LT3972
prop
33V, 3.5A, 2.4MHz
Step-Down Switching Regulator
with 75µA Quiescent Current
FEATURES
■
Wide Input Range:
Operation from 3.6V to 33V
Over-Voltage Lockout Protects Circuits
Through 62V Transients
■
3.5A Maximum Output Current
■
Low Ripple (<15mV
I
■
■
■
■
■
■
■
■
■
■
= 75μA at 12VIN to 3.3V
Q
Adjustable Switching Frequency: 200kHz to 2.4MHz
Low Shutdown Current: IQ < 1μA
Integrated Boost Diode
Synchronizable Between 250kHz to 2MHz
Power Good Flag
Saturating Switch Design: 95mΩ On-Resistance
Output Voltage: 0.79V to 30V
Thermal Protection
Soft-Start Capability
Small 10-Pin Thermally Enhanced MSOP and
) Burst Mode® Operation:
P-P
OUT
(3mm × 3mm) DFN Packages
APPLICATIONS
■
Automotive Battery Regulation
■
Distributed Supply Regulation
■
Industrial Supplies
■
Wall Transformer Regulation
DESCRIPTION
The LT®3972 is an adjustable frequency (200kHz to 2.4MHz)
monolithic buck switching regulator that accepts input
voltages up to 33V (62V Maximum). A high effi ciency
95mΩ switch is included on the die along with a boost
Schottky diode and the necessary oscillator, control, and
logic circuitry. Current mode topology is used for fast
transient response and good loop stability. Low ripple
Burst Mode operation maintains high effi ciency at low
output currents while keeping output ripple below 15mV
in a typical application. In addition, the LT3972 can further enhance low output current effi ciency by drawing
bias current from the output when V
Shutdown reduces input supply current to less than 1μA
while a resistor and capacitor on the RUN/SS pin provide a
controlled output voltage ramp (soft-start). A power good
fl ag signals when V
reaches 91% of the programmed
OUT
output voltage. The LT3972 is available in 10-Pin MSOP
and 3mm × 3mm DFN packages with exposed pads for
low thermal resistance.
, 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
erty of their respective owners.
is above 3V.
OUT
TYPICAL APPLICATION
5V Step-Down Converter
V
IN
6.3V TO 33V
TRANSIENT
TO 62V
10μF
OFF ON
15k
680pF
63.4k
V
RUN/SS BOOST
V
C
RT
PG
SYNC
IN
LT3972
GND
BD
SW
Effi ciency
V
OUT
5V
3.5A
0.47μF
4.7μH
100k
536k
47μF
3680 TA01
FB
100
90
80
70
EFFICIENCY (%)
60
V
OUT
L = 4.7μH
f = 600kHz
50
00.5
VIN = 24V
= 5V
1.5
1
OUTPUT CURRENT (A)
VIN = 12V
VIN = 30V
2
3.5
2.5
3
3680 TA01a
3972f
1
LT3972
C
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, RUN/SS Voltage (Note 5) ...................................62V
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin .........................................30V
FB, RT, V
Voltage .......................................................5V
C
PG, BD, SYNC Voltage ..............................................30V
PIN CONFIGURATION
TOP VIEW
10
BD
1
BOOST
2
11
3
SW
4
V
IN
5
RUN/SS
10-LEAD (3mm s 3mm) PLASTIC DFN
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
DD PACKAGE
θJA = 45°C/W, θJC = 10°C/W
RT
9
V
C
FB
8
7
PG
6
SYNC
Operating Junction Temperature Range (Note 2)
LT3972E .............................................–40°C to 125°C
LT3972I ..............................................– 40°C to 125°C
Storage Temperature Range ...................–65°C to 150°C
