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.
, LTC and LT 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
3725f
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
●
●
●
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
7.4
13.4
7.0
7.7
14
7.2
V
V
V
V
V
Gate Drive Undervoltage
V
GDUV
Gate Drive Undervoltage Threshold
2
U
mA
mA
V
mA
3725f
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
V
UVLOR
V
UVLOF
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
Undervoltage Lockout Threshold Rising
Undervoltage Lockout Threshold Falling
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
Rising
Falling
V
UVLO
= 1V
Primary-Side Control
I
OUT
= 100mA
I
OUT
= –100mA
●
●
●
ELECTRICAL CHARACTERISTICS
MIN
1.220
1.205
4.2
V
UVLOF(MIN)
TYP
1.242
1.226
4.9
MAX
1.280
1.265
5.6
3.75
UNITS
V
V
µA
V
Ω
V
A
ns
ns
Undervoltage Lockout (UVLO)
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:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
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
.
3725f
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
3725f
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
3725f
5
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