e
V
= 0.0289 + 0.003 = 32mV
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by the maximum switch current rating (IPK). The current rating for the LT1976 is 1.5A. Unlike most current mode converters, the LT1976 maximum switch current limit does not fall off at high duty cycles. Most current mode converters suffer a drop off of peak switch current for duty cycles above 50%. This is due to the effects of slope compensation required to prevent subharmonic oscilla­tions in current mode converters. (For detailed analysis, see Application Note 19.)
The LT1976 is able to maintain peak switch current limit over the full duty cycle range by using patented circuitry to cancel the effects of slope compensation on peak switch current without affecting the frequency compensation it provides.
12 33 6 200 3
()
12
==
33 5
.–
= (0.362A)(0.08) + (10e – 9)(363e3)
RIPPLE
= 12V, V
IN
33 12 33
.–.
()( )
ee
()()
363 5
.
e
= 3.3V, L = 33μH, ESR =
OUT
.
=
0 362
P-P
A
For V
Note that there is less load current available at the higher input voltage because inductor ripple current increases. At V
IN
conditions:
To calculate actual peak switch current in continuous mode with a given set of conditions, use:
If a small inductor is chosen which results in discontinous mode operation over the entire load range, the maximum load current is equal to:
= 5V, V
OUT
I
OUT MAX()
= 15V, duty cycle is 33% and for the same set of
I
OUT MAX()
II
SW PK OUT
I
OUT MAX
=+
()
()
= 8V and L = 20μH:
IN
58 5
.–
=
15
.–. .
==
15 024 126
.–
=
15
.–. .
==
15 042 108
VVV
IfLV
PK IN
=
2
VVV
()( )
OUT IN OUT
()( )
ee
2 20 6 200 3 8
()()()
A
515 5
()( )
ee
2 20 6 200 3 15
()()()
A
OUT IN OUT
2
2
2
()( )( )
()
LfV
()()( )
IN
Maximum load current would be equal to maximum switch current for an infinitely large inductor, but with finite inductor size, maximum load current is reduced by one-half peak-to-peak inductor current. The following formula assumes continuous mode operation, implying that the term on the right (I
VVV
()( )
II
OUT MAX PK
()
Discontinuous operation occurs when:
VVV
I
OUT DIS
()
OUT IN OUT
OUT IN OUT
()
2
()()( )
LfV
/2) is less than I
P-P
2
LfV
()()( )
IN
IN
.
OUT
I
-P
P
=
=
I
PK
2
CHOOSING THE INDUCTOR
For most applications the output inductor will fall in the range of 15μH to 100μH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1976 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage and reduce core loss.
When choosing an inductor you might have to consider maximum load current, core and copper losses, allow­able component height, output voltage ripple, EMI, fault current in the inductor, saturation and of course cost. The following procedure is suggested as a way of han­dling these somewhat complicated and conflicting requirements.
1976bfg
15
LT1976/LT1976B
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APPLICATIO S I FOR ATIO
1. Choose a value in microhenries from the graph of maximum load current. Choosing a small inductor with lighter loads may result in discontinuous mode of operation, but the LT1976 is designed to work well in either mode.
Table 4. Inductor Selection Criteria
VENDOR/ PART NUMBER VALUE (μH) I
Coiltronics
UP2B-150 15 2.4 0.041 6
UP2B-330 33 1.7 0.062 6
UP2B-470 47 1.4 0.139 6
UP2B-680 68 1.2 0.179 6
UP2B-101 100 0.95 0.271 6
UP3B-150 15 3.9 0.032 6.8
UP3B-330 33 2.4 0.069 6.8
UP3B-470 47 1.9 0.101 6.8
UP3B-680 68 1.6 0.156 6.8
UP3B-101 100 1.4 0.205 6.8
Sumida
CDRH8D28-150M 15 2.2 0.053 3
CDRH124-150M 15 3.2 0.05 4.5
CDRH127-150M 15 4.5 0.02 8
CDRH8D28-330M 33 1.4 0.122 3
CDRH124-330M 33 2.7 0.97 4.5
CDRH127-330M 33 3.0 0.048 8
CDRH8D28-470M 47 1.25 0.150 3
CDRH125-470M 47 1.8 0.058 6
CDRH127-470M 47 2.5 0.076 8
CDRH124-680M 68 1.5 0.228 4.5
CDRH127-680M 68 2.1 0.1 8
CDRH124-101M 100 1.2 0.30 4.5
CDRH127-101M 100 1.7 0.17 8
Coilcraft
DT3308P-153 15 2.0 0.1 3
DT3308P-333 33 1.4 0.3 3
DT3308P-473 47 1 0.47 3
(A) DCR (Ω) HEIGHT (mm)
RMS
Assume that the average inductor current is equal to load current and decide whether or not the inductor must withstand continuous fault conditions. If maxi­mum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 2A overload condition.
