LT1372/LT1377
500kHz and 1MHz
High Efficiency
1.5A Switching Regulators
FEATURES
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DESCRIPTION
The LT
®
1372/LT1377 are monolithic high frequency
switching regulators. They can be operated in all standard
switching configurations including boost, buck, flyback,
forward, inverting and “Cuk.” A 1.5A high efficiency switch
is included on the die, along with all oscillator, control and
protection circuitry. All functions of the LT1372/LT1377
are integrated into 8-pin SO/PDIP packages.
The LT1372/LT1377 typically consumes only 4mA quies-
cent current and has higher efficiency than previous parts.
High frequency switching allows for very small inductors
to be used. All surface mount components consume less
than 0.5 square inch of board space.
New design techniques increase flexibility and maintain
ease of use. Switching is easily synchronized to an exter-
nal logic level source. A logic low on the shutdown pin
reduces supply current to 12µA. Unique error amplifier
circuitry can regulate positive or negative output voltage
while maintaining simple frequency compensation tech-
niques. Nonlinear error amplifier transconductance re-
duces output overshoot on start-up or overload recovery.
Oscillator frequency shifting protects external compo-
nents during overload conditions.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Faster Switching with Increased Efficiency
Uses Small Inductors: 4.7µH
All Surface Mount Components
Only 0.5 Square Inch of Board Space
Low Minimum Supply Voltage: 2.7V
Quiescent Current: 4mA Typ
Current Limited Power Switch: 1.5A
Regulates Positive or Negative Outputs
Shutdown Supply Current: 12µA Typ
Easy External Synchronization
8-Pin SO or PDIP Packages
APPLICATIONS
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Boost Regulators
CCFL Backlight Driver
Laptop Computer Supplies
Multiple Output Flyback Supplies
Inverting Supplies
TYPICAL APPLICATION
5V-to-12V Boost Converter
5V
5
OFF
ON 4 S/S
V
IN
V
SW
8
R1
53.6k
1%
L1*
4.7µH
D1
MBRS120T3
100
V
OUT
†
12V
90
EFFICIENCY (%)
80
LT1372/LT1377
+
C1**
22µF
FB
GND
6, 7
C2
0.047µF
R3
2k
V
C
1
2
+
R2
6.19k
1%
C3
0.0047µF
C4**
22µF
*COILCRAFT DO1608-472 (4.7µH) OR
COILCRAFT DT3316-103 (10µH) OR
SUMIDA CD43-4R7 (4.7µH) OR
SUMIDA CD73-100KC (10µH) OR
**AVX TPSD226M025R0200
†
MAX I
OUT
L1
I
OUT
4.7µH 0.25A
10µH 0.35A
LT1372 • TA01
70
60
50
0.01
U
U
U
12V Output Efficiency
V
IN
= 5V
0.1
OUTPUT CURRENT (A)
1
LT1372 • TA02
1
LT1372/LT1377
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
C
1
FB 2
NFB 3
S/S 4
N8 PACKAGE
8-LEAD PDIP
8
7
6
5
V
SW
GND
GND S
V
IN
Supply Voltage ....................................................... 30V
Switch Voltage
LT1372/LT1377 .................................................. 35V
LT1372HV .......................................................... 42V
S/S Pin Voltage ....................................................... 30V
Feedback Pin Voltage (Transient, 10ms) ..............
±10V
Feedback Pin Current ........................................... 10mA
Negative Feedback Pin Voltage
(Transient, 10ms) .............................................
±10V
Operating Junction Temperature Range
Commercial ........................................ 0°C to 125°C*
Industrial ......................................... – 40°C to 125°C
Short Circuit ......................................... 0°C to 150°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART NUMBER
LT1372CN8
LT1372HVCN8
LT1372CS8
LT1372HVCS8
LT1372IN8
1372
1372I
LT1372HVIN8
LT1372IS8
LT1372HVIS8
LT1377CS8
LT1377IS8
1377
1377I
S8 PACKAGE
8-LEAD PLASTIC SO
S8 PART MARKING
1372H
1372HI
T
JMAX
= 125°C,
θ
JA
= 100°C/ W (N8)
T
JMAX
= 125°C,
θ
JA
= 120°C/ W (S8)
Consult factory for Military grade parts.
*Units shipped prior to Date Code 9552 are rated at 100°C maximum
operating temperature.
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.6V, V
FB
= V
REF
, V
SW
, S/S and NFB pins open, unless otherwise noted.
