LT3710
1
3710f
The LT
3710 is a high efficiency step-down switching
regulator intended for auxiliary outputs in single second-
ary winding, multiple output power supplies.
The LT3710 drives dual synchronous N-channel MOSFETs
and achieves high efficiency. With leading edge modula-
tion, it operates well with either primary side peak current
or voltage mode control. It is synchronized to the falling
edge of the transformer secondary winding and can be
used in both single-ended and double-ended isolated
power converter topologies. A high speed operational
amplifier is incorporated to achieve optimum compensa-
tion and fast transient response. A user selectable discon-
tinuous conduction mode improves light load efficiency.
The LT3710 is available in a thermally enhanced TSSOP-16
exposed pad power package.
s
48V Isolated DC/DC Converters
s
Multiple Output Supplies
s
Offline Converters
, LTC and LT are registered trademarks of Linear Technology Corporation.
s
Generates a Regulated Auxiliary Output in Isolated
DC/DC Converters
s
0.8V
1.5% Accurate Voltage Reference
s
Dual N-Channel MOSFET Synchronous Drivers
s
High Switching Frequency: Up to 500kHz
s
Programmable Current Limit Protection
s
Programmable Soft-Start
s
Automatic Frequency Synchronization
s
Small 16-Pin Thermally Enhanced TSSOP Package
Figure 1. Simplified Single Secondary Winding 3.3V and 1.8V Output Isolated DC/DC Converter
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
Secondary Side
Synchronous Post Regulator
V
COMP
V
DD
LTC1698
GND
V
FB
CG
FG
I
SNS
SYNC
OPTODRV
V
CC
BIAS
L1
V
OUT1
3.3V
AT 10A
BG
LT3781
SG
ISOLATION
BOUNDARY
C
OUT2
: POSCAP, 680
F/4V
L2: SUMIDA CEP125-IR8MC-H
Q1, Q2: SILICONIX Si7440DP
PLEASE REFER TO FIGURE 3
IN THE APPLICATIONS SECTION
FOR THE COMPLETE SCHEMATIC
3710 F01
V
IN
36V
TO 72V
V
REF
V
C
V
FB
TG
GBIAS
4.7
F
CMDSH-3
Q1
Q2
B340A
3.01k
2.32k
L2
1.8
H
0.006
V
CC
BOOST
CL
CL
+
LT3710
SYNC
TG
CSET
ILCOMP
SS
+
0.1
F
3.3k
33nF
10pF
C
S
680pF
10k
10k
180pF
0.01
F
BGS
PGND
SW
BG
VA
OUT
V
FB
4700pF
V
OUT2
1.8V
AT 10A
C
OUT2
+
220
LT3710
2
3710f
V
CC
Supply Voltage .................................................. 26V
BOOST Pin Voltage With Respect to SW pin ........... 10V
BOOST Pin Voltage With Respect to GND pin .......... 35V
SYNC Pin Voltage .................................................... 30V
Operating Junction Temperature Range
(Notes 2, 3) ...................................... 40
C to 125
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
Note: If higher than 30V on SYNC pin is needed, add a 10k resistor in series with the pin.
ORDER PART
NUMBER
T
JMAX
= 125
C,
JA
= 38
C/W
EXPOSED PAD IS SGND (PIN 17) MUST BE
CONNECTED TO PGND AND SOLDERED TO PCB
Consult LTC Marketing for parts specified with wider operating temperature ranges.
LT3710EFE
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
CC
= 11V, operating maximum V
CC
= 24V, no load on any outputs
unless otherwise noted.
