FEATURES
s
s
LT1681
Dual Transistor
Synchronous Forward Controller
DESCRIPTIO
The LT
®
1681 controller simplifies the design of high power
synchronous dual transistor forward DC/DC converters. The
part employs fixed frequency current mode control and
supports both isolated and nonisolated topologies. The IC
drives external N-channel power MOSFETs and operates with
input voltages up to 72V.
The LT1681’s operating frequency is programmable and can
be synchronized up to 350kHz. Switch phase is also con-
trolled during synchronized operation to accommodate mul-
tiple converter systems. Internal logic guarantees 50% maxi-
mum duty cycle operation to prevent transformer saturation.
The LT1681 incorporates a soft-start feature that provides a
controlled increase in supplied current during start-up and
after an undervoltage lockout or overvoltage/overcurrent
event.
The part is available in a 20-lead wide SO package to support
high voltage pin-to-pin clearance.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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High Voltage: Operation Up to 72V
Synchronizable Operating Frequency and Output
Switch Phase for Multiple Controller Systems
Fixed Frequency Operation to 350kHz
Adaptive and Adjustable Blanking
Synchronous Rectifier Driver
Local 1% Voltage Reference
Undervoltage Lockout Protection with Hysteresis
Input Overvoltage Protection
Programmable Start Inhibit
Transformer Primary Saturation Protection
Optocoupler Feedback Support
Soft-Start Control
APPLICATIO S
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Isolated Telecommunication Systems
Personal Computers and Peripherals
Lead Acid Battery Backup Systems
Automotive and Heavy Equipment
TYPICAL APPLICATIO
L1
4.7µH
V
IN+
36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)
L2
4.1µH
6
Q1
MURS120T3
C2
22µF
100V
C3
1.5µF
100V
V
IN–
C2:SANYO 100MV22AX
C3, C4: VITRAMON VJ1825Y155MXB
C5: 4X KEMET T510X337KO10AS
L1: COILCRAFT DO1608C-472
L2: PANASONIC ETQP6F4R1LF4
Q1,Q3:100V SILICONIX SUD40N10-25
Q5,Q6: SILICONIX Si4450
T1:COILTRONICS VP5-1200
Q10: ON SEMI MMBT3906LTI
73.2k
1%
270k
0.25W
20k
CMPZ-
5248B
15V
C4
1.5µF
100V
MURS120T3
7
5
8
4
0.025Ω
1/2W
9
T1 Q5
•
+
•
•
Q3
•
ZVN3310F
BAT54
1OV
BIAS
MMBD914LT1
0.1µF
100V
10k
330pF
5V
OUT
3.3Ω
0.047µF
8
1OV
BIAS
BAS21
14
20
17
19
18
16
11
12
15
V
CC
V
BST
BLKSENS TG BSTREF BG SENSE T
MAX
PGND 13
SG
2
LT1681
OVLO
9
1
THERM SYNC SGND SS V
C
V
FB
SHDN 5V
REF
F
SET
6
5
52.3k
1µF
150pF
3
7
4
8
10
100Ω
4.7nF
0.01µF
CMPZ5248B
18V
0.1µF
10k
1nF 24k
56k
1.24k
1%
3.01k
1%
1k
1%
51Ω
+
68µF
20V
Q10
LTC1693-2
6
V
CC1
V
CC2
5
3
IN2
OUT2
7
1
IN1
OUT1
2
4
GND2
GND1
U
U
U
V
OUT
= 5V
I
OUT
= 7A
V
OUT+
1
2
3
10Ω
0.25W
1nF
100V
10
1nF
11
100V
10Ω
0.25W
12
MBR-
0540T1
Q6
+
C5
330µF
10V
4.7Ω
V
OUT–
100Ω
FZT690
4.7µF
16V
CMPZ5242B
12V
2k
0.22µF
50V
1681 TA01
1681f
1
LT1681
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
SHDN 1
OVLO 2
THERM 3
SGND 4
5V
REF
5
FSET 6
SYNC 7
SS 8
V
FB
9
V
C
10
SW PACKAGE
20-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 85°C/ W
20 V
BST
19 TG
18 BSTREF
17 BLKSENS
16 BG
15 PWRGND
14 V
CC
13 SG
12 I
MAX
11 SENSE
Supply Voltages
Power Supply (V
CC
) ............................. – 0.3V to 20V
Topside Supply (V
BST
) ................... V
BSTREF
– 0.3V to
V
BSTREF
+ 20V (V
BST(MAX)
= 90V)
Topside Reference Pin (V
BSTREF
) .......... – 0.6V to 75V
Input Voltages
SHDN Pin .................................. – 0.3V to V
CC
+ 0.3V
All Other Inputs ..................... – 0.3V to 5V
REF
+ 0.3V
Maximum Currents
5V
REF
Pin ........................................ – 85mA to 10mA
FSET Pin ............................................. – 2mA to 5mA
All Other Inputs .................................. – 2mA to 2mA
Operating Ambient Temperature Range
LT1681E (Note 4) .............................. – 40°C to 85°C
LT1681I ............................................. – 40°C to 85°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
LT1681ESW
LT1681ISW
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
CC
= V
BST
= 12V, V
BSTREF
= 0V, V
VC
= 2V, V
FB
= V
REF
= 1.25V, C
TG
= C
BG
= C
SG
= 1000pF.
