FEATURES
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LT1952/LT1952-1
Single Switch Synchronous
Forward Controller
DESCRIPTION
The LT
®
1952/LT1952-1 are current mode PWM controllers
optimized to control the forward converter topology, using
one primary MOSFET. The LT1952/LT1952-1 provide
synchronous rectifier control, resulting in extremely
high efficiency. A programmable Volt-Second clamp
provides a safeguard for transformer reset that prevents
saturation. This allows a single MOSFET on the primary
side to reliably run at greater than 50% duty cycle for high
MOSFET, transformer and rectifier utilization. The devices
include soft-start for controlled exit from shutdown and
undervoltage lockout. A precision 107mV current limit
threshold, independent of duty cycle, combines with soft-
start to provide hiccup short-circuit protection. The LT1952
is optimized for micropower bootstrap start-up from high
input voltages. The LT1952-1 allows start-up from lower
input voltages. Programmable slope compensation and
leading edge blanking allow optimization of loop bandwidth
with a wide range of inductors and MOSFETs. Each device
can be programmed over a 100kHz to 500kHz frequency
range and the part can be synchronized to an external
clock. The error amplifier is a true op amp, allowing a wide
range of compensation networks. The LT1952/LT1952-1
are available in a small 16-pin SSOP package.
L,
LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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Synchronous Rectifier Control for High Efficiency
Programmable Volt-Second Clamp
Output Power Levels from 25W to 500W
Low Current Start-Up
(LT1952: 460μA; V
IN
On/Off = 14.25V/8.75V)
(LT1952-1: 400μA; V
IN
On/Off = 7.75V/6.5V)
True PWM Soft-Start
Low Stress Short-Circuit Protection
Precision 107mV Current Limit Threshold
Adjustable Delay for Synchronous Timing
Accurate Shutdown Threshold with Programmable
Hysteresis
Programmable Slope Compensation
Programmable Leading Edge Blanking
Programmable Frequency (100kHz to 500kHz)
Synchronizable to an External Clock up to 1.5 • f
OSC
Internal 1.23V Reference
2.5V External Reference
Current Mode Control
Small 16-Pin SSOP Package
APPLICATIONS
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Telecommunications Power Supplies
Industrial and Distributed Power
Isolated and Non Isolated DC/DC Converters
TYPICAL APPLICATION
36V to 72V Input, 12V at 20A Semi-Regulated Bus Converter
SUPPLY FROM BIAS
WINDING OF T1
40k
V
IN
T1
PA0905
L1
PA1494.242
V
OUT
12V
20A
47μF
16V
X5R
2
V
OUT
(V)
12V Bus Converter
V
OUT
vs V
IN
16
16V
V
REF
52.3k
V
IN
340k
100k
13k
COMP
SS_MAXDC
LT1952/
LT1952-1
SD_V
SEC
FB
OUT
OC
I
SENSE
V
IN
10μF
14
Si7450
Si7370
2
PH4840
2
12
0.005Ω
LTC3900
T2
FG
SYNC
220pF
560Ω
CG
10
SYNC
GND
0.1μF
0.1μF
40k
SOUT
R
OSC
8
36
42
48
54
V
IN
(V)
60
66
72
PGND BLANK DELAY
40k
178k
1952 TA01
1952 TA01b
19521fd
1
LT1952/LT1952-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
COMP
FB
R
OSC
SYNC
SS_MAXDC
V
REF
SD_V
SEC
GND
1
2
3
4
5
6
7
8
16 SOUT
15 V
IN
14 OUT
13 PGND
12 DELAY
11 OC
10 I
SENSE
9
BLANK
V
IN
(Note 8) ............................................... –0.3V to 25V
SYNC, SS_MAXDC, SD_V
SEC
, I
SENSE
, OC ....– 0.3V to 6V
COMP BLANK, DELAY ............................... –0.3V to 3.5V
,
FB ................................................................– 0.3V to 3V
R
OSC
..................................................................... –50μA
V
REF
....................................................................–10mA
Operating Junction Temperature Range
(Notes 2, 5) ............................................–40°C to 125°C
Storage Temperature Range...................–65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
GN PACKAGE
16-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 110°C/W,
θ
JC
= 40°C/W
ORDER INFORMATION
LEAD FREE FINISH
LT1952EGN#PBF
LT1952IGN#PBF
LT1952EGN-1#PBF
LT1952IGN-1#PBF
LEAD BASED FINISH
LT1952EGN
LT1952IGN
LT1952EGN-1
LT1952IGN-1
TAPE AND REEL
LT1952EGN#TRPBF
LT1952IGN#TRPBF
LT1952EGN-1#TRPBF
LT1952IGN-1#TRPBF
TAPE AND REEL
LT1952EGN#TR
LT1952IGN#TR
LT1952EGN-1#TR
LT1952IGN-1#TR
PART MARKING
1952
1952I
19521
1952I1
PART MARKING
1952
1952I
19521
1952I1
PACKAGE DESCRIPTION
16-Lead Plastic SSOP
16-Lead Plastic SSOP
16-Lead Plastic SSOP
16-Lead Plastic SSOP
PACKAGE DESCRIPTION
16-Lead Plastic SSOP
16-Lead Plastic SSOP
16-Lead Plastic SSOP
16-Lead Plastic SSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. COMP = open, FB = 1.4V, R
OSC
= 178k, SYNC = 0V, SS_MAXDC = V
REF
, V
REF
= 0.1μF SD_V
SEC
= 2V, BLANK = 121k, DELAY = 121k, I
SENSE
= 0V, OC = 0V, OUT = 1nF V
IN
= 15V, SOUT = open, unless otherwise specified.
