LTC1698
Isolated Secondary
Synchronous Rectifier Controller
FEATURES
DESCRIPTIO
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High Efficiency Over Wide Load Current Range The LTC 1698 is a precision secondary-side forward
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0.8% Output Voltage Accuracy converter controller that synchronously drives external
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Dual N-Channel MOSFET Synchronous Drivers N-channel MOSFETs. It is designed for use with the
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Pulse Transformer Synchronization LT 3781 primary-side synchronous forward converter
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Optocoupler Feedback Driver controller to create a completely isolated power supply.
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Programmable Current Limit Protection The LT3781 synchronizes the LTC1698 through a small
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5% Margin Output Voltage Adjustment pulse transformer and the LTC1698 drives a feedback
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Adjustable Overvoltage Fault Protection optocoupler to close the feedback loop. Output accuracy
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Power Good Flag of 0.8% and high efficiency over a wide range of load
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Auxiliary 3.3V Logic Supply currents are obtained.
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Available in 16-Lead SSOP and SO Packages
The LTC1698 provides accurate secondary-side current
U
limit using an external current sense resistor. The input
APPLICATIO S
voltage at the MARGIN pin provides 5% output voltage
adjustment. A power good flag and overvoltage input are
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48V Input Isolated DC/DC Converters
provided to ensure proper power supply conditions. An
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Isolated Telecommunication Power Systems
auxiliary 3.3V logic supply is included that supplies up to
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Distributed Power Step-Down Converters
10mA of output current.
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Industrial Control Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Automotive and Heavy Equipment
TYPICAL APPLICATIO
L1
V
IN
V
OUT
36V to 72V
+
R2
C
OUT
Q1 R1
V BIAS
DD
+
D1
110
T1
R R5
V PWRGD C
DD
28
Q4
CG V
FB
C
C
Q3 16
Q2
D2 FG LTC1698
C
FB
6
12
R V
PRISEN COMP
I
SNS
R
SECSEN C
CILM
R
CILM
11
13
I I
SNSGND COMP
C C
SG SYNC
15 9
TG BG SG SYNC OVPIN
R4
T2
R
SYNC
7
MARGIN V
MARGIN
LT3781
R
K V
AUX
5 14
OPTODRV V 3.3V
AUX
C 10mA
K
O.1F
REF PGND GND
V
FB
34
1681 F01
V
C
R
E
R
F
C
F
ISOLATION PLEASE REFER TO FIGURE 12 IN THE TYPICAL APPLICATIONS
BOUNDARY SECTION FOR THE COMPLETE 3.3V/15A APPLICATION SCHEMATIC
Figure 1. Simplified 2-Transistor Isolated Forward Converter
1698f
1
+
U
ULTC1698
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
V , PWRGD ....................................................... 13.2V
TOP VIEW ORDER PART
DD
Input Voltage NUMBER
V 1 16 FG
DD
MARGIN, V , OVPIN, I , I ... 0.3V to 5.3V
CG 2 15 SYNC
FB SNSGND SNS
LTC1698EGN
SYNC ..................................................... 14V to 14V
PGND 3 14 V
AUX
LTC1698ES
Output Voltage GND 4 13 I
COMP
LTC1698IGN
OPTODRV 5 12 I
V , I (Note 2)......................... 0.3V to 5.3V
SNS
COMP COMP
LTC1698IS
V 6 11 I
Power Dissipation.............................................. 500mW COMP SNSGND
MARGIN 7 10 PWRGD
GN PART MARKING
Operating Temperature Range
V 8 9 OVPIN
FB
LTC1698E (Note 3) ............................ 40 C to 85 C
1698
GN PACKAGE S PACKAGE
LTC1698I........................................... 40 C to 85 C
1698I
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
Storage Temperature Range ................. 65 C to 150 C
T = 125 C, q = 130 C/W (GN)
JMAX JA
Lead Temperature (Soldering, 10 sec).................. 300 C T = 125 C, q = 110 C/W (SO)
JMAX JA
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The l indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25 C. V = 8V, unless otherwise noted. (Note 4)
A DD
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V Supply Voltage l 6 8 12.6 V
DD
V Undervoltage Lockout 4V
UVLO
I V Supply Current V , OVPIN, V , V = 0V,
VDD DD FB ISNS ISNSGND
C = C = 1000pF, C = 0.1m F,
FG CG VAUX
V = 0V l 1.8 4 mA
SYNC
f = 100kHz (Note 5) 5.0 mA
SYNC
MARGIN and Error Amplifier
V Feedback Voltage MARGIN = Open, V = 1V (Note 7) 1.223 1.233 1.243 V
FB COMP
l 1.215 1.233 1.251 V
I Feedback Input Current V = 1.233V l 0.05 1 m A
VFB FB
V MARGIN Voltage MARGIN = Open 1.65 V
MARGIN
R MARGIN Input Resistance 16.5 kW
MARGIN
D V Feedback Voltage Adjustment V = 3.3V l 456 %
FB MARGIN
V = 0V l 6 5 4 %
MARGIN
G Error Amplifier Open-Loop DC Gain V = 0.8V to 1.2V, Load = 2kW , 100pF l 65 90 dB
ERR COMP
BW Error Amplifier Unity-Gain Bandwidth No Load (Note 6) 2 MHz
ERR
V Error Amplifier Output Clamp Voltage V = 0V 2 V
CLAMP FB
I Error Amplifier Source Current V = 0V l 25 10 mA
VCOMP FB
Error Amplifier Sink Current V = 5V, V = 1.233V l 37 mA
FB COMP
OPTODRV
G Opto Driver DC Gain OVPIN, V , V = 0V l 4.75 5 5.25 V/V
OPTO ISNS ISNSGND
BW Opto Driver Unity-Gain Bandwidth No Load (Note 6) 1 MHz
OPTO
V Opto Driver Output High Voltage V , OVPIN, V = 0V, V = 50mV,
OPTOHIGH FB ISNSGND ISNS
I = 10mA l 45 V
OPTODRV
I Opto Driver Output Short-Circuit Current OVPIN, V , V = 0V, V = 1.233V l 50 25 10 mA
OPTOSC ISNSGND ISNS FB
1698f
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