For applications with a duty cycle above 50%, the inductor value should be chosen to obtain an inductor ripple current of less than 40% of the peak switch current.
2. Calculate peak inductor current at full load current to ensure that the inductor will not saturate. Peak current can be significantly higher than output current, especially with smaller inductors and lighter loads, so don’t omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core materials fall somewhere in between. The following formula assumes continuous mode of opera­tion, but it errs only slightly on the high side for discon­tinuous mode, so it can be used for all conditions.
II
=+
PEAK OUT
VVV
OUT IN OUT
2
()
fLV
()( )( )
IN
VIN = maximum input voltage f = switching frequency, 200kHz
3. Decide if the design can tolerate an “open” core geom­etry like a rod or barrel, which have high magnetic field radiation, or whether it needs a closed core like a toroid to prevent EMI problems. This is a tough decision because the rods or barrels are temptingly cheap and small and there are no helpful guidelines to calculate when the magnetic field radiation will be a problem.
4. After making an initial choice, consider the secondary things like output voltage ripple, second sourcing, etc. Use the experts in the Linear Technology’s applications department if you feel uncertain about the final choice. They have experience with a wide range of inductor types and can tell you about the latest developments in low profile, surface mounting, etc.
16
1976bfg
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APPLICATIO S I FOR ATIO
LT1976/LT1976B
Short-Circuit Considerations
The LT1976 is a current mode controller. It uses the V node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as this peak current is reached. The internal clamp on the V node, nominally 2.2V, then acts as an output switch peak current limit. This action becomes the switch current limit specification. The maximum available output power is then determined by the switch current limit.
A potential controllability prod}m could occur under short­circuit conditions. If the power supply output is short circuited, the feedback amplifier responds to the low output voltage by raising the control voltage, VC, to its peak current limit value. Ideally, the output switch would be turned on, and then turned off as its current exceeded the value indicated by VC. However, there is finite response time involved in both the current comparator and turn-off of the output switch. These result in a minimum on time t
ON(MIN).
(VF + I • R), the diode forward voltage plus inductor I • R voltage drop, the potential exists for a loss of control. Expressed mathematically the requirement to maintain control is:
where:
If this condition is not observed, the current will not be limited at IPK but will cycle-by-cycle ratchet up to some higher value. Using the nominal LT1976 clock frequency of 200kHz, a VIN of 40V and a (VF + I • R) of say 0.7V, the maximum tON to maintain control would be approximately 90ns, an unacceptably short time.
When combined with the large ratio of VIN to
VIR
•≤+
ft
ON
f = switching frequency tON = switch on time VF = diode forward voltage VIN = Input voltage I • R = inductor I • R voltage drop
F
V
IN
C
C
The solution to this dilemma is to slow down the oscillator to allow the current in the inductor to drop to a sufficiently low value such that the current doesn’t continue to ratchet higher. When the FB pin voltage is abnormally low thereby indicating some sort of short-circuit condition, the oscil­lator frequency will be reduced. Oscillator frequency is reduced by a factor of 4 when the FB pin voltage is below
0.4V and increases linearly to its typical value of 200kHz at
a FB voltage of 0.95V (see Typical Performance Character­istics). In addition, if the current in the switch exceeds 1.5
current demanded by the VC pin, the LT1976 will skip
• I
PK
the next on cycle effectively reducing the oscillator fre­quency by a factor of 2. These oscillator frequency reduc­tions during short-circuit conditions allow the LT1976 to maintain current control.
SOFT-START
For applications where [V large input surge currents can’t be tolerated, the LT1976 soft-start feature should be used to control the output capacitor charge rate during start-up, or during recovery from an output short circuit thereby adding additional control over peak inductor current. The soft-start function limits the switch current via the VC pin to maintain a constant voltage ramp rate (dV/dt) at the output capacitor. A capacitor (C1 in Figure 2) from the CSS pin to the regulated output voltage determines the output voltage ramp rate. When the current through the CSS capacitor exceeds the CSS threshold (I output capacitor is limited by reducing the VC pin voltage. The CSS threshold is proportional to the FB voltage (see Typical Performance Characteristics) and is defeated for FB voltages greater than 0.9V (typical). The output dV/dt can be approximated by:
dVdtI
CSS
=
C
SS
but actual values will vary due to start-up load conditions, compensation values and output capacitor selection.
