LTC3725
Single-Switch Forward
Controller and Gate Driver
FEATURES
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■
DESCRIPTIO
■
■
■
■
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■
■
High Speed Gate Driver for Forward Converter
On-Chip Rectifier and Self-Starting Architecture
Eliminates Need for Separate Gate Drive Bias
Supply
Wide Input Voltage Supply Range: 9V and Up
Linear Regulator Controller for Fast Start-Up
Precision UVLO with Adjustable Hysteresis
Overcurrent Protection
Volt-Second Limit Prevents Transformer Core
Saturation
Voltage Feedforward for Fast Transient Response
Available in 10-Lead MSOP Package
The LTC
®
3725 is a controller for a single-switch forward
converter and includes an on-chip gate driver.
For secondary-side control, combine the LTC3725 with
the LTC3706 PolyPhase
®
secondary-side synchronous
forward controller to create a complete forward converter
using a minimum of discrete parts. A proprietary scheme
is used to multiplex gate drive signals across the isolation
barrier through a tiny pulse transformer. The on-chip
rectifier and the same pulse transformer provide gate drive
bias power.
Alternatively, the LTC3725 can be used as a standalone
voltage mode controller in a primary-side control architec-
ture with optoisolator feedback. Voltage feedforward pro-
vides excellent line regulation and transient response.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
PolyPhase is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Patent Pending
APPLICATIO S
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Isolated 48V Telecommunication Systems
Internet Servers and Routers
Distributed Power Step-Down Converters
Automotive and Heavy Equipment
TYPICAL APPLICATIO
V
IN+
36V TO 72V
36V-72V to 3.3V/30A Isolated Single-Switch Forward Converter
T1
•
•
1µF
100V
×2
Si7450DP
0.030Ω
1W
2.2nF
200V
•
HAT2165
×2
0.0012Ω
1W
HAT2165
×2
V
IN–
FDC2512
100k
GATE I
S
FB/IN
+
0.1µF
T2
5.1k
1µF
PT
+
NDRV
365k
1µF
15k
V
CC
UVLO
FG SW
I
S–
I
S +
SG V
IN
NDRV V
CC
MODE
•
•
LTC3706
PT
–
GND PGND RUN/SS SLP REGSD FS/SYNC PHASE
FB
LTC3725
FS/IN
–
SSFLT
V
SLMT
PGND GND
33nF
162k
33nF
100k
470pF
L1: PULSE PA1294.910
T1: PULSE PA0815
T2: PULSE PA0297
U
L1
0.85µH
V
OUT+
3.3V
30A
100µF
6.3V
×2
B0540W
U
U
+
1.2Ω
1/4W
10µF
220µF
6.3V
V
OUT–
FCX491A
2.2µF
2.74k
I
TH
604Ω
3.3k
47nF
3725 TA01
3725fa
1
LTC3725
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
UVLO
SSFLT
NDRV
FB/IN
+
FS/IN
–
1
2
3
4
5
10
9
8
7
6
I
S
V
SLMT
V
CC
GATE
PGND
Power Supply (V
CC
) ...................................– 0.3V to 15V
External NMOS Drive (NDRV) ....................– 0.3V to 20V
NDRV to V
CC ...........................................................
– 0.3V to 5V
Soft-Start Fault, Feedback,
Frequency Set, Transformer
Inputs (SSFLT, FB/IN
+
, FS/IN
–
) ..................– 0.3V to 15V
All Other Pins (V
SLMT
, I
S
, UVLO) .................– 0.3V to 5V
Peak Output Current <1µs (GATE) ............................. 2A
Operating Ambient Temperature Range .. – 40°C to 85°C
Operating Junction Temperature (Note 2) ............ 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
11
MSE PACKAGE
10-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 45°C/W,
θ
JC
= 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER
LTC3725EMSE
LTC3725IMSE
MSE PART MARKING
LTBSV
LTBSW
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 12V, GND = PGND = 0V, T
A
= 25°C, unless otherwise noted.
