LTC3726
Secondary-Side Synchronous
Forward Controller
FEATURES
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DESCRIPTIO
Secondary-Side Control for Fast Transient Response
Self-Starting Architecture Eliminates Need for
Separate Bias Regulator
Proprietary Gate Drive Encoding Scheme Reduces
System Complexity
Current Mode Control Ensures Current Sharing
PLL Fixed Frequency: 100kHz to 500kHz
±1%
Output Voltage Accuracy
OPTI-LOOP Compensation
Available in a Narrow 16-Lead SSOP Package
The LTC
®
3726 is a secondary-side controller for synchro-
nous forward converters. When used in conjunction with
the LTC3705/LTC3725 gate driver and primary-side con-
trollers, the part creates a complete isolated power supply
that combines the simplicity of OPTI-LOOP
®
compensa-
tion with the speed of secondary-side control.
The LTC3726 has been designed to simplify the design of
highly efficient, secondary-side forward converters. Work-
ing in concert with the LTC3705 or LTC 3725, the LTC3726
forms a robust, self-starting converter that eliminates the
need for the separate bias regulator that is commonly used
in secondary-side control applications. In addition, a pro-
prietary scheme is used to multiplex gate drive signals and
DC bias power across the isolation barrier through a
single, tiny pulse transformer.
The LTC3726 is available in a 16-lead SSOP package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
OPTI-LOOP is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 6144194. Other patents 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+
Si7852DP
36V-72V to 3.3V/20A Isolated Forward Converter
T1
L1
1.2µH
5k
FZT690B
330µF
6.3V
×3
V
OUT+
•
MURS120
Si7852DP
•
CMPSH1-4
1µF
100V
x3
Si7336ADP
MURS120
30mΩ
1W
2mΩ
2W
V
IN–
100k
BAS21 0.22µF
FQT7N10
L1: COILCRAFT SER2010-122
T1: PULSE PA0807
T2: PULSE PA0297
TS BG IS
FB/IN
+
1µF
T2
I
S –
I
S +
PT
+
PT
–
162k
FG
SW SG
365k
1%
NDRV
UVLO
2.2µF
25V
V
CC
SS/FLT
BOOST TG
•
•
LTC3726
LTC3705
FS/IN
–
GND PGND VSLMT
33nF
RUN/SS GND
PGND
SLP
15k
1%
33nF
U
1.2Ω
Si7336ADP
×2
10µF
25V
7.5V
2.2µF
V
OUT–
102k
1%
V
CC
FS/SYNC
FB/PHASE
ITH
680pF
22.6k
1%
3726 TA01
U
U
MODE
20k
3726fb
1
LTC3726
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
SG
FG
MODE
FB/PHASE
ITH
RUN/SS
SLP
GND
1
2
3
4
5
6
7
8
16 V
CC
15 PT
+
14 PT
–
13 PGND
12 SW
11 IS
+
10 IS
–
9
FS/SYNC
V
CC
........................................................... – 0.3V to 10V
SW ............................................................... –5V to 50V
ITH, RUN/SS ............................................... – 0.3V to 7V
All Other Pins ............................................ – 0.3V to 10V
Operating Ambient Temperature Range (Note 2)
LTC3726EGN ...................................... – 40°C to 85°C
LTC3726IGN ....................................... – 40°C to 85°C
Operating Junction Temperature (Note 3) ........... 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
= 130°C/W
ORDER PART NUMBER
LTC3726EGN
LTC3726IGN
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.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
FB
∆V
FB(LINREG)
∆V
FB(LOADREG)
V
ISMAX
V
ISOC
g
m
I
RUN/SS(C)
I
RUN/SS(D)
V
RUN/SS
t
ON,MIN
FG, SG R
UP
FG, SG R
DOWN
PT
+
, PT
–
R
UP
∆V
FB(OV)
PARAMETER
Regulated Feedback Voltage
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
Maximum Current Sense Threshold
Over-Current Shutdown Threshold
Transconductance Amplifier g
m
Soft-Start Charge Current
Soft-Start Discharge Current
RUN/SS Pin ON Threshold
Minimum ON Time
FG, SG Driver Pull-Up On Resistance
FG, SG Driver Pull-Down On Resistance
PT
+
, PT
–
Driver Pull-Up Resistance
Output Overvoltage Threshold
Main Control Loop
The
●
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 7V, GND = PGND = 0V, unless otherwise noted.