IN
/(V
+ VF)] ratios > 10 or
OUT
), the voltage ramp of the
CSS
1976bfg
17
LT1976/LT1976B
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APPLICATIO S I FOR ATIO
C
= GND
V
0.5V/DIV
SS
OUT
C
= 0.1μFC
SS
C
= 47μF TIME (1ms/DIV) 1976 F04
OUT
I
= 200mA
LOAD
= 12V
V
IN
Figure 4. V
OUT
dV/dt
= 0.1μF
SS
Burst Mode OPERATION (LT1976 ONLY)
To enhance efficiency at light loads, the LT1976 automati­cally switches to Burst Mode operation which keeps the output capacitor charged to the proper voltage while mini­mizing the input quiescent current. During Burst Mode operation, the LT1976 delivers short bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capaci­tor. In addition, VIN and BIAS quiescent currents are re­duced to typically 45μA and 125μA respectively during the sleep time. As the load current decreases towards a no load condition, the percentage of time that the LT1976 operates in sleep mode increases and the average input current is greatly reduced resulting in higher efficiency.
The minimum average input current depends on the VIN to V
ratio, VC frequency compensation, feedback divider
OUT
network and Schottky diode leakage. It can be approxi­mated by the following equation:
Example: For V
45 5
IAA
IN AVG()
=μ+μ+
45 5 47 97
=μ+μ+μ=μ
During the sleep portion of the Burst Mode cycle, the V
= 3.3V, VIN = 12V
OUT
μ+ μ+
125 12 5 0
AA
()
33
.
12
AA A A
.
08
()
...5
μ
A
C
pin voltage is held just below the level needed for normal operation to improve transient response. See the Typical Performance Characteristics section for burst and tran­sient response waveforms.
If a no load condition can be anticipated, the supply current can be further reduced by cycling the SHDN pin at a rate higher than the natural no load burst frequency. Figure 6 shows Burst Mode operation with the SHDN pin. V
OUT
burst ripple is maintained while the average supply current
150
125
100
75
50
SUPPLY CURRENT (μA)
25
0
10
0
30 40
20
INPUT VOLTAGE (V)
Figure 5. IQ vs V
V
IN
OUT
T
= 3.3V
= 25°C
A
50
60
1976 F05
III
III
IN AVG VINS SHDN
≅+ +
()
⎛ ⎜
V
()
OUT
⎟ ⎠
V
IN
++
BIASS FB S
η
()
where
I
= input pin current in sleep mode
VINS
V
= output voltage
OUT
V
input voltage
IN =
I
= BIAS pin current in sleep mode
BIASS
IFB = feedback network current IS = catch diode reverse leakage at V
OUT
η = low current efficiency (non Burst Mode operation)
18
V
OUT
50mV/DIV
V
SHDN
2V/DIV
I
SW
500mA/DIV
V
= 12V TIME (50ms/DIV) 1976 G16
IN
V
= 3.3V
OUT
= 15μA
I
Q
Figure 6. Burst Mode with Shutdown Pin
1976bfg
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APPLICATIO S I FOR ATIO
LT1976/LT1976B
drops to 15μA. The PG pin will be active low during the “on” portion of the SHDN waveform due to the C
capaci-
T
tor discharge when SHDN is taken low. See the Power Good section for further information.
CATCH DIODE
The catch diode carries load current during the SW off time. The average diode current is therefore dependent on the switch duty cycle. At high input to output voltage ratios the diode conducts most of the time. As the ratio ap­proaches unity the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short circuited. Under this condition the diode must safely handle I
at maximum duty cycle.
PEAK
To maximize high and low load current efficiency a fast switching diode with low forward drop and low reverse leakage should be used. Low reverse leakage is critical to maximize low current efficiency since its value over tem­perature can potentially exceed the magnitude of the LT1976 supply current. Low forward drop is critical for high current efficiency since the loss is proportional to forward drop.
These requirements result in the use of a Schottky type diode. DC switching losses are minimized due to its low forward voltage drop and AC behavior is benign due to its lack of a significant reverse recovery time. Schottky diodes are generally available with reverse voltage ratings of 60V and even 100V and are price competitive with other types.
Table 5. Catch Diode Selection Criteria
IQ at 125°C EFFICIENCY
LEAKAGE V
V
= 3.3V VF AT 1A V
OUT
DIODE 25°C 125°C25°C 125°CIL = 0A IL = 1A
IR 10BQ100 0.0μA59μA 0.72V 0.58V 125μA 74.1% Diodes Inc. 0.1μA 242μA 0.48V 0.41V 215μA 82.8%
B260SMA Diodes Inc. 0.2μA 440μA 0.45V 0.36V 270μA 83.6%
B360SMB IR 1μA 1.81mA 0.42V 0.34V 821μA 83.7%
MBRS360TR IR 30BQ100 1.7μA 2.64mA 0.40V 0.32V 1088μA 84.5%
=12V VIN =12V
IN
= 3.3 V
OUT
OUT
= 3.3V
The effect of reverse leakage and forward drop on effi­ciency for various Schottky diodes is shown in Table 4. As can be seen these are conflicting parameters and the user must weigh the importance of each specification in choos­ing the best diode for the application.