,
,
CONDITIONS
I(V
REF
) = 0μA
I(V
REF
) = 0μA, I
SENSE
= OC = Open
FB = 0V, SS_MAXDC = 0V (Notes 4, 9)
FB = 0V, SS_MAXDC = 0V (Notes 4, 9)
SD_V
SEC
= 0V
10V < V
IN
< 25V
SD_V
SEC
= SD_V
SEC
Threshold + 100mV
SD_V
SEC
= SD_V
SEC
Threshold – 100mV
8.3
l
l
l
l
MIN
V
IN
OFF
TYP
MAX
25
UNITS
V
mA
μA
μA
μA
V
μA
μA
19521fd
PWM CONTROLLER
Operational Input Voltage
V
IN
Quiescent Current
V
IN
Start-up Current (LT1952)
V
IN
Start-up Current (LT1952-1)
V
IN
Shutdown Current
SD_V
SEC
Threshold
SD_V
SEC (ON)
Current
SD_V
SEC (OFF)
Current
5.2
460
400
240
1.261
1.32
0
10
11.7
6.5
700
575
350
1.379
2
LT1952/LT1952-1
ELECTRICAL CHARACTERISTICS
PARAMETER
V
IN ON
(LT1952)
V
IN OFF
(LT1952)
V
IN HYSTERESIS
(LT1952)
V
IN ON
(LT1952-1)
V
IN OFF
(LT1952-1)
V
IN HYSTERESIS
(LT1952-1)
V
REF
Output Voltage
Line Regulation
Load Regulation
OSCILLATOR
Frequency: f
OSC
Minimum Programmable f
OSC
Maximum Programmable f
OSC
SYNC Input Resistance
SYNC Switching Threshold
SYNC Frequency/f
OSC
f
OSC
Line Reg
V
ROSC
ERROR AMPLIFIER
FB Reference Voltage
FB Input Bias Current
Open Loop Voltage Gain
Unity Gain Bandwidth
COMP Source Current
COMP Sink Current
COMP Current (Disabled)
COMP High Level: V
OH
COMP Active Threshold
COMP Low Level: V
OL
CURRENT SENSE
I
SENSE
Maximum Threshold
I
SENSE
Input Current (Duty Cycle = 0%)
I
SENSE
Input Current (Duty Cycle = 80%)
OC Threshold
OC Input Current
Default Blanking Time
Adjustable Blanking Time
V
BLANK
COMP = 2.5V, FB = 1V
COMP = 2.5V, FB = 1V (Note 4)
COMP = 2.5V, FB = 1V (Note 4)
COMP = 2.5V, FB = 1V
(OC = 100mV)
COMP = 2.5V, FB = 1V, R
BLANK
= 40k (Note 10)
COMP = 2.5V, FB = 1V, R
BLANK
= 120k
197
220
–8
–35
107
–50
180
540
1
243
mV
μA
μA
mV
nA
ns
ns
V
19521fd
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. COMP = open, FB = 1.4V, R
OSC
= 178k, SYNC = 0V, SS_MAXDC = V
REF
, V
REF
= 0.1μF SD_V
SEC
= 2V, BLANK = 121k, DELAY = 121k, I
SENSE
= 0V, OC = 0V, OUT = 1nF V
IN
= 15V, SOUT = open, unless otherwise specified.