Lead Temperature (Soldering, 10 sec)
(MSE Only) ....................................................... 300°C
TOP VIEW
10
1
BD
2
BOOST
SW
V
IN
RUN/SS
10-LEAD PLASTIC MSOP
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θJA = 45°C/W, θJC = 10°C/W
3
4
5
MSE PACKAGE
11
RT
9
V
C
FB
8
PG
7
SYN
6
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3972EDD#PBF LT3972EDD#TRPBF LDXR
LT3972IDD#PBF LT3972IDD#TRPBF LDXR
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
LT3972EMSE#PBF LT3972EMSE#TRPBF LTDXS 10-Lead Plastic MSOP –40°C to 125°C
LT3972IMSE#PBF LT3972IMSE#TRPBF LTDXS 10-Lead Plastic MSOP – 40°C to 125°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based fi nish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The ● denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T
= 25°C. VIN = 10V, V
A
RUN/SS
= 10V, V
= 15V, VBD = 3.3V unless otherwise
BOOST
noted. (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage
Overvoltage Lockout
V
IN
Quiescent Current from V
IN
Quiescent Current from BD V
V
= 0.2V 0.01 0.5 μA
RUN/SS
= 3V, Not Switching
V
BD
= 0, Not Switching 120 160 μA
V
BD
= 0.2V 0.01 0.5 μA
RUN/SS
●
●
33 35 37 V
●
–40°C to 125°C
–40°C to 125°C
3 3.6 V
30 65 μA
3972f
2
LT3972
ELECTRICAL CHARACTERISTICS
The ● denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T
noted. (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Bias Voltage (BD Pin) 2.7 3 V
Feedback Voltage
FB Pin Bias Current (Note 3) V
FB Voltage Line Regulation 4V < V
Error Amp g
Error Amp Gain 2000
Source Current 60 μA
V
C
Sink Current 60 μA
V
C
Pin to Switch Current Gain 5.3 A/V
V
C
Clamp Voltage 2V
V
C
Switching Frequency R
Minimum Switch Off-Time
Switch Current Limit Duty Cycle = 5% 4.6 5.4 6.2 A
Switch V
Boost Schottky Reverse Leakage V
Minimum Boost Voltage (Note 4)
BOOST Pin Current I
RUN/SS Pin Current V
RUN/SS Input Voltage High 2.5 V
RUN/SS Input Voltage Low 0.2 V
PG Threshold Offset from Feedback Voltage VFB Rising 65 mV
PG Hysteresis 10 mV
PG Leakage V
PG Sink Current V
SYNC Low Threshold 0.5 V
SYNC High Threshold 0.7 V
SYNC Pin Bias Current V
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 LT3972E is guaranteed to meet performance specifi cations
from 0°C to 125°C. Specifi cations over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3972I specifi cations are
guaranteed over the –40°C to 125°C temperature range.
m
CESAT
= 25°C. VIN = 10V, V
A
= 3V, Not Switching
V
BD
= 0, Not Switching 15 μA
V
BD
= 0.8V, VC = 1.2V
FB
< 33V 0.002 0.01 %/V
IN
RUN/SS
= 10V, V
= 15V, VBD = 3.3V unless otherwise
BOOST
●
780
775
●
●
90 130 μA
790
790
10 40 nA
800
805
mV
mV
500 μMho
= 8.66k
T
R
= 29.4k
T
R
= 187k
T
2.2
1.0
200
●
2.45
1.1
230
2.7
1.25
260
MHz
MHz
60 150 nS
kHz
ISW = 3.5A 335 mV
= 0V 0.02 2 μA
BD
●
= 1A 35 50 mA
SW
= 2.5V 5 8 μA
RUN/SS
= 5V 0.1 1 μA
PG
= 0.4V
PG
= 0V 0.1 μA
SYNC
●
200 800 μA
1.5 2 V
Note 3: Bias current fl ows out of the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 5: Absolute Maximum at V
and RUN/SS Pins is 62V for non-
IN
repetitive 1 minute transients, and 40V for continuous operation.
3972f
3
LT3972
TYPICAL PERFORMANCE CHARACTERISTICS
= 25°C unless otherwise noted.