The use of so-called “ultrafast” recovery diodes is gener­ally not recommended. When operating in continuous mode, the reverse recovery time exhibited by “ultrafast” diodes will result in a slingshot type effect. The power internal switch will ramp up VIN current into the diode in an attempt to get it to recover. Then, when the diode has finally turned off, some tens of nanoseconds later, the V
SW
node voltage ramps up at an extremely high dV/dt, per­haps 5V to even 10V/ns! With real world lead inductances the VSW node can easily overshoot the VIN rail. This can result in poor RFI behavior and, if the overshoot is severe enough, damage the IC itself.
BOOST PIN
For most applications the boost components are a 0.33μF capacitor and a MMSD914 diode. The anode is typically connected to the regulated output voltage to generate a voltage approximately V
above VIN to drive the output
OUT
stage (Figure 7a). However, the output stage discharges the boost capacitor during the on time of the switch. The output driver requires at least 2.5V of headroom through­out this period to keep the switch fully saturated. If the output voltage is less than 3.3V it is recommended that an alternate boost supply is used. The boost diode can be connected to the input (Figure 7b) but care must be taken to prevent the boost voltage (V
= VIN • 2) from
BOOST
exceeding the BOOST pin absolute maximum rating. The additional voltage across the switch driver also increases power loss and reduces efficiency. If available, an inde­pendent supply can be used to generate the required BOOST voltage (Figure 7c). Tying BOOST to VIN or an independent supply may reduce efficiency but it will re­duce the minimum VIN required to start-up with light loads. If the generated BOOST voltage dissipates too much power at maximum load, the BOOST voltage the LT1976 sees can be reduced by placing a Zener diode in series with the BOOST diode (Figure 7a option).
1976bfg
19
LT1976/LT1976B
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APPLICATIO S I FOR ATIO
OPTIONAL
BOOST
V
IN
V
IN
V
IN
Figure 7. BOOST Pin Configurations
V
V
BOOST
V
BOOST(MAX)
V
V
BOOST
V
BOOST(MAX)
V
V
BOOST
V
BOOST(MAX)
IN
LT1976
– VSW = V
= VIN + V
BOOST
IN
LT1976
– VSW = V
BOOST
IN
LT1976
– VSW = V
= VDC + V
SWGND
SWGND
= 2V
SWGND
OUT
(7a)
IN IN
(7b)
DC
(7c)
OUT
D
SS
IN
1976 F07
SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT
V
OUT
The SHDN pin on the LT1976 controls the operation of the IC. When the voltage on the SHDN pin is below the 1.2V shutdown threshold the LT1976 is placed in a “zero” supply current state. Driving the SHDN pin above the shutdown threshold enables normal operation. The SHDN pin has an internal sink current of 3μA.
In addition to the shutdown feature, the LT1976 has an undervoltage lockout function. When the input voltage is below 2.4V, switching will be disabled. The undervoltage lockout threshold doesn’t have any hysteresis and is
V
OUT
mainly used to insure that all internal voltages are at the correct level before switching is enabled. If an undervolt­age lockout function with hysteresis is needed to limit input current at low VIN to V
ratios refer to Figure 8 and
OUT
the following:
V
DC
VR
V
OUT
UVLO
V
HYST
V
=++
=
SHDN SHDN
1
R
VR
()
OUT
R
3
V
IV
32
R
SHDN SHDN
1
+
⎟ ⎠
R1 should be chosen to minimize quiescent current during normal operation by the following equation:
A 0.33μF boost capacitor is recommended for most appli- cations. Almost any type of film or ceramic capacitor is suitable but the ESR should be <1Ω to ensure it can be fully recharged during the off time of the switch. The capacitor value is derived from worst-case conditions of 4700ns on time, 42mA boost current and 0.7V discharge ripple. The boost capacitor value could be reduced under less de­manding conditions but this will not improve circuit opera­tion or efficiency. Under low input voltage and low load conditions a higher value capacitor and Schottky boost diode will reduce discharge ripple and improve start-up and dropout operation.
20
R
1
=
Example:
R
1
=
R
=
3
R2 =
=
VV
2
IN
.