SYMBOL
V
CC
I
CC
PARAMETER
Operating Supply Voltage Range
DC Active Supply Current
DC Active UVL Supply Current
DC Standby Supply Current
I
BST
V
SHDN
I
SS
V
SS
V
CCUVLO
DC Active Supply Current
DC Standby Supply Current
Shutdown Rising Threshold
Shutdown Threshold Hysteresis
Soft-Start Charge Current
Soft-Start Reset Threshold
Undervoltage Lockout Threshold
Undervoltage Lockout Hysteresis
V
BSTUVLO
Boost Undervoltage Lockout
(V
BST
-BSTREF)
Boost UVLO Hysteresis
Falling Edge
Rising Edge
Falling Edge
Rising Edge
q
q
q
q
q
q
CONDITIONS
q
MIN
9
TYP
12
17
MAX
18
22
25
1200
8.5
1.35
200
–6
8.60
8.95
7.1
7.5
UNITS
V
mA
mA
µA
µA
mA
µA
V
mV
µA
mV
V
V
V
V
V
V
Supply and Protection
(Note 2)
q
V
SHDN
> 1.35V, V
CC
= 8V
V
SHDN
< 0.3V
TG Logic High (Note 2)
V
SHDN
< 0.3V
q
800
0.5
5
0.1
1.15
100
–14
8.0
8.3
0.25
5.7
6.5
0.3
1.25
150
– 10
225
8.40
8.75
0.35
6.4
7.0
0.6
q
q
q
V
SS
= 2V
q
2
U
W
U
U
W W
W
1681f
LT1681
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
CC
= V
BST
= 12V, V
BSTREF
= 0V, V
VC
= 2V, V
FB
= V
REF
= 1.25V, C
TG
= C
BG
= C
SG
= 1000pF.
SYMBOL
V
5VREF
I
5VREFSC
R
5VREF
Error Amp
V
FB
I
FB
A
V
I
VC
V
VC
GBW
A
V
I
SENSE
V
SENSE
t
D
I
BLKSENS
t
MIN
I
MAX
Sense
I
IMAX
V
IMAX
t
P
V
THERM
/
V
OVLO
t
P
f
OSC
I
FSET
V
SYNC
f
SYNC
t
H, L
Input Bias Current
I
MAX
Threshold (Rising Edge)
I
MAX
Threshold Hysteresis
I
MAX
Output Switch Disable Delay
Threshold (Rising Edge)
Threshold Hysteresis
Fault Delay to Output Disable
Oscillator Frequency, Free Run
Frequency Programming Error, Free Run
FSET Input Bias Current
SYNC Logic High Input Threshold
SYNC Logic Low Input Threshold
SYNC Frequency
Maximum SYNC Pulse Width
(Logic High or Logic Low)
f
OSC
= Oscillator Free-Run Frequency
50mV Overdrive
Measured at FSET Pin
f
OSC
≤
500kHz (Note 3)
FSET Charging, V
FSET
= 2V
Positive-Going Edge
Negative-Going Edge
q
q
q
q
PARAMETER
5V Reference Voltage
Short-Circuit Current
Output Impedance
Error Amplifier Reference Voltage
Feedback Input Current
Error Amplifier Voltage Gain
Error Amplifier Current Limit
Zero Current Output Voltage
Maximum Output Voltage
Gain Bandwidth Product
Amplifier DC Gain
Input Bias Current
Current Limit Threshold
Current Sense to Switch Delay
CONDITIONS
0
≤
(I
5VREF
– I
VC
) < 20mA
q
MIN
4.85
4.80
20
TYP
5
45
1
MAX
5.10
5.15
UNITS
V
V
mA
Ω
5V External Reference
Source, I
VC
= 0
0
≤
(I
5VREF
– I
VC
) < 20mA
Measured at Feedback Pin
q
q
1.242
1.225
1.250
–50
72
1.258
1.265
V
V
nA
dB
mA
mA
V
V
MHz
V/V
µA
V
FB
= V
REF
Source
Sink
q
q
10
0.5
25
1
1.4
3.2
(Note 3)
1
12
– 275
Current Sense and Blanking
Measured at SENSE Pin
q
135
130
4.5
150
175
165
170
5.5
mV
mV
ns
V
µA
ns
µA
V
BLKSENS
Blanking Input Threshold
Blanking Input Bias Current
Switch Minimum On Time
V
BLKSENS
= V
BG
, Measured at BG Output
q
5
–2
250
– 250
Measured at I
MAX
Input
Measured at I
MAX
Input
Measured at BG Output
q
320
360
140
130
400
mV
mV
ns
THERM and OVLO Fault Detectors
q
q
1.2
20
1.25
40
650
1.3
60
V
mV
ns
Oscillator and Synchronization Decoder
700
–10
50
0.8
f
OSC
/2
1.4
1.4
2
350
1/f
OSC
5
kHz
%
nA
V
V
kHz
s
1681f
3
LT1681
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
CC
= V
BST
= 12V, V
BSTREF
= 0V, V
VC
= 2V, V
TS
= 0V, V
FB
= V
REF
= 1.25V, C
TG
= C
BG
= C
SG
= 1000pF.
