LT1371
500kHz High Efficiency
3A Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1371 is a monolithic high frequency current mode
switching regulator. It can be operated in all standard
switching configurations including boost, buck, flyback,
forward, inverting and “Cuk.” A 3A high efficiency switch
is included on the die, along with all oscillator, control and
protection circuitry.
The LT1371 typically consumes only 4mA quiescent
current and has higher efficiency than previous parts.
High frequency switching allows for very small inductors
to be used.
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
Low Minimum Supply Voltage: 2.7V
Quiescent Current: 4mA Typ
Current Limited Power Switch: 3A
Regulates Positive or Negative Outputs
Shutdown Supply Current: 12µA Typ
Easy External Synchronization
APPLICATIONS
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Boost Regulators
Laptop Computer Supplies
Multiple Output Flyback Supplies
Inverting Supplies
TYPICAL APPLICATION
5V to 12V Boost Converter
5V
L1*
4.7µH
D1
MBRS330T3
V
OUT
12V
R1
53.6k
1%
†
V
IN
OFF
ON
S/S
LT1371
V
SW
EFFICIENCY (%)
+
C1**
22µF
25V
FB
GND
C2
0.047µF
R3
2k
V
C
+
R2
6.19k
1%
C3
0.0047µF
LT1371 • TA01
C4**
22µF
25V
×
2
*COILCRAFT DO3316P-472 (4.7µH),
DO3316P-103 (10µH) OR
SUMIDA CD104-100MC (10µH)
**AVX TPSD226M025R0200
†
MAX I
OUT
L1
I
OUT
4.7µH 0.7A
10µH 0.8A
U
U
U
12V Output Efficiency
100
V
IN
= 5V
90
80
70
60
50
0.01
0.1
OUTPUT CURRENT (A)
1
LT1371 • TA02
1
LT1371
ABSOLUTE
AXI U
RATI GS
Operating Ambient Temperature Range ...... 0°C to 70°C
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
Supply Voltage ....................................................... 30V
Switch Voltage
LT1371 ............................................................... 35V
LT1371HV .......................................................... 42V
S/S, SHDN, SYNC Pin Voltage ................................ 30V
Feedback Pin Voltage (Transient, 10ms) ..............
±10V
Feedback Pin Current ........................................... 10mA
Negative Feedback Pin Voltage
(Transient, 10ms) .............................................
±10V
PACKAGE/ORDER I FOR ATIO
FRONT VIEW
7
6
5
4
3
2
1
R PACKAGE
7-LEAD PLASTIC DD
T
JMAX
= 125°C,
θ
JA
= 30°C/W
WITH PACKAGE SOLDERED TO 0.5 INCH
2
COPPER
AREA OVER BACKSIDE GROUND PLANE OR INTERNAL
POWER PLANE.
θ
JA
CAN VARY FROM 20°C/W TO
> 40°C/W DEPENDING ON MOUNTING TECHNIQUE
TAB
IS
GND
V
IN
S/S
V
SW
GND
NFB
FB
V
C
ORDER PART
NUMBER
TOP VIEW
LT1371CR
LT1371HVCR
LT1371IR
LT1371HVIR
FRONT VIEW
7
6
5
4
3
2
1
T7 PACKAGE
7-LEAD TO-220
T
JMAX
= 125°C,
θ
JA
= 50°C/W,
θ
JC
= 4°C/W
TAB
IS
GND
V
IN
S/S
V
SW
GND
NFB
FB
V
C
ORDER PART
NUMBER
LT1371CT7
LT1371HVCT7
LT1371IT7
LT1371HVIT7
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.6V, V
FB
= V
REF
, V
SW
, S/S, SHDN, SYNC and NFB pins open, unless otherwise noted.
SYMBOL
V
REF
I
FB
PARAMETER
Reference Voltage
Feedback Input Current
Reference Voltage Line Regulation
CONDITIONS
Measured at Feedback Pin
V
C
= 0.8V
V
FB
= V
REF
q
q
2.7V
≤
V
IN
≤
25V, V
C
= 0.8V
2
U
U
W
W W
U
W
ORDER PART
NUMBER
V
C
1
FB 2
NFB 3
GND 4
GND 5
GND 6
GND 7
SHDN 8
SYNC 9
V
IN
10
20 V
SW
19 NC
18 V
SW
17 GND
16 GND
15 GND
14 GND
13 NC
12 NC
11 GND
LT1371CSW
LT1371HVCSW
LT1371ISW
LT1371HVISW
SW PACKAGE
20-LEAD PLASTIC SO WIDE
T
JMAX
= 125°C,
θ
JA
= 50°C/W
θ
JA
WILL VARY FROM APPROXIMATELY 40°C/W WITH
0.75 INCH
2
OF 1 OZ COPPER TO 50°C/W WITH 0.33 INCH
2
OF 1 OZ COPPER ON A DOUBLE-SIDED BOARD
MIN
1.230
1.225
TYP
1.245
1.245
250
MAX
1.260
1.265
550
900
0.03
UNITS
V
V
nA
nA
%/V
q
0.01
LT1371
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.6V, V
FB
= V
REF
, V
SW
, S/S, SHDN, SYNC and NFB pins open, unless otherwise noted.