SYMBOL
V
CCOP
V
CCLR
I
NDRV
t
r(VCC)
I
NDRVTO
I
CC
I
CCM
V
CCSR
I
CCSR
PARAMETER
Operating Voltage Range
Output Voltage
Current into NDRV Pin
Rise Time of V
CC
Linear Regulator Time Out Current Threshold
Supply Current
Maximum Supply Current
Maximum Supply Voltage
Minimum Current into NDRV/V
CC
Linear Regulator in Operation
Linear Regulator in Operation
Linear Regulator Charging (0.5V to 7.5V)
Primary-Side Operation
V
UVLO
= 1.5V, Linear Regulator in
Operation (Note 3)
V
UVLO
= 1.5V, Trickle Charger in Operation,
V
CC
= 13.2V (Note 3)
Trickle Charger Shunt Regulator
Trickle Charger Shunt Regulator, V
CC
= 15V
(Note 3)
V
CC
Rising
V
CC
Falling
V
CC
Rising (Linear Regulator)
V
CC
Rising (Trickle Charger)
V
CC
Falling
Rising
Falling
●
●
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
7
0.1
TYP
12
8
MAX
15
1
UNITS
V
V
mA
µs
mA
V
CC
Supply, Linear Regulator and Trickle Charger Shunt Regulator
45
0.27
1.4
1.7
14.25
10
2.1
2.5
15
Internal Undervoltage
V
CCUV
Internal Undervoltage Threshold
5.3
4.7
7.2
13.1
6.8
1.220
1.205
7.4
13.4
7.0
1.242
1.226
7.7
14
7.2
1.280
1.265
V
V
V
V
V
V
V
3725fa
Gate Drive Undervoltage
V
GDUV
Gate Drive Undervoltage Threshold
Undervoltage Lockout (UVLO)
V
UVLOR
V
UVLOF
Undervoltage Lockout Threshold Rising
Undervoltage Lockout Threshold Falling
2
U
mA
mA
V
mA
W
U
U
W W
W
LTC3725
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 12V, GND = PGND = 0V, T
A
= 25°C, unless otherwise noted.
SYMBOL
I
HUVLO
V
UVLOOP
R
OS
V
OH
I
PU
t
r
t
f
Rectifier
I
RECT
Oscillator
f
OSC(P)
Oscillator Frequency
Primary-Side Control, R
FS(P)
= 100kΩ
Primary-Side Control, R
FS(P)
= 25kΩ
Primary-Side Control, R
FS(P)
= 300kΩ
Primary-Side Control
25k < R
FSET
< 300k
Secondary-Side Control (During Start-Up),
R
FS(S)
= 100kΩ
Primary-Side Control, V
SSFLT
= 2V
Secondary-Side Control, V
UVLO
= 1.3V,
V
SSFLT
= 2V
Secondary-Side Control, V
UVLO
= 3.75V,
V
SSFLT
= 2V
V
CC
= 8V
Timing Out After Fault, V
SSFLT
= 2V
200
700
70
±15
300
kHz
kHz
kHz
%
kHz
Maximum Rectifier DC Output Current
25
mA
PARAMETER
Hysteresis Current
Voltage Feedforward Operating Range
Output Pull-Down Resistance
High Output Voltage
Peak Pull-Up Current
Output Rise Time
Output Fall Time
10% to 90%, C
OUT
= 4.7nF
10% to 90%, C
OUT
= 4.7nF
CONDITIONS
V
UVLO
= 1V
Primary-Side Control
I
OUT
= 100mA
I
OUT
= –100mA
●
ELECTRICAL CHARACTERISTICS
MIN
4.2
V
UVLOF(MIN)
TYP
4.9
MAX
5.6
3.75
UNITS
µA
V
Ω
V
A
ns
ns
Gate Driver (GATE)
1.9
11
1.7
40
70
∆f
RFS(P)
f
OSC(S)
Oscillator Resistor Set Accuracy
Oscillator Frequency
Soft-Start/Fault (SSFLT)
I
SS(C)
Soft-Start Charge Current
–5.2
–4
–1.6
3.9
6.7
1
300
1.26
0.25
Primary-Side Control I
FB
= 0V
Primary-Side Control V
FB
= 2.5V
Primary-Side Control V
FB
= 0V
3.3
0.5
1.6
µA
µA
µA
V
V
µA
mV
V
mA
V
mA
mA
V
LRTO
V
FLTH
I
SS(D)
V
IS(MAX)
V
VSL(MAX)
I
VSLMT(MAX)
V
OPTO
I
OPTO
Linear Regulator Time Out-Threshold
Fault Output High
Soft-Start Discharge Current
Overcurrent Threshold
Volt-Second Limit Threshold
Maximum Volt-Second Limit Resistor Current
Open Circuit Optoisolator Voltage
Optoisolator Bias Current
Current Sense Input (I
S
)
Volt Second Limit (V
SLMT
)
Optoisolator Bias Current
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:
Operating junction temperature T
J
(in
°C)
is calculated from the
ambient temperature T
A
and the average power dissipation PD (in watts)
by the formula: T
J
= T
A
+
θ
JA
• PD. Refer to the Applications Information
section for details.