I
15
()
()
SHDN MAX
()
12 2
155
A
.
μ
()
M
513
Ω
.
()
M
13
.
M
Ω
65
.
(Nearest 1% 6.49M )
1
1.3 13
649
.
.
M
Ω
7 – 1.3
Ω
1.3M k
408
(Nearest 1% 412k)
A
μ
1
––
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APPLICATIO S I FOR ATIO
LT1976
V
IN
4
R1
R3
V
OUT
SHDN
15
R2
3μA
+
V
IN
COMP
2.4V
+
SHDN COMP
1.3V
Figure 8. Undervoltage Lockout
LT1976/LT1976B
ENABLE
1976 F08
See the Typical Performance Characteristics section for graphs of SHDN and VIN currents verses input voltage.
SYNCHRONIZING
Oscillator synchronization to an external input is achieved by connecting a TTL logic-compatible square wave with a duty cycle between 20% and 80% to the LT1976 SYNC pin. The synchronizing range is equal to initial operating frequency up to 700kHz. This means that minimum practical sync frequency is equal to the worst-case high self-oscillating frequency (230kHz), not the typical oper­ating frequency of 200kHz. Caution should be used when synchronizing above 230kHz because at higher sync frequencies the amplitude of the internal slope compen­sation used to prevent subharmonic switching is re­duced. This type of subharmonic switching only occurs at input voltages less than twice output voltage. Higher inductor values will tend to eliminate this problem. See Frequency Compensation section for a discussion of an entirely different cause of subharmonic switching before assuming that the cause is insufficient slope compensa­tion. Application Note 19 has more details on the theory of slope compensation.
If the FB pin voltage is below 0.9V (power-up or output short-circuit conditions) the sync function is disabled. This allows the frequency foldback to operate to avoid and hazardous conditions for the SW pin.
If the synchronization signal is present during Burst Mode operation, synchronization will occur during the burst portion of the output waveform. Synchronizing the LT1976 during Burst Mode operation may alter the natural burst frequency which can lead to jitter and increased ripple in the burst waveform. Synchronizing the LT1976B during pulse skip operation may also increase output ripple.
If no synchronization is required this pin should be con­nected to ground.
POWER GOOD
The LT1976 contains a power good block which consists of a comparator, delay timer and active low flag that allows the user to generate a delayed signal after the power good threshold is exceeded.
Referring to Figure 2, the PGFB pin is the positive input to a comparator whose negative input is set at V PGFB is taken above V
, current (I
PGFB
) is sourced into
CSS
PGFB
. When
the CT pin starting the delay period. When the voltage on the PGFB pin drops below V
the CT pin is rapidly
PGFB
discharged resetting the delay period. The PGFB voltage is typically generated by a resistive divider from the regu­lated output or input supply.
The capacitor on the CT pin determines the amount of delay time between the PGFB pin exceeding its threshold (V
) and the PG pin set to a high impedance state.
PGFB
When the PGFB pin rises above V
current is sourced
PGFB
1976bfg
21
LT1976/LT1976B
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APPLICATIO S I FOR ATIO
(ICT) from the CT pin into the external capacitor. When the voltage on the external capacitor reaches an internal clamp (VCT), the PG pin becomes a high impedance node. The
V
OUT
500mV/DIV
PG
100k TO V
IN
V
CT
500mV/DIV
V
SHDN
2V/DIV
TIME (10ms/DIV) 1976 F09
Figure 9. Power Good
PG at 80% V
V
LT1976
PGFB
with 100ms Delay
OUT
IN
PG
FB
C
T
0.27μF
200k
153k
12k
100k
V
= 3.3V
OUT
C
OUT
resultant PG delay time is given by t = CCT •(VCT)/(ICT). If the voltage on the PGFB pin drops below its V
PGFB
, CCT will
be discharged rapidly and PG will be active low with a 200μA sink capability. If the SHDN pin is taken below its threshold during normal operation, the C
pin will be
T
discharged and PG inactive, resulting in a non Power Good cycle when SHDN is taken above its threshold. Figure 9 shows the power good operation with PGFB connected to FB and the capacitance on C
= 0.1μF. The PGOOD pin has
T
a limited amount of drive capability and is susceptible to noise during start-up and Burst Mode operation. If erratic operation occurs during these conditions a small filter capacitor from the PGOOD pin to ground will ensure proper operation. Figure 10 shows several different con­figurations for the LT1976 Power Good circuitry.
