ANALOG DEVICES LT 1376 CS8 Datasheet

FEATURES
LOAD CURRENT (A)
0
EFFICIENCY (%)
100
90
80
70
60
50
1.00
1375/76 TA02
0.25
0.50
0.75
1.25
V
OUT
= 5V
V
IN
= 10V
L = 10µH
Constant 500kHz Switching Frequency
Uses All Surface Mount Components
Inductor Size Reduced to 5µH
Easily Synchronizable
Saturating Switch Design: 0.4
Effective Supply Current: 2.5mA
Shutdown Current: 20µA
Cycle-by-Cycle Current Limiting
U
APPLICATIO S
Portable Computers
Battery-Powered Systems
Battery Charger
Distributed Power
U
DESCRIPTIO
The LT®1375/LT1376 are 500kHz monolithic buck mode switching regulators. A 1.5A switch is included on the die along with all the necessary oscillator, control and logic circuitry. High switching frequency allows a considerable reduction in the size of external components. The topology
LT1375/LT1376
1.5A, 500kHz Step-Down Switching Regulators
is current mode for fast transient response and good loop stability. Both fixed output voltage and adjustable parts are available.
A special high speed bipolar process and new design techniques achieve high efficiency at high switching fre­quency. Efficiency is maintained over a wide output cur­rent range by using the output to bias the circuitry utilizing a supply boost
capacitor to saturate the power switch. A shutdown signal will reduce supply current to 20µA on both parts. The LT1375 can be externally syn- chronized from 580kHz to 900kHz with logic level inputs.
The LT1375/LT1376 fit into standard 8-pin PDIP and SO packages, as well as a fused lead 16-pin SO with much lower thermal resistance. Full cycle-by-cycle short-cir­cuit protection and thermal shutdown are provided. Standard surface mount external parts are used, includ­ing the inductor and capacitors.
For low input voltage applications with 3.3V output, see LT1507. This is a functionally identical part that can operate with input voltages between 4.5V and 12V.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
and by
U
5V Buck Converter
D2
1N914
C2
0.1µF
L1**
5µH
D2 1N5818
V
IN
SHDN
BOOST
LT1376-5
GND
V
SW
BIAS
FB
V
C
C
C
3.3nF
INPUT
TO 25V
6V
* RIPPLE CURRENT I
** INCREASE L1 TO 10µH FOR LOAD CURRENTS ABOVE 0.6A AND TO 20µH ABOVE 1A
FOR INPUT VOLTAGE BELOW 7.5V, SOME RESTRICTIONS MAY APPLY. SEE APPLICATIONS INFORMATION.
C3*
10µF TO
50µF
+
DEFAULT
= ON
/2
OUT
+
Efficiency vs Load Current
OUTPUT** 5V, 1.25A
C1 100µF, 10V SOLID TANTALUM
1375/76 TA01
13756fd
1
LT1375/LT1376
WW
W
ABSOLUTE MAXIMUM RATINGS
U
(Note 1)
Input Voltage
LT1375/LT1376 .................................................. 25V
LT1375HV/LT1376HV ........................................ 30V
BOOST Pin Voltage
LT1375/LT1376 .................................................. 35V
LT1375HV/LT1376HV ........................................ 40V
SHDN Pin Voltage ..................................................... 7V
BIAS Pin Voltage ...................................................... 7V
FB Pin Voltage (Adjustable Part) ............................ 3.5V
U
W
U
PACKAGE/ORDER INFORMATION
TOP VIEW
BOOST
1
V
2
IN
V
3
SW
SHDN
4
N8 PACKAGE 8-LEAD PDIP
θJA = 100°C/ W (N8) θ
= 120°C/ W TO 150°C/W DEPENDING ON
JA
PC BOARD LAYOUT (S8)
8-LEAD PLASTIC SO
ORDER PART
NUMBER
LT1375CN8
LT1375CN8-5
