LTC3705
2-Switch Forward
Controller and Gate Driver
FEATURES
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DESCRIPTIO
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High-Speed Top and Bottom Gate Drivers for
2-Switch Forward Converter
On-Chip Rectifier and Self-Starting Architecture
Eliminate Need for Separate Gate Drive Bias
Supply
Wide Input Voltage Supply Range: 18V to 80V
Tolerant of 100V Input Voltage Transients
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 16-Lead Narrow SSOP Package
The LTC
®
3705 is a controller for a 2-switch forward
converter and includes on-chip bottom and top gate
drivers that do not require external transformers.
For secondary-side control, combine the LTC3705 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 LTC3705 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 –72V to 3.3V/20A Isolated 2-Switch Forward Converter
MURS120
T1
Si7852DP
1µF
100V
x3
1.2Ω
L1
1.2µH
V
OUT+
•
•
Si7336ADP
×2
Si7336ADP
CMPSH1-4
330µF
6.3V
×3
Si7852DP
MURS120
30mΩ
1W
2mΩ
2W
CZT3019
FQT7N10
100k
BAS21 0.22µF
L1: COILCRAFT SER2010-122
T1: PULSE PA0807
T2: PULSE PA0297
1µF
T2
I
S–
I
S+
PT
+
–
V
IN–
FG
SW SG
365k
NDRV
UVLO
V
CC
2.2µF
SS/FLT
BOOST TG TS BG IS
FB/IN
+
LTC3705
FS/IN
GND PGND VSLMT
–
•
•
LTC3706
15k
33nF
162k
PT
RUN/SS GND PGND PHASE SLP MODE REGSD
33nF
U
10µF
V
OUT–
2.2µF
V
IN
NDRV
V
CC
FS/SYNC
FB
ITH
680pF
102k
20k
22.6k
3705 TA01
U
U
3705fb
1
LTC3705
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
GND 1
I
S
2
V
SLMT
3
UVLO 4
SSFLT 5
NDRV 6
FB/IN
+
7
FS/IN
–
8
16 TS
15 TG
14 BOOST
13 NC
12 NC
11 V
CC
10 BG
9
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
Bootstrap Supply (BOOST) ......................– 0.3V to 115V
Top Source (TS) .......................................... -5V to 100V
BOOST to TS .............................................– 0.3V to 15V
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 (TG, BG) ........................... 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
ORDER PART
NUMBER
LTC3705EGN
LTC3705IGN
GN PART
MARKING
3705
3705I
GN PACKAGE
16-LEAD NARROW PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 110°C/W
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
= V
BOOST
= 12V, GND = PGND = V
TS
= 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
3705fb
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
LTC3705
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= V
BOOST
= 12V, GND = PGND = V
TS
= 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 Drivers (TG and BG)
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 LTC3705EGN 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 LTC3705IGN is
guaranteed and tested over the – 40°C to 85°C operating temperature
range.
3705fb
3
LTC3705
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs V
CC
2.0
TRICKLE CHARGER
350
1.5
CURRENT (mA)
LINEAR REGULATOR
1.0
I
BOOST
(µA)
250
V
TS
= 0V
200
150
100
50
UVLO THRESHOLD (V)
0.5
0
0
5
V
CC
(V)
10
UVLO Hysteresis Current vs
Temperature
5.05
800
700
5.00
600
I
HUVLO
(µA)
f
OSC
(kHz)
OSCILLATOR FREQUENCY f
OSC(P)
(kHz)
4.95
4.90
4.85
100
PRIMARY-SIDE CONTROL
4.80
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
0
0
100
200
R
FSET
(kΩ)
300
400
3705 G03
Shunt Regulator Current I
CC
vs V
CC
18
15
12
I
CCSR
(mA)
I
CC
(mA)
V
GDUV
(V)
9
6
3
0
14.00
14.25
14.50
V
CC
(V)
14.75
4
U W
15
3705 G01
3705 G10
3705 G04
(T
A
= 25°C unless otherwise specified)
UVLO Voltage Threshold vs
Temperature
1.245
Boost Current vs Boost – TS
Voltage
400
V
TS
= 80V
1.240
300
V
UVLOR
1.235
1.230
1.225
V
UVLOF
0
10
V
BOOST
– V
TS
(V)
5
15
3705 G02
0
1.220
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3705 G09
Oscillator Frequency
f
OSC
vs R
FSET
203
202
201
200
199
198
Oscillator Frequency vs
Temperature
500
400
300
200
SECONDARY-SIDE CONTROL
PRIMARY-SIDE CONTROL
R
FS(P)
= 100kΩ
197
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3705 G11
Shunt Regulator Current vs
Temperature
25
24
23
22
21
20
19
18
17
16
15.00
15
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
7
11
10
9
8
14
13
12
V
GDUV
vs Temperature
V
CC
RISING (TRICKLE CHARGER)
V
CC
RISING (LINEAR REGULATOR)
V
CC
FALLING (BOTH)
6
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3705 G12
3705 G13
3705fb
LTC3705
TYPICAL PERFOR A CE CHARACTERISTICS
Optoisolator Bias V
FB/IN
+ vs
I
FB/IN
+
3.5
3.0
2.5
V
FB/IN
+ (V)
2.0
1.5
1.0
0.5
0
GATE DRIVE RESISTANCE R
OS
(Ω)
2.00
I
PU
(A)
0
0.5
1.0
–I
FB/IN
+ (mA)
1.5
Linear Regulator Start-Up
V
IN
5V/DIV
NDRV
V
CC
25µs/DIV
3705 G06
U W
3705 G05
(T
A
= 25°C unless otherwise specified)
Gate Drive Pull-Down Resistance
vs Temperature
2.50
2.0
Gate Drive Peak Pull-Up Current
vs Temperature
1.9
2.25
1.8
1.7
1.75
1.6
2.0
1.50
20 40 60
–60
–40 –20 0
TEMPERATURE (°C)
80
100
1.5
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
100
3705 G14
3705 G15
Gate Drive Encoding
TG
FB/IN
10V/DIV
10V/DIV
Fault Operation
BG
SSFLT
FS/IN–
2V/DIV
1µs/DIV
3705 G07
40ms/DIV
3705 G08
3705fb
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