CONDITIONS
(Note 4) ITH = 1.2V
V
CC
= 5V to 10V, ITH = 1.2V
Measured in Servo Loop,
ITH = 0.5V to 2V
R
SENSE
Mode, V
IS
= 0V
CT Mode, V
IS
= 0V
R
SENSE
Mode, V
IS
= 0V
CT Mode, V
IS
= 0V
V
RUN/SS
= 2V
V
RUN/SS
Rising
FG, SG Low
FG, SG High
PT
+
, PT
–
Low
PT
+
, PT
–
High
V
FB
Rising
15
●
●
●
MIN
0.594
TYP
0.600
0.001
–0.01
MAX
0.606
–0.1
88
1.4
113
1.85
3.10
–6
0.5
2.3
2.3
2.3
2.3
19
UNITS
V
%/V
%
mV
V
mV
V
mS
µA
µA
V
ns
Ω
Ω
Ω
Ω
%
3726fb
68
1.15
87
1.45
2.40
–4
0.4
78
1.28
100
1.65
2.75
–5
3
0.45
200
1.5
1.5
1.5
1.5
17
PR
+
, PT
–
R
DOWN
PT
+
, PT
–
Driver Pull-Down Resistance
2
U
W
U
U
W W
W
LTC3726
ELECTRICAL CHARACTERISTICS
SYMBOL
V
CC
Supply
V
CCOP
I
CC
Operating Voltage Range
Supply Current
Operating
Shutdown
UV Lockout
UV Hysteresis
FS/SYNC Pin Sourcing Current
Oscillator Low Frequency Set Point
Oscillator High Frequency Set Point
Oscillator Resistor Set Accuracy
Maximum PLL Sync Frequency
Minimum PLL Sync Frequency
PARAMETER
The
●
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 7V, GND = PGND = 0V, unless otherwise noted.
CONDITIONS
MIN
5
f
OSC
= 200kHz (Note 5)
V
RUN/SS
= GND
V
CC
Rising
●
TYP
MAX
10
UNITS
V
mA
µA
4.2
700
4.52
4.60
0.4
20
4.70
V
UVLO
V
HYS
I
FS
f
LOW
f
HIGH
∆f
(R
FS
)
f
PLL(MAX)
f
PLL(MIN)
V
V
µA
Oscillator and Phase-Locked Loop
V
FS/SYNC
= GND
V
FS/SYNC
= V
CC
75kΩ < R
FS/SYNC
< 175kΩ
170
255
–20
500
75
200
300
230
345
20
kHz
kHz
%
kHz
kHz
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 LTC3726E is guaranteed to meet the performance
specifications from 0°C to 85°C junction temperature. Specifications over
the –40°C to 85°C operating temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3726I is guaranteed and tested over the full – 40°C to 85°C operating
temperature range.
Note 3:
Operating junction temperature T
J
(in
°C)
is calculated from the
ambient temperature T
A
and the average power dissipation P
D
(in Watts)
by the formula:
T
J
= T
A
+
θ
JA
• P
D
Refer to the Applications Information section for details.
Note 4:
The LTC3726 is tested in a feedback loop that servos V
FB
to a
voltage near the internal 0.6V reference voltage to obtain the specified ITH
voltage (V
ITH
= 1.2V).
Note 5:
Operating supply current is measured in test mode. Dynamic
supply current is higher due to the internal gate charge being delivered at
the switching frequency. See Typical Performance Characteristics.