,
,
CONDITIONS
l
l
l
l
l
l
l
MIN
TYP
14.25
8.75
MAX
15.75
9.25
6.75
8.13
6.82
UNITS
V
V
V
V
V
V
3.75
5.5
7.75
6.5
0.95
2.425
1.25
2.5
1
1
2.575
10
10
240
120
560
2.2
1.5
0.33
I(V
REF
) = 0μA
I(V
REF
) = 0μA, 10V < V
IN
< 25V
0μA < I(V
REF
) < 2.5mA
R
OSC
= 178k, FB = 1V, SS_MAXDC = 1.84V
R
OSC
= 365k, FB = 1V
R
OSC
= 64.9k, COMP = 2.5V, SD_V
SEC
= 2.64V
FB = 1V
FB = 1V (Note 7)
FB = 1V, R
OSC
= 178k; 10V < V
IN
< 25V,
SS_MAXDC = 1.84V
R
OSC
Pin voltage
10V < V
IN
< 25V, V
OL
+ 0.2V < COMP < V
OH
– 0.2
FB = FB Reference Voltage
V
OL
+ 0.2V < COMP < V
OH
– 0.2
(Note 6)
FB = 1V, COMP = 1.6V
COMP = 1.6V
FB = V
REF
, COMP = 1.6V
FB = 1V, I
(COMP)
= –250μA
FB = 1V, SOUT Duty Cycle > 0 %
I
(COMP)
= 250μA
V
mV
mV
kHz
kHz
kHz
kΩ
V
%/V
V
l
165
80
440
200
100
500
18
1.5
1.25
0.05
1
l
1.201
65
–4
4
18
2.7
0.7
1.226
–75
85
3
–9
10
23
3.2
0.8
0.15
1.250
–200
V
nA
dB
MHz
mA
mA
28
μA
V
V
0.4
V
98
116
–100
3
LT1952/LT1952-1
ELECTRICAL CHARACTERISTICS
PARAMETER
SOUT DRIVER
SOUT Clamp Voltage
SOUT Low Level
SOUT High Level
SOUT Active Pull-Off in Shutdown
SOUT to OUT (Rise) DELAY (t
DELAY
)
V
DELAY
OUT DRIVER
OUT Rise Time
OUT Fall Time
OUT Clamp Voltage
OUT Low Level
OUT High Level
FB = 1V, CL = 1nF (Notes 3, 6)
FB = 1V, CL = 1nF (Notes 3, 6)
I
(GATE)
= 0μA, COMP = 2.5V, FB = 1V
I
(GATE)
= 20mA
I
(GATE)
= 200mA
I
(GATE)
= –20mA, V
IN
= 12V, COMP = 2.5V,
FB = 1V
I
(GATE)
= –200mA, V
IN
= 12V, COMP = 2.5V,
FB = 1V
V
IN
= 5V, SD_V
SEC
= 0V, OUT = 1V
COMP = 2.5V, FB = 1V, R
DELAY
= 10k
(f
OSC
= 200kHz), V
IN
= 10V
SD_V
SEC
= 1.4V, SS_MAXDC = V
REF
COMP = 2.5V, FB = 1V, R
DELAY
= 10k
(f
OSC
= 200kHz), V
IN
= 10V
SD_V
SEC
= 1.32V, SS_MAXDC = 1.84V
SD_V
SEC
= 2.64V, SS_MAXDC = 1.84V
I
(SS_MAXDC)
= 150μA, OC = 1V
Measured on SS_MAXDC
FB = 1V, DC > 0%
SS_MAXDC = 1V, SD_V
SEC
= 1.4V, OC = 1V
11.5
50
30
13
0.45
1.25
9.9
9.75
20
14.5
0.75
1.8
ns
ns
V
V
V
V
V
mA
I
(GATE)
= 0μA, COMP = 2.5V, FB = 1V
I
(GATE)
= 25mA
I
(GATE)
= –25mA, V
IN
= 12V, COMP = 2.5V,
FB = 1V
V
IN
= 5V, SD_V
SEC
= 0V, SOUT = 1V
COMP = 2.5V, FB = 1V (Note 10)
R
DELAY
= 120k
10
1
40
120
0.9
10.5
12
0.5
13.5
0.75
V
V
V
mA
ns
ns
V
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. COMP = open, FB = 1.4V, R
OSC
= 178k, SYNC = 0V, SS_MAXDC = V
REF
, V
REF
= 0.1μF SD_V
SEC
= 2V, BLANK = 121k, DELAY = 121k, I
SENSE
= 0V, OC = 0V, OUT = 1nF V
IN
= 15V, SOUT = open, unless otherwise specified.