PG at VIN > 4V with 100ms Delay
V
LT1976
PGFB
PG
IN
FB
C
T
511k
200k
0.27μF
200k
165k
100k
V
= 3.3V
OUT
C
OUT
22
V
Disconnect at 80% V
OUT
with 100ms Delay
V
IN
PG
LT1976
PGFB
FB
C
T
200k
0.27μF
153k
12k
100k
OUT
V
= 3.3V
OUT
C
OUT
V
OUT
Figure 10. Power Good Circuits
Disconnect 3.3V Logic Signal
with 100μs Delay
V
IN
PG
LT1976
PGFB
FB
C
200k
C
OUT
866k
T
100k
270pF
1976 F10
V
= 12V
OUT
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APPLICATIO S I FOR ATIO
LT1976/LT1976B
LAYOUT CONSIDERATIONS
As with all high frequency switchers, when considering layout, care must be taken in order to achieve optimal electrical, thermal and noise performance. For maximum efficiency switch rise and fall times are typically in the nanosecond range. To prevent noise both radiated and conducted the high speed switching current path, shown in Figure 11, must be kept as short as possible. This is implemented in the suggested layout of Figure 12. Short­ening this path will also reduce the parasitic trace induc­tance of approximately 25nH/inch. At switch off, this parasitic inductance produces a flyback spike across the
LT1976
V
42
IN
V
IN
+
C2 C1
Figure 11. High Speed Switching Path
SW
HIGH
FREQUENCY
CIRCULATION
PATH
LT1976 switch. When operating at higher currents and input voltages, with poor layout, this spike can generate voltages across the LT1976 that may exceed its absolute maximum rating. A ground plane should always be used under the switcher circuitry to prevent interplane coupling and overall noise.
The VC and FB components should be kept as far away as possible from the switch and boost nodes. The LT1976 pinout has been designed to aid in this. The ground for these components should be separated from the switch current path. Failure to do so will result in poor stability or subharmonic like oscillation.
L1
D1
1976 F11
V
OUT
LOAD
GND
C2 D2
V
OUT
C1
C3
GND
MINIMIZE
D1-C3
LOOP
V
IN
L1
D1
CONNECT PIN 8 GND TO THE
PIN 17 EXPOSED PAD GND
PLACE VIA's UNDER EXPOSED
PAD TO A BOTTOM PLANE TO
ENHANCE THERMAL
CONDUCTIVITY
1
NC
SW
2
LT1976
NC
3
V
4
IN
NC
5
BOOST
6
C
7
C4
T
GND
8
SHDN
SYNC
PGFB
BIAS
C
PG
KELVIN SENSE
FEEDBACK
TRACE AND
KEEP SEPARATE
16
15
14
13
12
FB
11
V
C
10
9
SS
FROM BIAS TRACE
R3
R1
R2
C2
C5
1976 F12
Figure 12. Suggested Layout
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LT1976/LT1976B
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APPLICATIO S I FOR ATIO
Board layout also has a significant effect on thermal resistance. Pin 8 and the exposed die pad, Pin 17, are a continuous copper plate that runs under the LT1976 die. This is the best thermal path for heat out of the package. Reducing the thermal resistance from Pin 8 and exposed pad onto the board will reduce die temperature and in­crease the power capability of the LT1976. This is achieved by providing as much copper area as possible around the exposed pad. Adding multiple solder filled feedthroughs under and around this pad to an internal ground plane will also help. Similar treatment to the catch diode and coil terminations will reduce any additional heating effects.
THERMAL CALCULATIONS
Power dissipation in the LT1976 chip comes from four sources: switch DC loss, switch AC loss, boost circuit current, and input quiescent current. The following formu­las show how to calculate each of these losses. These formulas assume continuous mode operation, so they should not be used for calculating efficiency at light load currents.
Example: with VIN = 40V, V
03 1 5
Pee
004 0388 043
PW
PW
Total power dissipation is:
P
Thermal resistance for the LT1976 package is influenced by the presence of internal or backside planes. With a full plane under the FE16 package, thermal resistance will be about 45°C/W. No plane will increase resistance to about 150°C/W. To calculate die temperature, use the proper thermal resistance number for the desired package and add in worst-case ambient temperature:
TJ = TA + QJA (P
()()()
=
SW
.. .
+=
BOOST
Q
TOT
=
=
40 0 0015 5 0 003 0 08
()
= 0.43 + 0.02 + 0.08 = 0.53W
2
.
40
5136
()
...
+
W
2
/
()
40
=
+
()
)
TOT
= 5V and I
OUT
–/
97 9 1 2 1 40 200 3
()
.