SYMBOL
V
REF
I
FB
PARAMETER
Reference Voltage
Feedback Input Current
Reference Voltage Line Regulation
V
NFB
I
NFB
Negative Feedback Reference Voltage
Negative Feedback Input Current
Negative Feedback Reference Voltage
Line Regulation
g
m
Error Amplifier Transconductance
Error Amplifier Source Current
Error Amplifier Sink Current
Error Amplifier Clamp Voltage
A
V
f
Error Amplifier Voltage Gain
V
C
Pin Threshold
Switching Frequency
Duty Cycle = 0%
2.7V
≤
V
IN
≤
25V
LT1372
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
< 0°C (I Grade)
LT1377
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
< 0°C (I Grade)
0.8
450
430
400
0.90
0.86
0.80
CONDITIONS
Measured at Feedback Pin
V
C
= 0.8V
V
FB
= V
REF
q
q
MIN
1.230
1.225
TYP
1.245
1.245
250
MAX
1.260
1.265
550
900
0.03
– 2.440
– 2.410
– 15
0.05
1900
2300
350
2400
2.30
0.52
1.25
550
580
580
1.10
1.16
1.16
UNITS
V
V
nA
nA
%/V
V
V
µA
%/V
µmho
µmho
µA
µA
V
V
V/ V
V
kHz
kHz
kHz
MHz
MHz
MHz
2.7V
≤
V
IN
≤
25V, V
C
= 0.8V
Measured at Negative Feedback Pin
Feedback Pin Open, V
C
= 0.8V
V
NFB
= V
NFR
2.7V
≤
V
IN
≤
25V, V
C
= 0.8V
∆I
C
=
±25µA
q
0.01
– 2.490
– 2.490
– 30
0.01
1100
700
120
1.70
0.25
1500
200
1400
1.95
0.40
500
1
500
500
1
1
– 2.540
q
– 2.570
q
q
– 45
q
V
FB
= V
REF
– 150mV, V
C
= 1.5V
V
FB
= V
REF
+ 150mV, V
C
= 1.5V
High Clamp, V
FB
= 1V
Low Clamp, V
FB
= 1.5V
q
q
q
q
2
U
W
U
U
W W
W
LT1372/LT1377
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.6V, V
FB
= V
REF
, V
SW
, S/S and NFB pins open, unless otherwise noted.
SYMBOL
PARAMETER
Maximum Switch Duty Cycle
Switch Current Limit Blanking Time
BV
Output Switch Breakdown Voltage
LT1372/LT1377
LT1372HV
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
< 0°C (I Grade)
I
SW
= 1A
Duty Cycle = 50%
Duty Cycle = 80% (Note 1)
q
q
q
q
q
CONDITIONS
q
MIN
90
35
42
40
1.5
1.3
TYP
95
130
47
47
0.5
1.9
1.7
15
2
MAX
260
UNITS
%
ns
V
V
V
V
SAT
I
LIM
∆I
IN
∆I
SW
Output Switch “On” Resistance
Switch Current Limit
Supply Current Increase During Switch On-Time
Control Voltage to Switch Current
Transconductance
Minimum Input Voltage
0.8
2.7
2.5
25
Ω
A
A
mA/A
A/V
q
2.4
4
12
0.6
5
– 10
600
1.2
1.3
12
2.7
5.5
30
50
2
25
15
800
1.6
V
mA
µA
µA
V
µs
µA
kHz
MHz
I
Q
Supply Current
Shutdown Supply Current
2.7V
≤
V
IN
≤
25V
2.7V
≤
V
IN
≤
25V, V
S/S
≤
0.6V
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
< 0°C (I Grade)
2.7V
≤
V
IN
≤
25V
0V
≤
V
S/S
≤
5V
LT1372
LT1377
q
q
q
q
q
q
q
Shutdown Threshold
Shutdown Delay
S/S Pin Input Current
Synchronization Frequency Range
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
For duty cycles (DC) between 50% and 90%, minimum
guaranteed switch current is given by I
LIM
= 0.667 (2.75 – DC).