LT1375IN8
LT1375IN8-5
LT1375CS8
LT1375CS8-5
LT1375HVCS8
LT1375IS8
LT1375IS8-5
LT1375HVIS8
V
8
FB/SENSE
7
GND
6
SYNC
5
S8 PACKAGE
S8 PART
MARKING
1375 13755 1375HV 1375I 1375I5 375HVI
C
BOOST
V
V
SW
BIAS
N8 PACKAGE 8-LEAD PDIP
θJA = 100°C/ W (N8) θ
= 120°C/ W TO 150°C/W DEPENDING ON
JA
ORDER PART
NUMBER
LT1376CN8 LT1376CN8-5 LT1376IN8 LT1376IN8-5 LT1376CS8 LT1376CS8-5 LT1376HVCS8 LT1376IS8 LT1376IS8-5 LT1376HVIS8
TOP VIEW
1
2
IN
3
4
PC BOARD LAYOUT (S8)
FB Pin Current (Adjustable Part) ............................ 1mA
Sense Voltage (Fixed 5V Part) .................................. 7V
SYNC Pin Voltage ..................................................... 7V
Operating Junction Temperature Range
LT1375C/LT1376C ............................... 0°C to 125° C
LT1375I/LT1376I............................. – 40°C to 125°C
Storage Temperature Range ................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
TOP VIEW
V
8
C
FB/SENSE
7
GND
6
SHDN
5
S8 PACKAGE
8-LEAD PLASTIC SO
S8 PART
MARKING
1
GND
2
NC
3
BOOST
4
V
IN
5
V
SW
6
BIAS
7
NC
8
GND
S PACKAGE
16-LEAD PLASTIC NARROW SO
θJA =50°C/ W WITH FUSED CORNER PINS CONNECTED TO GROUND PLANE OR LARGE LANDS
16
15
14
13
12
11
10
9
GND
NC
V
C
FB/SENSE
GND
SHDN
NC
GND
ORDER PART NUMBER
1376 13765 1376HV 1376I
LT1376CS LT1376IS LT1376HVCS LT1376HVIS
1376I5 376HVI
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.
2
13756fd
LT1375/LT1376
ELECTRICAL CHARACTERISTICS
temperature range, otherwise specifications are at T
The ● denotes specifications which apply over the full operating
= 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open,
A
unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Reference Voltage (Adjustable) 2.39 2.42 2.45 V
2.36 2.48 V
Sense Voltage (Fixed 5V) 4.94 5.0 5.06 V
4.90 5.10 V
Sense Pin Resistance 71014 k Reference Voltage Line Regulation 5V ≤ VIN 25V 0.01 0.03 %/ V
30V (LT1375HV/LT1376HV) 0.01 0.03 %/V
5V V
IN
Feedback Input Bias Current
Error Amplifier Voltage Gain V
Error Amplifier Transconductance V
= 1V (Notes 2, 8) 200 400
SHDN
= 1V, ∆I (VC) = ±10µA (Note 8) 1500 2000 2700 µMho
SHDN
VC Pin to Switch Current Transconductance 2A/V
Error Amplifier Source Current V
Error Amplifier Sink Current V
= 1V, VFB = 2.1V or V
SHDN
= 1V, VFB = 2.7V or V
SHDN
= 4.4V
SENSE
= 5.6V 2 mA
SENSE
VC Pin Switching Threshold Duty Cycle = 0 0.9 V
VC Pin High Clamp V
Switch Current Limit VC Open, VFB = 2.1V or V
= VIN + 5V DC 50%
V
BOOST
= 1V 2.1 V
SHDN
= 4.4V,
SENSE
DC = 80%
Switch On Resistance (Note 7) ISW = 1.5A, V
Maximum Switch Duty Cycle VFB = 2.1V or V
= VIN + 5V 0.3 0.4
BOOST
= 4.4V
SENSE
Switch Frequency VC Set to Give 50% Duty Cycle 460 500 540 kHz
125°C 440 560 kHz
0°C T
J
Switch Frequency Line Regulation 5V ≤ VIN 25V
5V V
30V (LT1375HV/LT1376HV)
IN
Frequency Shifting Threshold on FB Pin ∆f = 10kHz
Minimum Input Voltage (Note 3)
Minimum Boost Voltage (Note 4) ISW 1.5A
Boost Current (Note 5) V
Input Supply Current (Note 6) V
Output Supply Current (Note 6) V
Shutdown Supply Current V
= VIN + 5V ISW = 500mA
BOOST
= 5V
BIAS
= 5V
BIAS
= 0V, VIN 25V, VSW = 0V, VC Open 15 50 µA
SHDN
V
= 0V, VIN 30V, VSW = 0V, VC Open
SHDN
I
SW
= 1.5A
(LT1375HV/LT1376HV) 20 75 µA
Lockout Threshold VC Open
