LT1952
Single Switch Synchronous
Forward Controller
FEATURES
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DESCRIPTIO
Synchronous Rectifier Control for High Efficiency
Programmable Volt-Second Clamp
Output Power Levels from 25W to 500W
Low Current Start-Up
True PWM Soft-Start
Low Stress Short Circuit Protection
Precision 100mV 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
APPLICATIO S
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Telecommunications Power Supplies
Industrial and Distributed Power
Isolated and Non-Isolated DC/DC Converters
The LT
®
1952 is a current mode PWM controller optimized
to control the forward converter topology, using one
primary MOSFET. The LT1952 provides 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, trans-
former and rectifier utilization. The LT1952 includes
soft-start for controlled exit from shutdown, overcurrent
conditions and undervoltage lockout. A precision 100mV
current limit threshold, independent of duty cycle, com-
bines with soft-start to provide hiccup short circuit protec-
tion. Micropower start-up allows the LT1952 to be effi-
ciently started from high input voltages. Programmable
slope compensation and leading edge blanking allow
optimization of loop bandwidth with a wide range of
inductors and MOSFETs. The LT1952 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 is available in a small 16-pin SSOP
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
SUPPLY FROM BIAS
WINDING OF T1
36V to 72V Input, 12V at 20A Semi-Regulated Bus Converter
40k
V
IN
T1
PA0905
L1
PA1494.242
V
OUT
12V
20A
47µF
16V
X5R
×2
V
OUT
(V)
16V
V
REF
52.3k
V
IN
LT1952
340k
100k
13k
SD_V
SEC
FB
COMP
SS_MAXDC
OUT
OC
I
SENSE
V
IN
10µF
Si7450
Si7370
×2
PH4840
×2
0.005Ω
LTC3900
T2
FG
SYNC
220pF
560Ω
CG
SYNC
GND
0.1µF
0.1µF
40k
SOUT
R
OSC
PGND BLANK DELAY
40k
178k
1952 TA01
U
12V Bus Converter
V
OUT
vs V
IN
16
14
12
10
8
36
42
48
54
V
IN
(V)
60
66
72
1952 TA01b
U
U
1952f
1
LT1952
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
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, COMP, BLANK, DELAY ......................... –0.3V to 6V
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
ORDER PART
NUMBER
LT1952EGN
LT1952IGN
GN PART
MARKING
1952E
1952I
GN PACKAGE
16-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 110°C/W,
θ
JC
= 40°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
PWM CONTROLLER
Operational Input Voltage
V
IN
Quiescent Current
V
IN
Startup Current
V
IN
Shutdown Current
SD_V
SEC
Threshold
SD_V
SEC (ON)
Current
SD_V
SEC (OFF)
Current
V
IN ON
V
IN OFF
V
IN HYSTERESIS
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
The
●
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 = 40k, DELAY = 40k, I
SENSE
= 0V, OC = 0V, OUT = 1nF, V
IN
= 15V, SOUT = open, unless otherwise specified.
CONDITIONS
I
(VREF)
= 0µA
I
(VREF)
= 0µA, FB = 0V, I
SENSE
= OC = Open
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
●
●
●
●
●
●
MIN
V
IN OFF
TYP
MAX
25
UNITS
V
mA
µA
µA
V
µA
µA
V
V
V
V
mV
mV
kHz
kHz
kHz
kΩ
V
%/V
V
1952f
5.2
460
240
1.261
9.5
12.75
8.0
3.75
2.425
1.32
0
11.2
14.25
8.75
5.5
2.5
1
1
●
6.5
700
350
1.379
12.9
15.75
9.25
6.75
2.575
10
10
240
120
560
2.2
1.5
0.33
I
(VREF)
= 0µA
I
(VREF)
= 0µA, 10V < V
IN
< 25V
0µA < I
(VREF)
< 2.5mA
R
OSC
= 178k, FB = 1V
R
OSC
= 365k
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
●
165
80
440
200
100
500
18
1.5
1.25
0.05
1
2
U
W
U
U
W W
W
LT1952
ELECTRICAL CHARACTERISTICS
PARAMETER
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
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
The
●
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 = 40k, DELAY = 40k, I
SENSE
= 0V, OC = 0V, OUT = 1nF, V
IN
= 15V, SOUT = open, unless otherwise
specified.
CONDITIONS
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
COMP = 2.5V, FB = 1V
COMP = 2.5V, FB = 1V (Note 4)
COMP = 2.5V, FB = 1V (Note 4)
●
●
MIN
1.201
65
–4
4
18
2.7
0.7
TYP
1.226
–75
85
3
–9
10
23
3.2
1.0
0.15
MAX
1.250
–200
UNITS
V
nA
dB
MHz
mA
mA
28
µA
V
V
0.4
243
V
mV
µA
µA
197
220
–8
–35
98
107
–50
180
540
1
116
–100
mV
nA
ns
ns
V
(OC = 100mV)
COMP = 2.5V, FB = 1V (Note 10)
COMP = 2.5V, FB = 1V, R
BLANK
= 120k
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
10.5
10
1
12
0.5
13.5
0.75
V
V
V
mA
40
120
0.9
ns
ns
V
ns
ns
14.5
0.75
1.8
V
V
V
V
V
mA
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
9.9
9.75
20
11.5
50
30
13
0.45
1.25
OUT Active Pull-Off in Shutdown
1952f
3
LT1952
ELECTRICAL CHARACTERISTICS
PARAMETER
OUT Max Duty Cycle
The
●
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 = 40k, DELAY = 40k, I
SENSE
= 0V, OC = 0V, OUT = 1nF, V
IN
= 15V, SOUT = open, unless otherwise
specified.
CONDITIONS
COMP = 2.5V, FB = 1V, R
DELAY
= 10k
(f
OSC
= 200kHz)
SD_V
SEC
= 1.4V, SS_MAXDC = V
REF
COMP = 2.5V, FB = 1V, R
DELAY
= 10k
(f
OSC
= 200kHz)
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
MIN
TYP
MAX
UNITS
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 Pulldown: Idis)
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1952EGN is 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 is 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:
This 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.
1952f
4
LT1952
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage vs Temperature
1.25
245
V
IN
SHUTDOWN CURRENT (µA)
230
215
200
185
170
155
–50
1.24
FB VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
1.23
1.22
1.21
1.20
–50
–25
50
25
0
75
TEMPERATURE (°C)
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
5.5
5.0
4.5
SD_V
SEC
= 1.4V
6.5
6.0
SD_V
SEC
TURN ON THRESHOLD (V)
SD_V
SEC
Pin Current vs
Temperature
15
PIN CURRENT BEFORE
PART TURN ON
10
V
IN
(V)
SD_V
SEC
PIN CURRENT (µA)
COMP (V)
5
0µA PIN CURRENT AFTER
PART TURN ON
0
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
U W
100
1952 G01
1952 G04
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
125
–25
50
25
0
75
TEMPERATURE (°C)
100
125
100
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1952 G02
1952 G03
V
IN
I
Q
vs Temperature
OC = OPEN
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 G05
1952 G06
V
IN
Turn ON/OFF Voltage vs
Temperature
18
16
14
12
10
V
IN
TURN OFF VOLTAGE
8
6
–50
1.6
1.4
V
IN
TURN ON VOLTAGE
1.2
1.0
0.8
0.6
0.4
0.2
–25
50
25
0
75
TEMPERATURE (°C)
100
125
COMP Active Threshold vs
Temperature
R
ISENSE
= 0k
0
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1952 G07
1952 G08
1952 G09
1952f
5
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