002
()( )( )
()
=
OUT
= 1A:
Switch loss:
RI V
P
SW
Boost current loss:
P
BOOST
Quiescent current loss: (LT1976)
PQ = VIN (0.0015) + V
RSW = switch resistance (0.3 when hot ) t
= effective switch current/voltage overlap time
EFF
(tr + tf + tIR + tIF) tr = (VIN/1.7)ns tf = (VIN/1.2)ns tIR = tIF = (I f = switch frequency
SW OUT OUT
=
VI
()
=
OUT
2
()( )
V
IN
2
()
OUT OUT
V
IN
/0.05)ns
36/
OUT
+
tIVf
EFF OUT IN
(0.003)
12/
()()()
()
With the FE16 package (QJA = 45°C/W) at an ambient temperature of 70°C:
TJ = 70 + 45(0.53) = 94°C
If a more accurate die temperature is required, a measure­ment of the SYNC pin resistance to ground can be used. The SYNC pin resistance can be measured by forcing a voltage no greater than 0.25V at the pin and monitoring the pin current versus temperature in a controlled temperature environment. The measurement should be done with minimal device power dissipation (pull the VC pin to ground for sleep mode) in order to calibrate the SYNC pin resistance with the ambient temperature.
24
1976bfg
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APPLICATIO S I FOR ATIO
LT1976/LT1976B
HIGH TEMPERATURE OPERATION
Extreme care must be taken when designing LT1976 applications to operate at high ambient temperatures. The LT1976H grade is designed to work at elevated tempera­tures but erratic operation can occur due to external components. Each passive component should be checked for absolute value and voltage ratings to ensure loop stability at temperature. Boost and Catch diode leakages, as well as increased series resistance (Table 5), will adversely affect efficiency and low quiescent current op­eration. Junction temperature increase in the diodes due to self heating (leakage) and power dissipation should be measured to ensure their maximum temperature specifi­cations are not violated.
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT1976 is specified at 60V. This is based solely on internal semi­conductor junction breakdown effects. Due to internal power dissipation the actual maximum VIN achievable in a particular application may be less than this.
A detailed theoretical basis for estimating internal power loss is given in the section Thermal Considerations. Note that AC switching loss is proportional to both operating frequency and output current. The majority of AC switch­ing loss is also proportional to the square of input voltage.
where:
= input voltage
V
IN
V
= output voltage
OUT
VF = Schottky diode forward drop f
= switching frequency
OSC
A potential controllability problem arises if the LT1976 is called upon to produce an on time shorter than it is able to produce. Feedback loop action will lower then reduce the V
control voltage to the point where some sort of cycle-
C
skipping or Burst Mode behavior is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high VIN, high I
and high f
OUT
may not be achievable in
OSC
practice due to internal dissipation. The Thermal Con­siderations section offers a basis to estimate internal power. In questionable cases a prototype supply should be built and exercised to verify acceptable operation.
2. The simultaneous requirements of high VIN, low V and high f
can result in an unacceptably short
OSC
OUT
minimum switch on time. Cycle-skipping and/or Burst Mode behavior will result although correct output volt­age is usually maintained.
FREQUENCY COMPENSATION
For example, while the combination of VIN = 40V, V 5V at 1A and f simultaneously raising VIN to 60V and f
= 200kHz may be easily achievable,
OSC
to 700kHz is
OSC
OUT
=
not possible. Nevertheless, input voltage transients up to 60V can usually be accommodated, assuming the result­ing increase in internal dissipation is of insufficient time duration to raise die temperature significantly.
A second consideration is controllability. A potential limi­tation occurs with a high step-down ratio of VIN to V
OUT
, as this requires a correspondingly narrow minimum switch on time. An approximate expression for this (assuming continuous mode operation) is given as follows:
t
ON(MIN)
= V
+ VF/VIN(f
OUT
OSC
)
Before starting on the theoretical analysis of frequency response the following should be remembered—the worse the board layout, the more difficult the circuit will be to stabilize. This is true of almost all high frequency analog circuits. Read the Layout Considerations section first. Common layout errors that appear as stability problems are distant placement of input decoupling capacitor and/or catch diode and connecting the VC compensation to a ground track carrying significant switch current. In addi­tion the theoretical analysis considers only first order non­ideal component behavior. For these reasons, it is important that a final stability check is made with production layout and components.
1976bfg
25
LT1976/LT1976B
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APPLICATIO S I FOR ATIO
The LT1976 uses current mode control. This alleviates many of the phase shift problems associated with the inductor. The basic regulator loop is shown in Figure 12. The LT1976 can be considered as two gm blocks, the error amplifier and the power stage.