1100 3000 µMho
150 225 320 µA
1.50 2 3 A
1.35 3 A
440 570 kHz
0.5 1.5 µA
0.5
86 93 %
0.05 0.15 %/ V
0.05 0.15 %/V
0.8 1.0 1.3 V
5.0 5.5 V
3 3.5 V
12 22 mA 25 35 mA
0.9 1.4 mA
3.2 4.0 mA
75 µA
100 µA
2.3 2.38 2.46 V
13756fd
3
LT1375/LT1376
ELECTRICAL CHARACTERISTICS
temperature range, otherwise specifications are at T
The ● denotes specifications which apply over the full operating
= 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open,
A
unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Shutdown Thresholds VC Open Device Shutting Down
Device Starting Up
VC Open LT1375HV/LT1376HV Device Shutting Down
LT1375HV/LT1376HV Device Starting Up
Minimum Synchronizing Amplitude (LT1375 Only) VIN = 5V
Synchronizing Range (LT1375 Only) 580 900 kHz SYNC Pin Input Resistance 40 k
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: Gain is measured with a V
swing equal to 200mV above the low
C
clamp level to 200mV below the upper clamp level. Note 3: Minimum input voltage is not measured directly, but is guaranteed
by other tests. It is defined as the voltage where internal bias lines are still regulated so that the reference voltage and oscillator frequency remain constant. Actual minimum input voltage to maintain a regulated output will depend on output voltage and load current. See Applications Information.
Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the internal power switch.
Note 5: Boost current is the current flowing into the BOOST pin with the pin held 5V above input voltage. It flows only during switch-on time.
Note 6: Input supply current is the bias current drawn by the input pin when the BIAS pin is held at 5V with switching disabled. Output supply
5V. Total input referred supply current is calculated by summing input supply current (I
= ISI + (ISO)(V
I
TOT
= 15V, V
With V
IN
) with a fraction of output supply current (ISO):
SI
OUT/VIN
= 5V, ISI = 0.9mA, ISO = 3.6mA, I
OUT
For the LT1375, quiescent current is equal to:
= ISI + ISO(1.15)
I
TOT
because the BIAS pin is internally connected to V For LT1375 or BIAS open circuit, input supply current is the sum of input
+ output supply currents. Note 7: Switch-on resistance is calculated by dividing V
by the forced current (1.5A). See Typical Performance Characteristics for the graph of switch voltage at other currents.
Note 8: Transconductance and voltage gain refer to the internal amplifier exclusive of the voltage divider. To calculate gain and transconductance refer to sense pin on fixed voltage parts. Divide values shown by the ratio V
/2.42.
OUT
current is the current drawn by the BIAS pin when the bias pin is held at
)(1.15)
0.15 0.37 0.60 V
0.25 0.45 0.60 V
0.15 0.37 0.70 V
0.25 0.45 0.70 V
1.5 2.2 V
= 2.28mA.
TOT
.
IN
to VSW voltage
IN
UW
TYPICAL PERFORMANCE CHARACTERISTICS
Inductor Core Loss
1.0 V
= 5V, VIN = 10V, I
OUT
0.1
CORE LOSS (W)
0.01 CORE LOSS IS
INDEPENDENT OF LOAD CURRENT UNTIL LOAD CURRENT FALLS LOW ENOUGH FOR CIRCUIT TO GO INTO DISCONTINUOUS MODE
0.001 05
10 15 20
INDUCTANCE (µH)
= 1A
OUT
TYPE 52 POWDERED IRON
®
Kool Mµ
PERMALLOY µ = 125
1375/76 G01
20 12
8
CORE LOSS (% OF 5W LOAD)
4
2
1.2
0.8
0.4
0.2
0.12
0.08
0.04
0.02
25
Switch Peak Current Limit
2.5
2.0
1.5
1.0
