FEATURES
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LTC3706
Secondary-Side Synchronous
Forward Controller with
PolyPhase Capability
DESCRIPTION
The LTC
®
3706 is a PolyPhase capable secondary-side
controller for synchronous forward converters. When used
in conjunction with the LTC3705 gate driver and primary-
side controller, the part creates a complete isolated power
supply that combines the power of PolyPhase operation
with the speed of secondary-side control.
The LTC3706 has been designed to simplify the design
of highly efficient, secondary-side forward converters.
Working in concert with the LTC3705, the LTC3706 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 LTC3706 provides remote sensing, accurate power
good and overvoltage monitoring circuits to support preci-
sion, high current applications. A linear regulator controller
with thermal protection is also provided to simplify the
generation of secondary-side bias voltage.
The LTC3706 is available in a 24-lead SSOP package.
Secondary-Side Control for Fast Transient Response
Self-Starting Architecture Eliminates Need for
Separate Bias Regulator
Proprietary Gate Drive Encoding Scheme Reduces
System Complexity
PolyPhase
®
Operation Reduces C
IN
Requirements
Current Mode Control Ensures Current Sharing
PLL Fixed Frequency: 100kHz to 500kHz
±1%
Output Voltage Accuracy
True Remote Sense Differential Amplifier
Power Good Output Voltage Monitor
High Voltage Linear Regulator Controller
Wide Supply Range: 5V to 30V
Available in a Narrow 24-Lead SSOP Package
APPLICATIONS
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Isolated 48V Telecommunication Systems
Internet Servers and Routers
Distributed Power Step-Down Converters
Automotive and Heavy Equipment
L,
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.
Protected by U.S. Patents including 6144194, other patents pending.
TYPICAL APPLICATION
36V-72V to 3.3V/20A Isolated Forward Converter
V
IN+
Si7852DP
MURS120
1μF
100V
3
Si7852DP
Si7336ADP
MURS120
30mΩ
1W
2mΩ
2W
CZT3019
FQT7N10
100k
BAS21 0.22μF
L1: COILCRAFT SER2010-122
T1: PULSE PA0807
T2: PULSE PA0297
TS BG IS
FB/IN
LTC3705
FS/IN
–
GND PGND VSLMT
162k
33nF
–
+
T1
•
•
1.2Ω
Si7336ADP
2
L1
1.2μH
V
OUT+
CMPSH1-4
330μF
6.3V
3
V
IN–
10μF
25V
V
OUT–
2.2μF
16V
102k
1%
365k
1%
NDRV
UVLO
2.2μF
25V
V
CC
SS/FLT
BOOST TG
1μF
T2
I
S–
I
S+
PT
+
FG
SW SG
V
IN
NDRV
V
CC
FS/SYNC
FB
•
•
LTC3706
ITH
680pF
22.6k
1%
3706 TA01
15k
1%
PT
RUN/SS GND PGND PHASE SLP MODE REGSD
33nF
20k
3706fb
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LTC3706
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
SG
FG
PGOOD
MODE
PHASE
FB
ITH
RUN/SS
V
SOUT
1
2
3
4
5
6
7
8
9
24 V
CC
23 PGND
22 PT
+
21 PT
–
20 SW
19 V
IN
18 NDRV
17 REGSD
16 I
S+
15 I
S–
14 SLP
13 FS/SYNC
V
CC
........................................................... –0.3V to 10V
V
IN
........................................................... –0.3V to 33V
SW ............................................................... –5V to 50V