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage
vs Switch Current
1.0
3.0
150°C
100°C
25°C
2.5
SWITCH SATURATION VOLTAGE (V)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
SWITCH CURRENT LIMIT (A)
INPUT VOLTAGE (V)
–55°C
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
LT1372 • G01
U W
Switch Current Limit
vs Duty Cycle
3.0
2.8
Minimum Input Voltage
vs Temperature
2.0
–55°C
1.5
1.0
0.5
0
0
25°C AND
125°C
2.6
2.4
2.2
2.0
1.8
–50 –25
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
LT1372 • G02
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1372 • G03
3
LT1372/LT1377
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Delay and Threshold
vs Temperature
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
20
18
16
2.0
1.8
SHUTDOWN THRESHOLD
1.6
1.4
1.2
1.0
SHUTDOWN DELAY
0.8
0.6
0.4
0.2
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1372 • G04
2.5
2.0
f
SYNC
= 700kHz (LT1372)
f
SYNC
= 1.4MHz (LT1377)
ERROR AMPLIFIER OUTPUT CURRENT (µA)
SHUTDOWN DELAY (µs)
14
12
10
8
6
4
2
0
–50 –25
S/S Pin Input Current
vs Voltage
5
4
SWITCHING FREQUENCY (% OF TYPICAL)
V
IN
= 5V
TRANSCONDUCTANCE (µmho)
S/S PIN INPUT CURRENT (µA)
3
2
1
0
–1
–2
–3
–4
–5
–1
0
1
2 3 4 5 6
S/S PIN VOLTAGE (V)
7
8
9
V
C
Pin Threshold and High
Clamp Voltage vs Temperature
2.4
2.2
2.0
V
C
PIN VOLTAGE (V)
V
C
HIGH CLAMP
FEEDBACK INPUT CURRENT (nA)
700
600
500
400
300
200
100
V
FB
=V
REF
NEGATIVE FEEDBACK INPUT CURRENT (µA)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1372 • G10
V
C
THRESHOLD
4
U W
LT1372 • G07
Minimum Synchronization
Voltage vs Temperature
3.0
400
300
200
100
0
–100
–200
–300
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1372 • G05
Error Amplifier Output Current
vs Feedback Pin Voltage
25°C
–55°C
125°C
SHUTDOWN THRESHOLD (V)
LT1377
1.5
LT1372
1.0
0.5
0
–50 –25
–0.3
V
REF
–0.2
–0.1
FEEDBACK PIN VOLTAGE (V)
0.1
LT1372 • G06
Switching Frequency
vs Feedback Pin Voltage
110
100
90
80
70
60
50
40
30
20
10
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
FEEDBACK PIN VOLTAGE (V)
LT1372 • G08
Error Amplifier Transconductance
vs Temperature
2000
1800
1600
1400
1200
1000
800
600
400
200
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1372 • G09
g
m
=
∆I
(V
C
)
∆V
(FB)
Feedback Input Current
vs Temperature
800
Negative Feedback Input Current
vs Temperature
0
V
NFB
=V
NFR
–10
–20
–30
–40
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1372 • G11
–50
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1372 • G12
LT1372/LT1377
PIN FUNCTIONS
V
C
(Pin 1):
The Compensation pin is used for frequency
compensation, current limiting and soft start. It is the
output of the error amplifier and the input of the current
comparator. Loop frequency compensation can be per-
formed with an RC network connected from the V
C
pin to
ground.
FB (Pin 2):
The Feedback pin is used for positive output
voltage sensing and oscillator frequency shifting. It is the
inverting input to the error amplifier. The noninverting
input of this amplifier is internally tied to a 1.245V
reference. Load on the FB pin should not exceed 250µA
when the NFB pin is used. See Applications Information.
NFB (Pin 3):
The Negative Feedback pin is used for
negative output voltage sensing. It is connected to the
inverting input of the negative feedback amplifier through
a 100k source resistor.
S/S (Pin 4):
Shutdown and Synchronization Pin. The S/S
pin is logic level compatible. Shutdown is active low and
the shutdown threshold is typically 1.3V. For normal
operation, pull the S/S pin high, tie it to V
IN
or leave it
floating. To synchronize switching, drive the S/S pin be-
tween 600kHz and 800kHz (LT1372) or 1.2MHz to 1.6MHz
(LT1377).
V
IN
(Pin 5):
Bypass input supply pin with 10µF or more. The
part goes into undervoltage lockout when V
IN
drops below
2.5V. Undervoltage lockout stops switching and pulls the
V
C
pin low.
GND S (Pin 6):
The ground sense pin is a “clean” ground.
The internal reference, error amplifier and negative feed-
back amplifier are referred to the ground sense pin. Con-
nect it to ground. Keep the ground path connection to the
output resistor divider and the V
C
compensation network
free of large ground currents.
GND (Pin 7):
The ground pin is the emitter connection of
the power switch and has large currents flowing through it.
It should be connected directly to a good quality ground
plane.
V
SW
(Pin 8):
The switch pin is the collector of the power
switch and has large currents flowing through it. Keep the
traces to the switching components as short as possible to
minimize radiation and voltage spikes.
BLOCK DIAGRAM
S/S
100k
NFB
50k
FB
1.245V
REF
GND SENSE
+
–
W
U
U
U
V
IN
SHUTDOWN
DELAY AND RESET
LOW DROPOUT
2.3V REG
SW
ANTI-SAT
SYNC
OSC
LOGIC
DRIVER
SWITCH
5:1 FREQUENCY
SHIFT
+
NFBA
–
COMP
+
EA
V
C
A
V
≈
6
IA
0.08Ω
–
GND
LT1372 • BD
5
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