SWITCH PEAK CURRENT (A)
0.5
0
0
TYPICAL
GUARANTEED MINIMUM
20
40
DUTY CYCLE (%)
60
80
4
1375/76 G08
Feedback Pin Voltage and Current
2.44
2.43
2.42
FEEDBACK VOLTAGE (V)
2.41
2.40
100
–50
VOLTAGE
CURRENT
–25 0 25 50 75 125
JUNCTION TEMPERATURE (°C)
100
1375/76 G09
13756fd
2.0
1.5
CURRENT (µA)
1.0
0.5
0
UW
INPUT VOLTAGE (V)
0
INPUT SUPPLY CURRENT (µA)
30
25
20
15
10
5
0
5101520
1375/76 G06
25
V
SHUTDOWN
= 0V
TYPICAL PERFORMANCE CHARACTERISTICS
LT1375/LT1376
Shutdown Pin Bias Current
500
CURRENT REQUIRED TO FORCE SHUTDOWN (FLOWS OUT OF PIN). AFTER SHUTDOWN,
400
CURRENT DROPS TO A FEW µA
300
200
CURRENT (µA)
8
AT 2.38V STANDBY THRESHOLD (CURRENT FLOWS OUT OF PIN)
4
0
–50
–25 0
TEMPERATURE (°C)
50 100 125
25 75
Shutdown Supply Current
150
125
100
VIN = 25V
75
50
INPUT SUPPLY CURRENT (µA)
25
0
0
0.1 0.2 0.3 0.4 SHUTDOWN VOLTAGE (V)
VIN = 10V
1375/76 G04
1375/76 G07
0.5
Standby and Shutdown Thresholds
2.40
STANDBY
2.36
2.32
0.8
START-UP
0.4
SHUTDOWN PIN VOLTAGE (V)
0
–50
–25 0
SHUTDOWN
50 100 125
25 75
JUNCTION TEMPERATURE (°C)
Error Amplifier Transconductance
2500
2000
1500
1000
500
TRANSCONDUCTANCE (µMho)
0
–50
0
–25
JUNCTION TEMPERATURE (°C)
50
25
75
1375/76 G05
100
1375/76 G02
125
Shutdown Supply Current
Error Amplifier Transconductance
3000
2500
2000
Mho)
µ
1500
V
GAIN (
1000
500
2 • 10
FB
ERROR AMPLIFIER EQUIVALENT CIRCUIT
R
LOAD
100 10k 100k 10M
PHASE
GAIN
R
–3
)(
= 50
1k 1M
FREQUENCY (Hz)
OUT
200k
C
OUT
12pF
V
C
1375/76 G03
200
150
PHASE (DEG)
100
50
0
–50
LT1376 Minimum Input Voltage
Frequency Foldback
500
400
300
200
100
0
SWITCHING FREQUENCY (kHz) OR CURRENT (µA)
0
SWITCHING FREQUENCY
FEEDBACK PIN CURRENT
0.5 FEEDBACK PIN VOLTAGE (V)
1.0
1.5
2.0
2.5
1375/76 G10
Switching Frequency
600
550
500
FREQUENCY (kHz)
450
400
–25 0 25 50 75 125
–50
JUNCTION TEMPERATURE (°C)
100
1375/76 G11
with 5V Output
8.5 MINIMUM INPUT VOLTAGE CAN BE REDUCED BY ADDING A SMALL EXTERNAL
8.0 PNP. SEE APPLICATIONS INFORMATION
7.5
7.0
6.5
INPUT VOLTAGE (V)
6.0
5.5
5.0
MINIMUM VOLTAGE TO START WITH
STANDARD
CIRCUIT
MINIMUM VOLTAGE
TO RUN WITH
STANDARD CIRCUIT
0
10 100 1000
LOAD CURRENT (mA)
1375/76 G12
13756fd
5
LT1375/LT1376
SWITCH CURRENT (A)
0
SWITCH VOLTAGE (V)
0.8
0.6
0.4
0.2
0
0.25 0.50 0.75 1.00
1375/76 G18
1.25 1.50
TJ = 25°C
W
U
TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Load Current at V
= 10V
OUT
1.50 V
= 10V
OUT
1.25
1.00
0.75
CURRENT (A)
0.50
0.25
0
0
5101520
INPUT VOLTAGE (V)
BOOST Pin Current
12
TJ = 25°C
10
8
6
4
BOOST PIN CURRENT (mA)
2
L = 20µH
L = 10µH
L = 5µH
1375/76 G13
Maximum Load Current at V
= 3.3V
OUT
1.50
1.25
1.00
0.75
CURRENT (A)
0.50
0.25
V
= 3.3V
OUT
25
0
0
5101520
INPUT VOLTAGE (V)
V
Pin Shutdown Threshold
C
1.4 SHUTDOWN
1.2
1.0
0.8
THRESHOLD VOLTAGE (V)
0.6
L = 20µH
L = 10µH
L = 5µH
25
1375/76 G14
Maximum Load Current at V
= 5V
OUT
1.50
1.25
1.00
0.75
CURRENT (A)
0.50
0.25
V
= 5V
OUT
0
0
5101520
INPUT VOLTAGE (V)
Switch Voltage Drop
L = 20µH
L = 10µH
L = 5µH
25
1375/76 G15
0
0
PIN FUNCTIONS
BOOST: 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 voltage loss approximates that of a 0.3 FET structure, but with much smaller die area. Efficiency improves from 75% for conventional bipolar designs to > 87% for these new parts.