NDRV ........................................................ –0.3V to 13V
ITH, RUN/SS, V
SOUT
, V
S+
, V
S–
, REGSD ....... –0.3V to 7V
All Other Pins ............................................ –0.3V to 10V
Operating Temperature Range (Note 2)
LTC3706EGN ........................................ –40°C to 85°C
LTC3706IGN......................................... –40°C to 85°C
Junction Temperature (Note 3) ............................ 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
V
S+
10
V
S–
11
GND 12
GN PACKAGE
24-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 130°C/W
ORDER INFORMATION
LEAD FREE FINISH
LTC3706EGN#PBF
LTC3706IGN#PBF
LEAD BASED FINISH
LTC3706EGN
LTC3706IGN
TAPE AND REEL
LTC3706EGN#TRPBF
LTC3706IGN#TRPBF
TAPE AND REEL
LTC3706EGN#TR
LTC3706IGN#TR
PART MARKING
LTC3706EGN
LTC3706IGN
PART MARKING
LTC3706EGN
LTC3706IGN
PACKAGE DESCRIPTION
24-Lead Plastic SSOP
24-Lead Plastic SSOP
PACKAGE DESCRIPTION
24-Lead Plastic SSOP
24-Lead Plastic SSOP
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
3706fb
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LTC3706
ELECTRICAL CHARACTERISTICS
SYMBOL
Main Control Loop
V
FB
I
FB
ΔV
FB(LINREG)
ΔV
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
PT
+
, PT
–
R
DOWN
ΔV
FB(OV)
V
CC
Supply
V
CCOP
V
CCREG
I
CC
Operating Voltage Range
Regulated Output Voltage
Supply Current
Operating
Shutdown
UV Lockout
UV Hysteresis
Operating Voltage Range
Supply Current
Normal Mode
Shutdown
UV Lockout
REGSD Shutdown Threshold
REGSD Transconductance
f
OSC
= 200kHz
V
RUN/SS
= GND
V
IN
Rising
V
REGSD
Rising
l
The
l
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 7V, V
IN
= 15V, GND = PGND = 0V, unless otherwise noted.
PARAMETER
Regulated Feedback Voltage
Feedback Input Current
Feedback Voltage Line Regulation
Feedback Voltage Load Regulation
Maximum Current Sense Threshold
Over-Current Shutdown Threshold
Transconductance Amplifier gm
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
PT
+
, PT
–
Driver Pull-Down Resistance
Output Overvoltage Threshold
FG, SG Low
FG, SG High
PT
+
, PT
–
Low
PT
+
, PT
–
High
V
FB
Rising
15
5
6.6
f
OSC
= 200kHz (Note 5)
V
RUN/SS
= GND
V
CC
Rising
l
CONDITIONS
(Note 4) ITH = 1.2V
(Note 4)
V
IN
= 6V to 30V, ITH = 1.2V
Measured in Servo Loop,
ITH = 0.5V to 2V
R
SENSE
Mode, 0V < V
IS–
< 5V
V
IS–
= V
CC
, 0V < V
IS+
< 2V (CT Mode)
R
SENSE
Mode, 0V < V
IS–
< 5V
V
IS–
= V
CC
, 0V < V
IS+
< 2V (CT Mode)
V
RUN/SS
= 2V
V
RUN/SS
Rising
l
l
l
MIN
0.594
TYP
0.600
2
0.001
–0.01
MAX
0.606
100
–0.1
88
1.4
113
1.85
3.10
–6
0.5
2.3
2.3
2.3
2.3
19
10
UNITS
V
nA
%/V
%
mV
V
mV
V
mS
μA
μA
V
ns
Ω
Ω
Ω
Ω
%
V
V
mA
μA
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
7.0
4.2
240
7.4
V
UVLO
V
HYS
V
IN
Supply
V
INOP
I
IN
4.52
4.60
0.4
4.70
V
V
5
900
460
3.90
4.30
0.2
4
5
30
V
μA
μA
V
INUVLO
V
INHYS
V
REGSD
g
m,REGSD
4.51
V
V
V
μS
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LTC3706
ELECTRICAL CHARACTERISTICS
SYMBOL
I
FS
f
LOW
f
HIGH
Δf
(R
FS
)
f