SYMBOL
V
TG
t
TGr/f
V
BG
t
BGr/f
V
SG
t
SGr/f
t
SG-BG
t
TG-BG
PARAMETER
TG On Voltage
TG Off Voltage
TG Rise/Fall Times
BG On Voltage
BG Off Voltage
BG Rise/Fall Times
SG On Voltage
SG Off Voltage
SG Rise/Fall Times
SG to BG Enable Lag Time
TG to BG Enable Lag Time
10% to 90%/90% to 10%
4V On/Off Thresholds
4V On/Off Thresholds
q
CONDITIONS
q
q
MIN
11
TYP
11.5
0.1
35
11.5
0.1
35
11.5
0.1
35
150
100
MAX
UNITS
V
V
ns
V
V
ns
V
V
ns
ns
ns
Output Drivers
0.5
10% to 90%/90% to 10%
q
q
11
0.5
10% to 90%/90% to 10%
q
q
11
0.5
300
80
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
Supply current specification does not include external FET gate
charge currents. Actual supply currents will be higher and vary with
operating frequency, operating voltages and the type of external switch
elements used. See Applications Information.
Note 3:
Guaranteed but not tested.
Note 4:
The LT1681E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the – 40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. For guaranteed performance to
specifications over the –40°C to 85°C range, the LT1681I is available.
TYPICAL PERFOR A CE CHARACTERISTICS
I
CC
Supply Current
vs Temperature
20
V
CC
= 12V
I
CC
SUPPLY CURRENT (mA)
900
I
CC
SUPPLY CURRENT (mA)
I
CC
SUPPLY CURRENT (nA)
19
18
17
16
15
–55 –40
40
80
TEMPERATURE (°C)
0
4
U W
1681 G01
I
CC
Supply Current
vs SHDN Pin Voltage
1100
T
A
= 25°C
I
CC
Supply Current
vs V
CC
Supply Voltage
18
T
A
= 25°C
17
700
16
500
125
0
100
200
300
400
SHDN PIN VOLTAGE (mV)
500
1681 G02
15
9
10
14
16
SUPPLY VOLTAGE (V)
12
18
1681 G03
1681f
LT1681
TYPICAL PERFOR A CE CHARACTERISTICS
I
BST
Boost Supply Current
vs Temperature
5.2
I
BST
BOOST SUPPLY CURRENT (mA)
5.1
40
UVLO I
CC
SUPPLY CURRENT (mA)
I
CC
SUPPLY CURRENT (µA)
5.0
4.9
4.8
–55 –40
40
80
TEMPERATURE (°C)
0
5V
REF
Voltage vs Temperature
5V
REF
SHORT-CIRCUIT CURRENT LIMIT (mA)
5.10
60
5.05
ERROR AMP REFERENCE (V)
5V
REF
VOLTAGE (V)
5.00
4.95
4.90
–55 –40
40
80
TEMPERATURE (°C)
0
V
C
Pin Short-Circuit Current Limit
vs Temperature
V
C
PIN SHORT-CIRCUIT CURRENT LIMIT (mA)
25
12
SOFT-START OUTPUT CURRENT (µA)
11
20
SOFT-START OUTPUT CURRENT (µA)
15
10
–55 –40
40
80
TEMPERATURE (°C)
0
U W
1681 G04
1681 G07
1681 G10
I
CC
Supply Current
vs SHDN Pin Voltage
60
T
A
= 25°C
1
UVLO I
CC
Supply Current
vs Temperature
0.8
20
0.6
125
0
0
0.2
0.4
0.6
0.8
1.0
SHDN PIN CURRENT (V)
1.2
1681 G05
0.5
–55 –40
40
80
TEMPERATURE (°C)
0
125
1681 G06
5V
REF
Short-Circuit Current Limit
vs Temperature
1.260
Error Amp Reference
vs Temperature
1.255
50
1.250
40
1.245
125
30
–55 –40
40
80
TEMPERATURE (°C)
0
125
1681 G08
1.240
–55 –40
40
80
TEMPERATURE (°C)
0
125
1681 G09
Soft-Start Output Current
vs Temperature
60
Soft-Start Output Current
vs Soft-Start Pin Voltage
T
A
= 25°C
V
SS
= 2V
40
10
20
9
125
8
–55 –40
0
40
80
TEMPERATURE (°C)
0
125
1681 G11
0
100
200
300
400
SOFT-START PIN VOLTAGE (mV)
500
1681 G12
1681f
5
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