: The switch pin is the emitter of the on-chip power
V
SW
NPN switch. It is driven up to the input pin voltage during switch on time. Inductor current drives the switch pin negative during switch off time. Negative voltage is clamped
6
0.25 0.50 0.75 1.00 SWITCH CURRENT (A)
UUU
1375/76 G16
1.25
0.4 –25 0 25 50 75 125
–50
JUNCTION TEMPERATURE (°C)
with the external catch diode. Maximum negative switch voltage allowed is –0.8V.
SHDN: The shutdown pin is used to turn off the regulator and to reduce input drain current to a few microamperes. Actually, this pin has two separate thresholds, one at
2.38V to disable switching, and a second at 0.4V to force complete micropower shutdown. The 2.38V threshold functions as an accurate undervoltage lockout (UVLO). This is sometimes used to prevent the regulator from operating until the input voltage has reached a predeter­mined level.
100
1375/76 G11
13756fd
PIN FUNCTIONS
LT1375/LT1376
UUU
VIN: This is the collector of the on-chip power NPN switch. This pin powers the internal circuitry and internal regulator when the BIAS pin is not present. At NPN switch on and off, high dl/dt edges occur on this pin. Keep the external bypass and catch diode close to this pin. All trace induc­tance on this path will create a voltage spike at switch off, adding to the VCE voltage across the internal NPN.
BIAS (LT1376 Only): 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 is a much more efficient way of doing business if the input voltage is much higher than the output.
operation is 3.3V
V
SYNC (LT1375 Only): 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 10% and 90% duty cycle. The synchronizing range is equal to Synchronizing section in Applications Information for details.
FB/SENSE: The feedback pin is used to set output voltage, using an external voltage divider that generates 2.42V at the pin with the desired output voltage. The fixed voltage (-5) parts have the divider included on the chip, and the FB
Minimum output voltage setting for this mode of
. Efficiency improvement at VIN = 20V,
= 5V, and I
OUT
initial
= 25mA is over 10%.
OUT
operating frequency, up to 900kHz. See
pin is used as a SENSE pin, connected directly to the 5V output. Two additional functions are performed by the FB pin. When the pin voltage drops below 1.7V, switch current limit is reduced. Below 1V, switching frequency is also reduced. See Feedback Pin Function section in Appli­cations Information for details.
: The VC pin is the output of the error amplifier and the
V
C
input of the peak switch current comparator. It is normally used for frequency compensation, but can do double duty as a current clamp or control loop override. This pin sits at about 1V for very light loads and 2V at maximum load. It can be driven to ground to shut off the regulator, but if driven high, current must be limited to 4mA.
GND: The GND pin connection needs consideration for two reasons. First, it 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 point. Keep the ground path short between the GND pin and the load, and use a ground plane when possible. The second consideration is EMI caused by GND pin current spikes. Internal capacitance between the V current spikes in the GND pin. If the GND pin is connected to system ground with a long metal trace, this trace may radiate excess EMI. Keep the path between the input bypass and the GND pin short.
pin and the GND pin creates very narrow (<10ns)
SW
W
BLOCK DIAGRAM
The LT1376 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 flip-flop to turn the switch on. When switch current reaches a level set by the inverting input of the comparator, the flip-flop 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 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 LT1376 operates from an internal 2.9V bias line. The bias regulator normally draws power from the regulator input pin, but if the BIAS pin is
13756fd
7
LT1375/LT1376
BLOCK DIAGRAM
W
connected to an external voltage higher than 3V, bias power will be drawn from the external source (typically the regulated output voltage). This will improve efficiency if the BIAS pin voltage is lower than regulator input voltage.