1
2
3
4
5
6
7
8
TOP VIEW
FE PACKAGE
16-LEAD PLASTIC TSSOP
16
15
14
13
12
11
10
9
BOOST
TGATE
SW
CSET
SYNC
ILCOMP
SS
V
FB
GBIAS
BGATE
PGND
V
CC
CL
CL
+
VA
OUT
BGS
17
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Overall
Supply Voltage (V
CC
)
q
8
24
V
Supply Current (I
VCC
)
VA
OUT
1.2V (Switching Off)
7
12
mA
Boost Pin Current
V
BOOST
= V
SW
+ 8V, 0V
V
SW
24V
TGATE High
2
3
mA
TGATE Low
2
3
mA
Voltage Amplifier VA
Reference Voltage (V
REF
)
0.788
0.8
0.812
V
q
0.780
0.820
V
FB Pin Input Current
V
FB
= V
REF
0.2
0.5
A
VA
OUT
High
4.5
V
VA
OUT
Low
0.8
V
VA
OUT
Source Current
q
100
300
A
Open-Loop Gain
100
dB
Gain Bandwidth Product
10
MHz
Soft-Start Current
5
12
18
A
Current Limit Amplifier CA1
Current Limit Threshold at (V
CL
+
V
CL
)
Common Mode Voltage from 0V to V
CC
2.5V
q
50
70
85
mV
BGATE Off Threshold at (V
CL
+
V
CL
), BGS Pin Float
Common Mode Voltage from 0V to V
CC
2.5V
0
8
15
mV
Switching Off Threshold at ILCOMP
V
ILCOMP
0.15
V
Input Current (CL
+
, CL
)
V
CL
+
= V
CL
100
A
FE PART
MARKING
3710EFE
LT3710
3
3710f
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT3710E is guaranteed to meet performance specifications
from 0
C to 125
C. Specifications over the 40
C to 125
C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
CC
= 11V, operating maximum V
CC
= 24V, no load on any outputs
unless otherwise noted.
Oscillator
Switching Frequency
C
S
= 500pF (No SYNC)
q
170
200
240
kHz
C
S
= 333pF (No SYNC)
q
240
280
340
kHz
Synchronization Frequency Range
C
S
= 500pF
q
245
400
kHz
C
S
= 333pF
q
345
500
kHz
CSET Ramp Valley Voltage
C
S
= 1000pF (No SYNC)
0.90
1.15
1.4
V
CSET Peak-to-Peak Voltage
C
S
= 1000pF (No SYNC)
2.4
V
Synchronization Pulse Threshold on SYNC Pin
Falling Edge V
SYNC
2.5
V
Maximum Duty Cycle
V
FB
= V
REF
5mV, C
S
> 500pF
q
85
90
%
Gate Drivers (TGATE, BGATE)
V
GBIAS
I
GBIAS
< 25mA
q
7.5
8.0
8.5
V
V
TGATE
High (V
TGATE
V
SW
)
I
TGATE
< 50mA, V
BOOST
= V
GBIAS
0.5V
q
5
6
7
V
V
BGATE
High
I
BGATE
< 50mA
q
5
6
7.5
V
V
TGATE
Low (V
TGATE
V
SW
)
I
TGATE
< 50mA
q
0.5
V
V
BGATE
Low
I
BGATE
< 50mA
q
0.5
V
Peak Gate Drive Current
10nF Load
1
A
Gate Drive Rise and Fall Time
1nF Load
25
ns
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
I
GBIAS
(mA)
0
V
GBIAS
(V)
8.1
8.0
7.9
7.8
7.7
3710 G01
10
20
26
40
C
25
C
125
C
V
CC
(V)
8
10
12
14
16
18
20
22
24
I
CC
(mA)
13
12
11
10
9
8
7
6
5
3710 G02
T
A
= 25
C
FREQUENCY (Hz)
10
GAIN (dB)
120
80
40
0
20
PHASE (DEG)
0
50
100
150
180
3710 G03
PHASE
0dB, 10MHz
(111
)
GAIN
100
1k
10k
100k
1M
10M 100M
T
A
= 25
C
V
GBIAS
vs I
GBIAS
over Junction
Temperature
I
CC
vs V
CC
(Switching Off)
Voltage Amplifier VA Gain and
Phase
Note 3: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125
C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
LT3710
4
3710f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
V
CC
(V)
10
15
20
25
V
REF
(mV)
3
2
1
0
1
FREQ (kHz)
1
0
1
3710 G04
V
REF
CSET = 500pF
T
A
= 25
C
FREQ
JUNCTION TEMPERATURE (
C)
20
40
25
0
75
50
125
V
REF
(V)