T
A
Effi ciency
100
90
80
70
EFFICIENCY (%)
60
V
OUT
L = 4.7μH
f = 600kHz
50
0 0.5
VIN = 24V
= 5V
1.5
1
OUTPUT CURRENT (A)
No Load Supply Current
130
110
90
70
50
SUPPLY CURRENT (μA)
30
10
0
510 20
INPUT VOLTAGE (V)
VIN = 12V
15
VIN = 30V
2
2.5
25 30 35
3
3680 G01
3.5
Effi ciency
100
90
80
70
EFFICIENCY (%)
60
V
= 3.3V
OUT
L = 3.3μH
f = 600kHz
50
0 0.5
VIN = 12V
VIN = 30V
VIN = 24V
2
1.5
1
OUTPUT CURRENT (A)
2.5
3
3.5
3680 G02
Effi ciency
100
90
80
70
EFFICIENCY (%)
VIN = 12V
60
V
= 5V
OUT
L = 4.7μH
f = 600kHz
50
0 0.5
2
1.5
1
OUTPUT CURRENT (A)
2.5
3
3680 G03
3.0
2.5
TOTAL POWER LOSS (W)
2.0
1.5
1.0
0.5
3.5
No Load Supply Current Maximum Load Current
V
= 3.3V
OUT
3680 G04
400
CATCH DIODE: DIODES, INC. PDS360
350
VIN = 12V
= 3.3V
V
OUT
300
250
200
150
SUPPLY CURRENT (μA)
100
50
0
–50
INCREASED SUPPLY
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
HIGH TEMPERATURE
–25 0 50
25
TEMPERATURE (°C)
75 100 150125
3680 G05
5.5
5.0
4.5
4.0
3.5
LOAD CURRENT (A)
V
= 3.3V
OUT
= 25°C
T
A
3.0
L = 4.7μH
f = 600kHz
2.5
10 20
5
TYPICAL
MINIMUM
15
INPUT VOLTAGE (V)
25 30
3680 G06
Maximum Load Current
5.5
5.0
4.5
4.0
LOAD CURRENT (A)
V
= 5V
OUT
3.5
= 25°C
T
A
L = 4.7μH
f = 600kHz
3.0
10 20
5
4
TYPICAL
MINIMUM
15
INPUT VOLTAGE (V)
25 30
3680 G07
Switch Current Limit
6.0
5.5
5.0
4.5
4.0
SWITCH CURRENT LIMIT(A)
3.5
3.0
20 60
0
40
DUTY CYCLE (%)
80 100
3680 G08
Switch Current Limit
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
SWITCH CURRENT LIMIT (A)
2.5
2.0
–50 25–25 0 50 75 100 150125
DUTY CYCLE = 10 %
DUTY CYCLE = 90 %
TEMPERATURE (°C)
3680 G09
3972f
TYPICAL PERFORMANCE CHARACTERISTICS
= 25°C unless otherwise noted.
T
A
LT3972
Switch Voltage Drop
700
600
500
400
300
VOLTAGE DROP (mV)
200
100
0
12 45
0
SWITCH CURRENT (A)
3
Switching Frequency
1.20
1.15
1.10
1.05
1.00
0.95
FREQUENCY (MHz)
0.90
0.85
0.80
–50 25–25 0 50 75 100 150125
TEMPERATURE (°C)
3680 G10
3680 G13
Boost Pin Current Feedback Voltage
120
105
90
75
60
45
30
BOOST PIN CURRENT (mA)
15
0
0312 45
SWITCH CURRENT (A)
Frequency Foldback
1200
1000
800
600
400
SWITCHING FREQUENCY (kHz)
200
0
0
200 400
100 300
FB PIN VOLTAGE (mV)
500
700 900
600
3680 G11
800
3680 G14
840
820
800
780
FEEDBACK VOLTAGE (mV)
760
–50 25–25 0 50 75 100 150125
Minimum Switch On-Time
140
120
100
80
60
40
MINIMUM SWITCH ON TIME (ns)
20
0
–50 25–25 0 50 75 100 150125
TEMPERATURE (°C)
3680 G12
TEMPERATURE (°C)
3680 G15
Soft-Start
7
6
5
4
3
2
SWITCH CURRENT LIMIT (A)
1
0
0.5 1 2
0
1.5
RUN/SS PIN VOLTAGE (V)
2.5 3 3.5
3680 G16
RUN/SS Pin Current Boost Diode
12
10
8
6
4
RUN/SS PIN CURRENT (μA)
2
0
0
510
15 25
RUN/SS PIN VOLTAGE (V)
20 30 35
3680 G17
1.4
1.2
1.0
(V)
F
0.8
0.6
BOOST DIODE V
0.4
0.2
0
0
0.5 1.0 1.5
BOOST DIODE CURRENT (A)
2.0
3680 G18
3972f
5
LT3972
TYPICAL PERFORMANCE CHARACTERISTICS
Error Amp Output Current
50
40
30
20
10
0
–10
PIN CURRENT (μA)
C
–20
V
–30
–40
–50
–200
–100 100
FB PIN ERROR VOLTAGE (mV)
0 200
3680 G19
VC Voltages Power Good Threshold
2.50
2.00
1.50
VOLTAGE (V)
1.00
C
V
0.50
CURRENT LIMIT CLAMP
SWITCHING THRESHOLD
Minimum Input Voltage Minimum Input Voltage
5.0
4.5
4.0
3.5
3.0
INPUT VOLTAGE (V)
V
= 3.3V
OUT
2.5
= 25°C
T
A
L = 4.7μH
f = 800kHz
2.0
1
10 100 1000
LOAD CURRENT (mA)
95
90
85
80
THRESHOLD VOLTAGE (%)
= 25°C unless otherwise noted.