SYMBOL
V
NFB
I
NFB
PARAMETER
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
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
≤
0°C (I Grade)
q
q
CONDITIONS
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
MIN
– 2.540
q
– 2.570
q
q
TYP
– 2.490
– 2.490
– 30
0.01
MAX
– 2.440
– 2.410
– 15
0.05
1900
2300
350
2400
2.30
0.52
1.25
550
580
580
260
UNITS
V
V
µA
%/V
µmho
µmho
µA
µA
V
V
V/ V
V
kHz
kHz
kHz
%
ns
V
V
V
– 45
1100
700
120
1.70
0.25
0.8
450
430
400
85
35
42
40
3.0
2.6
1500
200
1400
1.95
0.40
500
1
500
500
95
130
47
47
0.25
3.8
3.4
15
4
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
Maximum Switch Duty Cycle
Switch Current Limit Blanking Time
BV
Output Switch Breakdown Voltage
LT1371
LT1371HV
0° C
≤
T
J
≤
125°C
– 40°C
≤
T
J
≤
0°C (I Grade)
I
SW
= 2A
Duty Cycle = 50%
Duty Cycle = 80% (Note 1)
q
q
q
q
q
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
0.45
5.4
5.0
25
Ω
A
A
mA/A
A/V
q
2.4
4
12
0.6
5
– 10
600
1.3
12
2.7
5.5
30
50
2
25
15
800
V
mA
µA
µA
V
µs
µA
kHz
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
or V
SHDN
≤
5V
q
q
q
q
q
q
Shutdown Threshold
Shutdown Delay
S/S or SHDN 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
= 1.33 (2.75 – DC).
3
LT1371
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage
vs Switch Current
1.0
6
150°C
100°C
25°C
5
4
–55°C
3
2
1
0
0
0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0
SWITCH CURRENT (A)
LT1371 • G01
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
Shutdown Delay and Threshold
vs Temperature
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
20
18
16
SHUTDOWN DELAY (µs)
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)
LT1371 • G04
f
SYNC
= 700kHz
ERROR AMPLIFIER OUTPUT CURRENT (µA)
14
12
10
8
6
4
2
0
–50 –25
S/S or SHDN Pin Input Current
vs Voltage
5
4
3
INPUT CURRENT (µA)
2
1
0
–1
–2
–3
–4
–5
–1
0
1
2
3 4 5 6
VOLTAGE (V)
7
8
9
SWITCHING FREQUENCY (% OF TYPICAL)
V
IN
= 5V
TRANSCONDUCTANCE (µmho)
4
U W
LT1371 • G07
Switch Current Limit
vs Duty Cycle
3.0
2.8
25°C AND
125°C
Minimum Input Voltage
vs Temperature
2.6
2.4
2.2
2.0
1.8
–50 –25
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
LT1371 • G02
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1371 • G03
Minimum Synchronization
Voltage vs Temperature
2.0
3.0
2.5
2.0
1.5
1.0
0.5
0
–50 –25
400
300
200
100
0
–100
–200
–300
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1371 • G05
Error Amplifier Output Current
vs Feedback Pin Voltage
25°C
–55°C
125°C
SHUTDOWN THRESHOLD (V)
–0.3
V
REF
–0.2
–0.1
FEEDBACK PIN VOLTAGE (V)
0.1
LT1371 • 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)
LT1371 • 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)
LT1371 • G09
g
m
=
∆I
(V
C
)
∆V
(FB)
LT1371
TYPICAL PERFORMANCE CHARACTERISTICS
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)
LT1371 • G10
V
C
THRESHOLD
PIN FUNCTIONS
V
C
:
The Compensation pin is used for frequency compen-
sation, 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 performed with an
RC network connected from the V
C
pin to ground.
FB:
The Feedback pin is used for positive output voltage
sensing and oscillator frequency shifting. It is the invert-
ing 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 NFB pin is
used. See Applications Information.
NFB:
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 (R and T7 Packages Only):
Shutdown and Synchroni-
zation 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 between 600kHz and 800kHz.
SHDN: (SW Package Only):
The Shutdown pin is active
low and the shutdown threshold is typically 1.3V. For
normal operation, pull the SHDN pin high, tie it to V
IN
or
leave it floating.
SYNC (SW Package Only):
To synchronize switching,
drive the SYNC pin between 600kHz and 800kHz. If not
used, the SYNC pin can be tied high, low or left floating.
V
IN
:
Bypass Input Supply pin with a low ESR capacitor,
10µF or more. The regulator goes into undervoltage lock-
out when V
IN
drops below 2.5V. Undervoltage lockout
stops switching and pulls the V
C
pin low.
V
SW
:
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.
GND:
Tie all Ground pins to a good quality ground plane.
U W
Feedback Input Current
vs Temperature
800
0
Negative Feedback Input Current
vs Temperature
V
NFB
=V
NFR
–10
–20
–30
–40
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1371 • G11
–50
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1371 • G12
U
U
U
5
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