0.801
0.800
0.799
0.798
3710 G05
CSET = 500pF
CSET (pF)
400
200
600
800
1000
FREQUENCY (kHz)
500
400
300
200
100
MAXIMUM DUTY CYCLE
1.00
0.95
0.90
0.85
0.80
0.75
0.70
3710 G07
MAXIMUM DUTY CYCLE
CSET
T
A
= 25
C
TIME
0
I
GBIAS
(mA)
300
250
200
150
100
50
0
V
GBIAS
(V)
12
10
8
6
4
2
0
500
s
3710 G08
1ms
V
GBIAS
C
GBIAS
= 2.2
F
I
GBIAS
V
CL
+
V
CL
(mV)
60
50
70
80
90
VA
OUT
(V)
8
7
6
5
4
3
2
1
0
3710 G09
CSET PEAK
V
CC
= 11V
V
CLN
= 5V
T
A
= 25
C
CSET VALLEY
V
REF
vs V
CC
,
FREQ vs V
CC
V
REF
vs Temperature
CSET vs Switching Frequency
G
BIAS
vs I
GBIAS
(Charging 2.2
F)
Current Limit Amplifier CA1 Gain
at V
CC
= 11V, V
CL
= 5V
JUNCTION TEMPERATURE (
C)
20
40
25
0
75
50
125
SWITCHING FREQUENCY (kHz)
195
200
205
210
215
3710 G06
CSET = 500pF
Frequency vs Temperature
LT3710
5
3710f
BOOST (Pin 1): Topside (Boosted) Driver Supply. This pin
is used to bootstrap and supply the topside power switch
gate drive circuitry. In normal operation V
BOOST
is powered
from the internally generated 8V GBIAS, V
BOOST
= V
SW
+
8.2V when TGATE is on.
TGATE (Pin 2): Topside (Boosted) N-Channel MOSFET
Driver. When TGATE is on, the voltage is equal to V
SW
+ 6V.
SW (Pin 3): Switch Node Connection to Inductor.
CSET (Pin 4): Oscillator Timing Pin. The capacitor on this
pin sets the PWM switching frequency.
SYNC (Pin 5): Synchronization Input. This pin should be
connected to the secondary side output of the power
transformer with a series resistor. A filtering capacitor of
10pF is recommended.
ILCOMP (Pin 6): Current Limit Amplifier Compensation
Node. At current limit, CA1 pulls down on this pin to
regulate the output current.
SS (Pin 7): Soft-Start. A capacitor on this pin sets the
output ramp up rate. The typical time for SS to reach the
programmed level is (C 0.8V)/10
A.
V
FB
(Pin 8): Voltage Amplifier Inverting Input. A resistor
divider to this pin sets the output voltage. Nominal voltage
at this pin is 0.8V.
PI FU CTIO S
U
U
U
BGS (Pin 9): Bottom Gate Switching Control. CA2 moni-
tors the inductor current and prohibits BGATE from turn-
ing on when the inductor current is low (below 8mV across
the current sense resistor RS1) to allow discontinous
mode operation. Grounding this pin disables comparator
CA2.
VA
OUT
(Pin 10): Voltage Amplifier Output.
CL
+
(Pin 11): Current Limit Amplifier Positive Input. The
threshold is set at 70mV.
CL
(Pin 12): Current Limit Amplifier Negative Input.
When used, CL
is connected to the output capacitor side
of the current + sense resistor and CL
+
is connected to the
inductor side of the current sense resistor.
V
CC
(Pin 13): Supply of the IC. For proper bypassing, a low
ESR capacitor is required.
PGND (Pin 14): Ground of the Bottom Side N-Channel
MOSFET Driver.
BGATE (Pin 15): Bottom Side N-Channel MOSFET Driver.
GBIAS (Pin 16): 8V Regulator Output for Boostrapping
V
BOOST
.
A bypass capacitor of at least 2
F is needed.
Exposed Pad (Pin 17): Connect to PGND (Pin 14).
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