LT1373
250kHz Low Supply Current
High Efficiency
1.5A Switching Regulator
FEATURES
s
s
s
s
s
s
s
s
s
s
s
DESCRIPTIO
1mA I
Q
at 250kHz
Uses Small Inductors: 15µH
All Surface Mount Components
Only 0.6 Square Inch of Board Space
Low Minimum Supply Voltage: 2.7V
Constant Frequency Current Mode
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
The LT
®
1373 is a low supply current high frequency
current mode switching regulator. It can be operated in all
standard switching configurations including boost, buck,
flyback, forward, inverting and “Cuk.” A 1.5A high effi-
ciency switch is included on the die, along with all oscilla-
tor, control and protection circuitry. All functions of the
LT1373 are integrated into 8-pin SO/PDIP packages.
Compared to the 500kHz LT1372, which draws 4mA of
quiescent current, the LT1373 switches at 250kHz, typi-
cally consumes only 1mA and has higher efficiency. High
frequency switching allows for small inductors to be used.
All surface mount components consume less than 0.6
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.
APPLICATIO S
s
s
s
s
s
Boost Regulators
CCFL Backlight Driver
Laptop Computer Supplies
Multiple Output Flyback Supplies
Inverting Supplies
TYPICAL APPLICATIO
5V
5
OFF
ON 4 S/S
V
IN
5V-to-12V Boost Converter
L1*
22µH
D1
MBRS120T3
R1
215k
1%
V
OUT
†
12V
EFFICIENCY (%)
100
V
IN
= 5V
f = 250kHz
90
V
SW
LT1373
8
80
+
†
+
C1**
22µF
FB
GND
6, 7
V
C
1
2
C4**
22µF
MAX I
OUT
70
C2
0.01µF
R3
5k
R2
24.9k
1%
L1
I
OUT
15µH 0.3A
22µH 0.35A
*SUMIDA CD75-220KC (22µH) OR
COILCRAFT D03316-153 (15µH)
**AVX TPSD226M025R0200
60
50
1
10
100
OUTPUT CURRENT (mA)
1000
LT1373 • TA02
LT1373 • TA01
U
12V Output Efficiency
U
U
1
LT1373
ABSOLUTE
(Note 1)
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
LT1373 ............................................................... 35V
LT1373HV .......................................................... 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
LT1373CN8
LT1373HVCN8
LT1373CS8
LT1373HVCS8
LT1373IN8
LT1373HVIN8
LT1373IS8
LT1373HVIS8
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 100°C/ W (N8)
T
JMAX
= 125°C,
θ
JA
= 120°C/ W (S8)
S8 PART MARKING
1373
1373I
1373H
1373HI
Consult factory for Military grade parts.
*Units shipped prior to Date Code 9552 are rated at 100°C maximum
operating temperature.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
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
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
≤
0°C (I Grade)
q
q
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
50
MAX
1.260
1.265
150
275
0.03
– 2.39
– 2.35
–2
0.05
500
600
90
1500
2.30
0.52
1.25
275
290
290
500
UNITS
V
V
nA
nA
%/V
V
V
µA
%/V
µmho
µmho
µA
µA
V
V
V/ V
V
kHz
kHz
kHz
%
ns
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
=
±5µA
q
q
q
q
0.01
– 2.51
– 2.55
– 12
– 2.45
– 2.45
–7
0.01
250
150
25
1.70
0.25
0.8
225
210
200
90
375
50
850
1.95
0.40
250
1
250
250
95
340
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
Maximum Switch Duty Cycle
Switch Current Limit Blanking Time
2
U
W
U
U
W W
W
LT1373
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
IN
= 5V, V
C
= 0.6V, V
FB
= V
REF
, V
SW
, S/S and NFB pins open, unless otherwise noted.
SYMBOL
BV
PARAMETER
Output Switch Breakdown Voltage
CONDITIONS
LT1373
LT1373HV
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
≤
0°C (I Grade)
I
SW
= 1A
Duty Cycle = 50%
Duty Cycle = 80% (Note 2)
q
q
q
MIN
35
42
40
TYP
47
47
0.5
MAX
UNITS
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
0.85
2.7
2.5
20
Ω
A
A
mA/A
A/V
q
q
1.5
1.3
1.9
1.7
10
2
q
2.4
1
12
0.6
5
– 10
300
1.3
12
2.7
1.5
30
50
2
100
15
340
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
≤
5V
q
q
q
q
q
q
Shutdown Threshold
Shutdown Delay
S/S Pin Input Current
Synchronization Frequency Range
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2:
For duty cycles (DC) between 50% and 90%, minimum
guaranteed switch current is given by I
LIM
= 0.667 (2.75 – DC).
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage
vs Switch Current
1.0
SWITCH SATURATION VOLTAGE (V)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
150°C
100°C
SWITCH CURRENT LIMIT (A)
INPUT VOLTAGE (V)
–55°C
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
LT1373 • G01
U W
25°C
Switch Current Limit
vs Duty Cycle
3.0
2.5
2.0
–55°C
1.5
1.0
0.5
0
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
LT1373 • G02
Minimum Input Voltage
vs Temperature
3.0
2.8
25°C AND
125°C
2.6
2.4
2.2
2.0
1.8
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1373 • G03
3
LT1373
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Delay and Threshold
vs Temperature
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
20
18
16
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)
LT1373 • G04
f
SYNC
= 330kHz
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
HIGH CLAMP
FEEDBACK INPUT CURRENT (nA)
V
FB
= V
REF
350
300
250
200
150
100
50
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1373 • G10
NEGATIVE FEEDBACK INPUT CURRENT (µA)
V
C
PIN VOLTAGE (V)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
–50 –25
V
C
THRESHOLD
4
U W
LT1373 • G07
Minimum Synchronization
Voltage vs Temperature
2.0
Error Amplifier Output Current
vs Feedback Pin Voltage
100
75
50
25
0
–25
–50
–75
–0.3
V
REF
–0.2
–0.1
FEEDBACK PIN VOLTAGE (V)
0.1
LT1373 • G06
3.0
2.5
2.0
1.5
1.0
0.5
25°C
–55°C
125°C
SHUTDOWN THRESHOLD (V)
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1373 • G05
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)
LT1373 • G08
Error Amplifier Transconductance
vs Temperature
500
g
m
=
400
∆I
(V
C
)
∆V
(FB)
300
200
100
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1373 • G09
Feedback Input Current
vs Temperature
400
Negative Feedback Input
Current vs Temperature
0
–2
–4
–6
–8
–10
–12
–14
–16
–18
–20
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1373 • G12
V
NFB
= V
NFR
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LT1373 • G11
LT1373
PI FU CTIO S
V
C
(Pin 1):
Compensation Pin. The V
C
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
performed 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 100µA
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 400k
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 300kHz and 340kHz.
V
IN
(Pin 5):
Input Supply Pin. Bypass V
IN
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 DIAGRA
S/S
400k
NFB
200k
FB
1.245V
REF
GND SENSE
+
W
–
U
U
U
V
IN
SHUTDOWN
DELAY AND RESET
250kHz
OSC
5:1 FREQUENCY
SHIFT
LOW DROPOUT
2.3V REG
SW
ANTI-SAT
SYNC
LOGIC
DRIVER
SWITCH
+
NEGATIVE
FEEDBACK
AMP
–
COMP
+
CURRENT
AMP
V
C
A
V
≈
6
0.08Ω
ERROR
AMP
–
GND
LT1373 • BD
5
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