Note 3:
I
CC
is the sum of current into NDRV and V
CC
.
Note 4:
The LTC3725EMSE is guaranteed to meet performance
specifications from 0°C to 85°C. Specifications over the –40°C to 85°C
operating temperature range are assured by design, characterization and
correlation with statistical process controls. The LTC3725IMSE is
guaranteed and tested over the – 40°C to 85°C operating temperature
range.
3725fa
3
LTC3725
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs V
CC
2.0
TRICKLE CHARGER
1.240
UVLO THRESHOLD (V)
1.5
CURRENT (mA)
LINEAR REGULATOR
1.0
1.235
I
HUVLO
(µA)
0.5
0
0
5
V
CC
(V)
10
Oscillator Frequency
f
OSC
vs R
FSET
800
700
600
OSCILLATOR FREQUENCY f
OSC(P)
(kHz)
f
OSC
(kHz)
400
300
200
100
PRIMARY-SIDE CONTROL
0
0
100
200
R
FSET
(kΩ)
300
400
3725 G04
200
199
198
PRIMARY-SIDE CONTROL
R
FS(P)
= 100kΩ
I
CC
(mA)
500
SECONDARY-SIDE CONTROL
Shunt Regulator Current vs
Temperature
25
24
23
12
22
14
13
I
CCSR
(mA)
V
FB/IN
+ (V)
V
GDUV
(V)
21
20
19
18
17
16
15
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
7
4
U W
15
3725 G01
3725 G07
UVLO Voltage Threshold vs
Temperature
1.245
5.05
UVLO Hysteresis Current vs
Temperature
V
UVLOR
5.00
4.95
1.230
4.90
1.225
V
UVLOF
1.220
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
4.85
4.80
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3725 G02
3725 G03
Oscillator Frequency vs
Temperature
203
202
201
18
15
12
9
6
3
Shunt Regulator Current I
CC
vs V
CC
197
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
0
14.00
14.25
14.50
V
CC
(V)
14.75
15.00
3725 G06
3725 G05
V
GDUV
vs Temperature
3.5
V
CC
RISING (TRICKLE CHARGER)
3.0
2.5
2.0
1.5
1.0
0.5
V
CC
FALLING (BOTH)
6
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
0
Optoisolator Bias V
FB/IN
+ vs
I
FB/IN
+
11
10
9
8
V
CC
RISING (LINEAR REGULATOR)
0
0.5
1.0
–I
FB/IN
+ (mA)
1.5
2.0
3725 G09
3725 G08
3725fa
LTC3725
TYPICAL PERFOR A CE CHARACTERISTICS
Gate Drive Pull-Down Resistance
vs Temperature
2.50
2.0
GATE DRIVE RESISTANCE R
OS
(Ω)
2.25
1.8
2.00
I
PU
(A)
1.75
1.50
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
Linear Regulator Start-Up
V
IN
5V/DIV
NDRV
V
CC
25µs/DIV
3725 G12
U W
Gate Drive Peak Pull-Up Current
vs Temperature
1.9
1.7
1.6
80
100
1.5
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3725 G10
3725 G11
Gate Drive Encoding
GATE
FB/IN
10V/DIV
10V/DIV
Fault Operation
GATE
SSFLT
FS/IN–
2V/DIV
3705 G13
1µs/DIV
40ms/DIV
3725 G14
3725fa
5
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