,
,
CONDITIONS
MIN
TYP
MAX
UNITS
OUT Active Pull-Off in Shutdown
OUT Max Duty Cycle
83
90
%
OUT Max Duty Cycle Clamp
63.5
25
72
33
0.2
0.45
0.8
800
80.5
41
%
%
V
V
V
μA
SOFT-START
SS_MAXDC Low Level: V
OL
SS_MAXDC Soft-Start Reset Threshold
SS_MAXDC Active Threshold
SS_MAXDC Input Current (Soft-Start Pull-Down: Idis)
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 LT1952EGN/LT1952EGN-1 are guaranteed to meet
performance specifications from 0°C to 125°C junction temperature.
Specifications over the –40°C to 125°C operating junction temperature
range are assured by design, characterization and correlation with
statistical process controls. The LT1952IGN/LT1952IGN-1 are guaranteed
over the full –40°C to 125°C operating junction temperature range.
Note 3:
Rise and Fall times are measured at 10% and 90% levels.
Note 4:
Guaranteed by correlation to static test.
Note 5:
Each IC includes over-temperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when over-temperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 6:
Guaranteed but not tested.
Note 7:
Maximum recommended SYNC frequency = 500kHz.
Note 8:
In applications where the V
IN
pin is supplied via an external RC
network from a SYSTEM V
IN
> 25V, an external zener with clamp voltage
V
IN ON(MAX)
< V
Z
< 25V should be connected from the V
IN
pin to ground.
Note 9:
V
IN
start-up current is measured at V
IN
= V
IN ON
– 0.25V and
scaled by x 1.18 (to correlate to worst case V
IN
start-up current at V
IN ON
).
Note 10:
Timing for R = 40k derived from measurement with R = 240k.
19521fd
4
LT1952/LT1952-1
TYPICAL PERFORMANCE CHARACTERISTICS
FB Voltage vs Temperature
1.25
245
V
IN
SHUTDOWN CURRENT (μA)
125
230
215
200
185
170
155
–50
Switching Frequency
vs Temperature
500
V
IN
Shutdown Current
vs Temperature
V
IN
= 15V
SD_V
SEC
= 0V
450
400
350
300
250
200
150
1.24
FB VOLTAGE (V)
1.23
1.22
1.21
1.20
–50
SWITCHING FREQUENCY (kHz)
–25
50
25
0
75
TEMPERATURE (°C)
100
125
–25
50
25
0
75
TEMPERATURE (°C)
100
100
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1952 G01
1952 G02
1952 G03
V
IN
Start-up Current
vs Temperature
600
550
V
IN
STARTUP CURRENT (μA)
500
V
IN
I
Q
(mA)
450
400
350
300
250
200
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
4.0
LT1952-1
LT1952
5.5
5.0
4.5
SD_V
SEC
= 1.4V
6.5
6.0
V
IN
I
Q
vs Temperature
OC = OPEN
SD_V
SEC
TURN ON THRESHOLD (V)
1.42
SD_V
SEC
Turn ON Threshold
vs Temperature
1.37
1.32
1.27
3.5
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1.22
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1952 G04
1952 G05
1952 G06
SD_V
SEC
Pin Current
vs Temperature
15
PIN CURRENT BEFORE
PART TURN ON
10
V
IN
(V)
18
16
14
V
IN
Turn ON/OFF Voltage
vs Temperature
1.6
1.4
LT1952 V
IN
TURN ON VOLTAGE
1.2
COMP (V)
1.0
0.8
0.6
LT1952 V
IN
TURN OFF VOLTAGE
0.4
0.2
125
COMP Active Threshold
vs Temperature
R
ISENSE
= 0k
SD_V
SEC
PIN CURRENT (μA)
12
10
8
LT1952-1 V
IN
ON
LT1952-1 V
IN
OFF
100
6
–50
5
0mA PIN CURRENT AFTER
PART TURN ON
0
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
–25
50
25
0
75
TEMPERATURE (°C)
0
–50
–25
50
25
75
0
TEMPERATURE (°C)
100
125
1952 G07
1952 G08
1952 G09
19521fd
5
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