High switch efficiency is attained by using the BOOST pin to provide a voltage to the switch driver which is higher
+
0.05
Σ
CURRENT SENSE AMPLIFIER VOLTAGE GAIN = 10
0.9V
+
CURRENT COMPARATOR
INPUT
BIAS
SYNC
SHUTDOWN
COMPARATOR
2.9V BIAS
REGULATOR
+
0.37V
INTERNAL V
CC
SLOPE COMP
500kHz
OSCILLATOR
than the input voltage, allowing switch to be saturated. This boosted voltage is generated with an external capaci­tor and diode. Two comparators are connected to the shutdown pin. One has a 2.38V threshold for undervoltage lockout and the second has a 0.4V threshold for complete shutdown.
BOOST
S
FLIP-FLOP
R
R
S
DRIVER
CIRCUITRY
Q1 POWER SWITCH
V
SW
SHDN
2.38V
3.5µA
+
LOCKOUT COMPARATOR
FOLDBACK
CURRENT
LIMIT
CLAMP
V
C
Figure 1. Block Diagram
U
WUU
APPLICATIONS INFORMATION
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1376 is used to set output voltage and also to 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 foldback frequency and current limiting created by the FB pin. Please read both parts before committing to a final design. The fixed 5V LT1376-5 has internal divider
FREQUENCY
SHIFT CIRCUIT
Q2
AMPLIFIER
= 2000µMho
g
m
ERROR
+
2.42V
FB
GND
1375/76 BD
resistors and the FB pin is renamed SENSE, connected directly to the output.
The suggested value for the output divider resistor (see Figure 2) from FB to ground (R2) is 5k or less, and a formula for R1 is shown below. The output voltage error caused by ignoring the input bias current on the FB pin is less than 0.25% with R2 = 5k. A table of standard 1% values is shown in Table 1 for common output voltages.
13756fd
8
LT1375/LT1376
U
WUU
APPLICATIONS INFORMATION
LT1375/LT1376
Q2
V
C
Please read the following if divider resistors are increased above the suggested values.
RV
2242
R
1
=
Table 1
OUTPUT R1 % ERROR AT OUTPUT
VOLTAGE R2 (NEAREST 1%) DUE TO DISCREET 1%
(V) (k
3 4.99 1.21 +0.23
3.3 4.99 1.82 +0.08
5 4.99 5.36 +0.39
6 4.99 7.32 –0.5
8 4.99 11.5 –0.04
10 4.99 15.8 + 0.83
12 4.99 19.6 – 0.62
15 4.99 26.1 + 0.52
()
OUT
.
242
.
)(k
) RESISTOR STEPS
More Than Just Voltage Feedback
The feedback (FB) pin is used for more than just output voltage sensing. It also reduces switching frequency and current limit when output voltage is very low (see the Frequency Foldback graph in Typical Performance Char­acteristics). This 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, and the average current through the diode and inductor is
TO FREQUENCY
SHIFTING
ERROR
AMPLIFIER
R5 5k
GND
Figure 2. Frequency and Current Limit Foldback
1.6V
+
Q1
2.4V
R3 1k
V
SW
R4 1k
FB
R1
R2 5k
OUTPUT 5V
+
1375/76 F02
equal to the short-circuit current limit of the switch (typi­cally 2A for the LT1376, folding back to less than 1A). Minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 500kHz, so fre­quency is reduced by about 5:1 when the feedback pin voltage drops below 1V (see Frequency Foldback graph). This does not affect operation with normal load condi­tions; one simply sees a gear shift in switching frequency during start-up as the output voltage rises.
In addition to lower switching frequency, the LT1376 also operates at lower switch current limit when the feedback pin voltage drops below 1.7V. Q2 in Figure 2 performs this function by clamping the VC pin to a voltage less than its normal 2.3V upper clamp level. This
foldback current limit
greatly reduces power dissipation in the IC, diode and inductor during short-circuit conditions. Again, it is nearly transparent to the user under normal load conditions. The only loads which may be affected are current source loads which maintain full load current with output voltage less than 50% of final value. In these rare situations the Feedback pin can be clamped above 1.5V with an external diode to defeat foldback current limit.
Caution:
clamping the feedback pin means that frequency shifting will also be defeated, so a combination of high input voltage and dead shorted output may cause the LT1376 to lose control of current limit.
The internal circuitry which forces reduced switching frequency also causes current to flow out of the feedback
13756fd
9
+ 19 hidden pages