PLL(MAX)
f
PLL(MIN)
PGOOD Output
V
FBH
/0.6
V
FBL1
/0.6
V
FBL2
/0.6
ADA
CMRR DA
R
IN
f
BW
Power Good Upper Threshold
Power Good Lower Threshold
Power Good Lower Threshold
Gain
Common Mode Rejection Ratio
Input Resistance
–3dB Bandwidth
V
FB
Rising
V
FB
Rising
V
FB
Falling
V
S–
= GND, 1V ≤ V
S+
≤ 5V
115
91.5
89.5
0.994
117
93
91
1
75
80
3
119
94.5
92.5
1.006
%
%
%
V/V
dB
kΩ
MHz
PARAMETER
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
V
FS/SYNC
= GND
V
FS/SYNC
= VCC
75kΩ < R
FS/SYNC
< 175kΩ
170
255
–20
500
75
CONDITIONS
MIN
TYP
20
200
300
230
345
20
MAX
UNITS
μA
kHz
kHz
%
kHz
kHz
Oscillator and Phase-Locked Loop
Differential Amplifier (V
SENSE
AMP)
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 LTC3706E is guaranteed to meet the performance specifica-
tions over the 0°C to 85°C operating temperature range. Specifications
over the –40°C to 85°C operating temperature range are assured by
design, characterization and correlation with statistical process controls.
The LTC3706I is guaranteed and tested over the full –40°C to 85°C
operating temperature range.
Note 3:
Junction temperature T
J
(in °C) is calculated from the ambient tem-
perature 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 LTC3706 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.
3706fb
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LTC3706
TYPICAL PERFORMANCE CHARACTERISTICS
V
CC
Supply Current
vs Input Voltage
7
f
OSC
= 200kHz
ALL GATES: C
LOAD
= 0
V
CC
OUTPUT VOLTAGE (V)
7.06
7.04
7.02
V
IS
/V
IS,MAX
(%)
7.00
6.98
6.96
6.94
6.92
6.90
6.88
3
5
6
8
INPUT VOLTAGE (V)
7
9
10
3706 G01
T
A
= 25°C, unless otherwise noted.
Maximum Current Sense
Threshold vs Duty Cycle
100
R
SLP
= 0
80
100kΩ
R
SLP
= 50k
40
V
CC
Regulator Output Voltage
vs Temperature
SUPPLY CURRENT (mA)
6
60
5
4
20
6.86
–50
0
–25
0
25
50
75
TEMPERATURE (°C)
100
125
0
20
40
DUTY CYCLE (%)
3706 G02
3706 G03
60
80
Maximum Current Sense
Threshold vs ITH Voltage
100
IS PIN SOURCE CURRENT (μA)
400
300
80
V
IS
/V
IS,MAX
(%)
200
100
IS Pins Source Current
265
IS PIN SOURCE CURRENT (μA)
260
255
250
245
240
235
IS Pins Source Current
vs Temperature
RS-MODE: (I
IS
+ + I
IS
–)
V
IS
+ = V
IS
– = 0V
60
CT-MODE: I
IS
+
0
–100
–200
–300
RS-MODE: (I
IS
+ + I
IS
–)
40
20
0
0
0.5
1.0
1.5
2.0
ITH VOLTAGE (V)
2.5
3.0
3706 G04
–400
–1
5
2
3
4
+
–
IS , IS COMMON-MODE VOLTAGE (V)
0
1
6
230
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3706 G05
3706 G06
Maximum Current Sense
Threshold vs Temperature
101.5
470
RUN/SS ON Threshold
vs Temperature
5
PERCENT CHANGE IN FREQUENCY (%)
4
3
2
1
0
–1
Oscillator Frequency
vs Temperature
101.0
V
IS
/V
IS,MAX
(%)
V
RUN/SS
(mV)
460
100.5
450
R = 175KΩ
f
OSC
= 500kHz
100.0
440
99.5
430
99.0
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
420
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
–2
–50
R = 75KΩ
f
OSC
= 100kHz
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3706 G07
3706 G08
3706 G14
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