T
A
6.5
6.0
5.5
5.0
INPUT VOLTAGE (V)
V
= 5V
OUT
4.5
= 25 °C
T
A
L = 4.7μH
f = 800kHz
10000
3680 G20
4.0
1 1000010 100 1000
LOAD CURRENT (mA)
Switching Waveforms; Burst Mode
V
SW
5V/DIV
I
L
0.2A/DIV
V
OUT
10mV/DIV
3680 G21
0
–50 25–25 0 50 75 100 150125
TEMPERATURE (°C)
Switching Waveforms; Transition
from Burst Mode to Full Frequency
V
SW
5V/DIV
I
L
0.2A/DIV
V
OUT
10mV/DIV
VIN = 12V
V
= 3.3V
OUT
I
= 110mA
LOAD
3680 G22
1μs/DIV
75
–50 25–25 0 50 75 100 150125
TEMPERATURE (°C)
V
5V/DIV
0.5A/DIV
V
10mV/DIV
3680 G25
VIN = 12V
= 3.3V
V
OUT
= 10mA
I
3680 G23
LOAD
Switching Waveforms; Full
Frequency Continuous Operation
SW
I
L
OUT
VIN = 12V
V
= 3.3V
OUT
I
= 1A
LOAD
1μs/DIV
5μs/DIV
3680 G26
3680 G24
3972f
6
PIN FUNCTIONS
LT3972
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode. BD also supplies current to the internal
regulator.
BOOST (Pin 2): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pin 3): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
(Pin 4): The VIN pin supplies current to the LT3972’s
V
IN
internal regulator and to the internal power switch. This
pin must be locally bypassed.
RUN/SS (Pin 5): The RUN/SS pin is used to put the
LT3972 in shutdown mode. Tie to ground to shut down
the LT3972. Tie to 2.5V or more for normal operation. If
the shutdown feature is not used, tie this pin to the V
IN
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.
SYNC (Pin 6): This is the external clock synchronization
input. Ground this pin for low ripple Burst Mode operation at
low output loads. Tie to a clock source for synchronization.
Clock edges should have rise and fall times faster than 1μs.
Tie Pin to GND if not used. See Synchronization section
in Applications Information.
PG (Pin 7): The PG pin is the open collector output of an
internal comparator. PG remains low until the FB pin is
within 9% of the fi nal regulation voltage. PG output is valid
when V
is above 3.6V and RUN/SS is high.
IN
FB (Pin 8): The LT3972 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
(Pin 9): The VC pin is the output of the internal error
V
C
amplifi er. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
RT (Pin 10): Oscillator Resistor Input. Connecting a resistor
to ground from this pin sets the switching frequency.
Exposed Pad (Pin 11): Ground. The Exposed Pad must
be soldered to PCB.
BLOCK DIAGRAM
V
V
IN
IN
4
C1
INTERNAL 0.79V REF
RUN/SS
5
RT
10
R
T
SYNC
6
SOFT-START
PG
7
GND
11 8
–
BOOST
BD
1
2
C3
SW
3
V
C
9
L1
D1
C
C
C
F
R
C
C2
3680 BD
V
OUT
3972f
+
SLOPE COMP
3
OSCILLATOR
200kHzTO2.4MHz
ERROR AMP
0.73V
+
–
FB
R1
+
–
R2
SWITCH
LATCH
R
S
DISABLE
BurstMode
DETECT
CLAMP
V
C
Q
7
LT3972
OPERATION
The LT3972 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT,
enables an RS fl ip-fl op, turning on the internal power
switch. An amplifi er and comparator monitor the current
fl owing between the V
off when this current reaches a level determined by the
voltage at V
voltage through an external resistor divider tied to the FB
pin and servos the V
increases, more current is delivered to the output; if it
decreases, less current is delivered. An active clamp on the
pin provides current limit. The VC pin is also clamped to
V
C
the voltage on the RUN/SS pin; soft-start is implemented
by generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the V
but if the BD 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
effi ciency. The RUN/SS pin is used to place the LT3972
in shutdown, disconnecting the output and reducing the
input current to less than 0.5μA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
. An error amplifi er measures the output
C
and SW pins, turning the switch
IN
pin. If the error amplifi er’s output
C
pin,
IN
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate
the internal bipolar NPN power switch for effi cient operation.
To further optimize effi ciency, the LT3972 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input supply
current to 75μA in a typical application.
The oscillator reduces the LT3972’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup
and overload.
The LT3972 contains a power good comparator which trips
when the FB pin is at 91% of its regulated value. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3972 is
enabled and V
The LT3972 has an overvoltage protection feature which
disables switching action when the V
typical (33V minimum). When switching is disabled, the
LT3972 can safely sustain input voltages up to 62V.
is above 3.6V.
IN
goes above 35V
IN
8
3972f
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