Figure 13 shows the overall loop response with a 330pF V
C
capacitor and a typical 100μF tantalum output capacitor. The response is set by the following terms:
Error amplifier: DC gain is set by gm and RO:
Ω
EA Gain = 650μ • 1.5M = 975
The pole set by CF and RL:
EA Pole = 1/(2π • 1.5M • 330pF) = 322Hz
Unity gain frequency is set by CF and gm:
EA Unity Gain Frequency = 650μF/(2π • 330pF)
= 313kHz
Powerstage: DC gain is set by gm and RL (assume 10Ω):
PS DC Gain = 3 • 10 = 30
Pole set by C
OUT
and RL:
PS Pole = 1/(2π • 100μF • 10) = 159Hz
Unity gain set by C
OUT
and gm:
Tantalum output capacitor zero is set by C
OUT
and C
OUT
ESR
Output Capacitor Zero = 1/(2π • 100μF • 0.1) = 15.9kHz
The zero produced by the ESR of the tantalum output capacitor is very useful in maintaining stability. If better transient response is required, a zero can be added to the loop using a resistor (R capacitor. As the value of R
) in series with the compensation
C
is increased, transient
C
response will generally improve but two effects limit its value. First, the combination of output capacitor ESR and a large RC may stop loop gain rolling off altogether. Second, if the loop gain is not rolled off sufficiently at the switching frequency output ripple will perturb the V
C
pin enough to cause unstable duty cycle switching similar to subharmonic oscillation. This may not be apparent at the output. Small-signal analysis will not show this since a continuous time system is assumed. If needed, an addi­tional capacitor (CF) can be added to form a pole at typically one-fifth the switching frequency (if RC = 10k, CE = 1500pF, CF = 330pF)
When checking loop stability the circuit should be oper­ated over the application’s full voltage, current and tem­perature range. Any transient loads should be applied and the output voltage monitored for a well-damped behavior.
PS Unity Gain Freq = 3/(2π • 100μF) = 4.7kHz.
LT1976
CURRENT MODE
POWER STAGE
Ω
= 3
g
m
V
C
11
C
C
F
C
C
gm = 650μ
1.6MR
Ω
ERROR
AMP
SW
2
R1
FB
12
R2
+
1.26V
Figure 13. Model for Loop Response
26
1976 F13
1976 F14
1M10
100
135
PHASE (DEG)
90
45
0
100
OUTPUT
C
FB
50
ESR
GAIN (dB)
–50
0
100 1k 10k 100k
C
OUT
V
OUT
C
OUT
= 330pF
C
F
R
L/CL
I
LOAD
FREQUENCY (Hz)
= 3.3V = 100μF, 0.1Ω
= NC
= 330mA
Figure 14. Overall Loop Response
1976bfg
TYPICAL APPLICATIO
LT1976/LT1976B
U
V
3.3V TO 60V
4700pF
8.06k
IN
4.7μF 100V CER
1μF
100pF
V
IN
SHDN
V
C
C
T
SYNC GND
BOOST
LT1976B
SW
C
V
BIAS
PGFB
PG
0.1μF
SS
FB
14V to 3.3V Non Burst Mode Step-Down Converter
PACKAGE DESCRIPTIO
0.33μF
V
OUT
16.5k 1%
10k 1%
3.3V 1A
100μF
6.3V TANT
1976 TA03
33μH
10MQ60N
4148
U
FE Package
16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation BC
LT1976B Efficiency and Power Loss vs Load Current
100
EFFICIENCY
75
5V
3.3V
50
EFFICIENCY (%)
25
0
0.1
1
10
LOAD CURRENT (mA)
TYPICAL POWER LOSS
100
1000
10000
1976 G25
10
1
POWER LOSS (W)
0.1
0.01
0.001
3.58
(.141)
6.60 ±0.10
4.50 ±0.10
RECOMMENDED SOLDER PAD LAYOUT
0.09 – 0.20
(.0035 – .0079)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
SEE NOTE 4
0.65 BSC
4.30 – 4.50* (.169 – .177)
0.50 – 0.75
(.020 – .030)
MILLIMETERS
(INCHES)
2.94
(.116)
0.45 ±0.05
4.90 – 5.10* (.193 – .201)
3.58
(.141)
16 1514 13 12 11
1.05 ±0.10
1345678
2
0.25 REF
0° – 8°
0.65
(.0256)
BSC
0.195 – 0.30
(.0077 – .0118)
TYP
4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE
10 9
2.94
(.116)
1.10
(.0433)
MAX
0.05 – 0.15
(.002 – .006)
FE16 (BC) TSSOP 0204
6.40
(.252)
BSC
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
1976bfg
27
LT1976/LT1976B
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28
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
www.linear.com
1976bfg
LT/CGRAFX 0407 REV G PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 2003
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