3726fb
3
LTC3726
TYPICAL PERFOR A CE CHARACTERISTICS
V
CC
Supply Current vs
Input Voltage
7
f
OSC
= 200kHz
ALL GATES: C
LOAD
= 0
100
R
SLP
= 0
80
V
IS
/V
IS,MAX
(%)
80
SUPPLY CURRENT (mA)
6
60
R
SLP
= 50k
40
100kΩ
V
IS
/V
IS,MAX
(%)
80
5
4
3
5
6
8
INPUT VOLTAGE (V)
7
9
10
3726 G01
IS Pins Source Current
400
300
265
260
255
IS PIN SOURCE CURRENT (µA)
IS PIN SOURCE CURRENT (µA)
200
100
CT-MODE: I
IS
+
0
–100
–200
–300
–400
–1
5
2
3
4
+
, IS
–
COMMON-MODE VOLTAGE (V)
IS
0
1
6
RS-MODE: (I
IS
+ + I
IS
–)
V
IS
/V
IS,MAX
(%)
RUN/SS ON Threshold
vs Temperature
470
5
PERCENT CHANGE IN FREQUENCY (%)
460
V
RUN/SS
(mV)
FREQUENCY (kHz)
450
440
430
420
–50
–25
0
25
50
75
TEMPERATURE (°C)
4
U W
100
T
A
= 25°C, unless otherwise noted.
Maximum Current Sense
Threshold vs ITH Voltage
100
Maximum Current Sense
Threshold vs Duty Cycle
60
40
20
20
0
0
20
40
DUTY CYCLE (%)
3726 G03
0
60
0
0.5
1.0
1.5
2.0
ITH VOLTAGE (V)
2.5
3.0
3726 G04
IS Pins Source Current
vs Temperature
RS-MODE: (I
IS
+ + I
IS
–)
V
IS
+ = V
IS
– = 0V
101.5
Maximum Current Sense
Threshold vs Temperature
101.0
250
245
240
100.5
100.0
99.5
235
230
–50
99.0
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3726 G05
3726 G06
3726 G07
Oscillator Frequency
vs Temperature
600
500
400
300
200
100
Oscillator Frequency vs R
FS
4
3
2
1
0
–1
–2
–50
R = 175KΩ
f
OSC
= 500kHz
R = 75KΩ
f
OSC
= 100kHz
–25
0
25
50
75
TEMPERATURE (°C)
100
125
0
50
75
100
125
150
R
FS
(kΩ)
175
200
125
3726 G08
3726 G14
3726 G09
3726fb
LTC3726
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage vs Temperature
601.0
601.0
UVLO THRESHOLD VOLTAGE (V)
600.5
V
FB
(mV)
V
FB
(mV)
600.0
599.5
599.0
–50
–25
0
25
50
75
TEMPERATURE (°C)
Gate Driver On-Resistance vs V
CC
1.8
1.7
1.6
R
DS,ON
(Ω)
1.5
PULL-DOWN
1.4
PULL-UP
1.3
1.2
5
6
8
V
CC
VOLTAGE (V)
7
9
10
3726 G12
R
DS,ON
(Ω)
Efficiency (Figure 5)
95
V
IN
= 36V
EFFICIENCY (%)
90
V
IN
= 72V
85
80
0
5
10
15
LOAD CURRENT (A)
20
25
3726 G17
U W
100
FB Voltage Line Regulation
4.65
4.60
4.55
4.50
4.45
4.40
4.35
4.30
599.0
125
0
5
15
20
25
V
IN
SUPPLY VOLTAGE (V)
10
30
3726 G11
Undervoltage Lockout
vs Temperature
600.5
V
CC
RISING
600.0
599.5
4.25
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3726 G15
3726 G16
Gate Driver On-Resistance
vs Temperature
2.50
2.25
2.00
1.75
1.50
1.25
1.00
–50
V
CC
= 7V
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3726 G13
Load Step (Figure 5)
V
OUT
100mV/DIV
I
OUT
10A/DIV
20µs/DIV
V
IN
= 48V
V
OUT
= 3.3V
LOAD STEP = 0A TO 20A
3726 G18
3726fb
5
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