LINEAR TECHNOLOGY LTC1698 Technical data

FEATURES

■

High Efficiency Over Wide Load Current Range

■

±0.8% Output Voltage Accuracy

■

Dual N-Channel MOSFET Synchronous Drivers

■

Pulse Transformer Synchronization

■

Optocoupler Feedback Driver

■

Programmable Current Limit Protection

■

±5% Margin Output Voltage Adjustment

■

Adjustable Overvoltage Fault Protection

■

Power Good Flag

■

Auxiliary 3.3V Logic Supply

■

Available in 16-Lead SSOP and SO Packages

U

APPLICATIO S

■

48V Input Isolated DC/DC Converters

■

Isolated Telecommunication Power Systems

■

Distributed Power Step-Down Converters

■

Industrial Control Systems

■

Automotive and Heavy Equipment

LTC1698

Isolated Secondary

Synchronous Rectifier Controller

U

DESCRIPTIO

The LTC®1698 is a precision secondary-side forward
converter controller that synchronously drives external
N-channel MOSFETs. It is designed for use with the
LT®3781 primary-side synchronous forward converter
controller to create a completely isolated power supply.
The LT3781 synchronizes the LTC1698 through a small
pulse transformer and the LTC1698 drives a feedback
optocoupler to close the feedback loop. Output accuracy
of ±0.8% and high efficiency over a wide range of load
currents are obtained.

The LTC1698 provides accurate secondary-side current
limit using an external current sense resistor. The input
voltage at the MARGIN pin provides ±5% output voltage
adjustment. A power good flag and overvoltage input are
provided to ensure proper power supply conditions. An
auxiliary 3.3V logic supply is included that supplies up to
10mA of output current.

, LTC and LT are registered trademarks of Linear Technology Corporation.

TYPICAL APPLICATIO

V

IN

36V to 72V

TG BG

LT3781

+
–

V

C

R

F

U

L1

+

C

OUT

+

110

V

PWRGD

DD

28

CG

16

FG

12

I

SNS

11

I

SNSGND

15

SYNC

5

OPTODRV

PGND GND

V

LTC1698

V

COMP

I

COMP

OVPIN

MARGIN V

V

AUX

34

Q2

R

REF

C

D1

PRISEN

SG

V

FB

F

Q1

T1

C

SG

T2

•

D2

•

R

E

ISOLATION
BOUNDARY

•

R

•

Q4
Q3

SECSEN

VDD BIAS

C

SYNC

R

SYNC

R

K

C

K

PLEASE REFER TO FIGURE 12 IN THE TYPICAL APPLICATIONS
SECTION FOR THE COMPLETE 3.3V/15A APPLICATION SCHEMATIC

Figure 1. Simplified 2-Transistor Isolated Forward Converter

R2

R1

R

FB

6

13

9

7

14

C

C

C

C

FB

C

CILM

R

CILM

MARGIN

V

AUX

3.3V
10mA

O.1µF

1681 F01

V

OUT

R5

R4

1698f

1

LTC1698

WW

W

ABSOLUTE MAXIMUM RATINGS

U

PACKAGE/ORDER INFORMATION

(Note 1)

VDD, PWRGD ....................................................... 13.2V

Input Voltage

MARGIN, VFB, OVPIN, I

SNSGND

, I

... –0.3V to 5.3V

SNS

SYNC ..................................................... –14V to 14V

Output Voltage

V

COMP

, I

(Note 2)......................... – 0.3V to 5.3V

COMP

Power Dissipation.............................................. 500mW

Operating Temperature Range

LTC1698E (Note 3)............................ –40°C to 85°C

LTC1698I........................................... – 40°C to 85°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

ELECTRICAL CHARACTERISTICS

The ● indicates specifications which apply over the full operating

V

DD

CG

PGND

GND

OPTODRV

V

COMP

MARGIN

V

FB

GN PACKAGE

16-LEAD PLASTIC SSOP

T

JMAX

T

JMAX

Consult LTC Marketing for parts specified with wider operating temperature ranges.

TOP VIEW

1
2
3
4
5
6
7
8

16-LEAD PLASTIC SO

= 125°C, θJA = 130°C/W (GN)
= 125°C, θJA = 110°C/W (SO)

FG

16

SYNC

15

V

14

I

13

I

12

I

11

PWRGD

10

OVPIN

9

S PACKAGE

AUX

COMP

SNS

SNSGND

temperature range, otherwise specifications are at TA = 25°C. VDD = 8V, unless otherwise noted. (Note 4)

U

W

ORDER PART

NUMBER

LTC1698EGN
LTC1698ES
LTC1698IGN
LTC1698IS

GN PART MARKING

1698
1698I

U

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

DD

V

UVLO

I

VDD

MARGIN and Error Amplifier

V

FB

I

VFB

V

MARGIN

R

MARGIN

∆V

FB

G

ERR

BW

ERR

V

CLAMP

I

VCOMP

OPTODRV

G

OPTO

BW

OPTO

V

OPTOHIGH

I

OPTOSC

Supply Voltage ● 6 8 12.6 V
Undervoltage Lockout 4V
VDD Supply Current VFB, OVPIN, V

C

= CCG = 1000pF, C

FG

V

= 0V ● 1.8 4 mA

SYNC

f

= 100kHz (Note 5) 5.0 mA

SYNC

Feedback Voltage MARGIN = Open, V

Feedback Input Current VFB = 1.233V ● 0.05 1 µA
MARGIN Voltage MARGIN = Open 1.65 V
MARGIN Input Resistance 16.5 kΩ
Feedback Voltage Adjustment V

Error Amplifier Open-Loop DC Gain V
Error Amplifier Unity-Gain Bandwidth No Load (Note 6) 2 MHz
Error Amplifier Output Clamp Voltage VFB = 0V 2 V
Error Amplifier Source Current VFB = 0V ● –25 –10 mA

Error Amplifier Sink Current V

Opto Driver DC Gain OVPIN, V
Opto Driver Unity-Gain Bandwidth No Load (Note 6) 1 MHz
Opto Driver Output High Voltage VFB, OVPIN, V

Opto Driver Output Short-Circuit Current OVPIN, V

= 3.3V ● 456 %

MARGIN

V

= 0V ● –6 –5 –4 %

MARGIN

= 0.8V to 1.2V, Load = 2kΩ, 100pF ● 65 90 dB

COMP

= 5V, V

FB

I

= –10mA ● 45 V

OPTODRV

, V

ISNS

COMP

= 1.233V ● 37 mA

COMP

, V

ISNS

ISNSGND

ISNSGND

, V

ISNSGND

ISNS

= 0V,

ISNSGND

= 0.1µF,

VAUX

= 1V (Note 7) 1.223 1.233 1.243 V

● 1.215 1.233 1.251 V

= 0V ● 4.75 5 5.25 V/V

= 0V, V

= 0V, VFB = 1.233V ● –50 –25 –10 mA

= –50mV,

ISNS

2

1698f

LTC1698

ELECTRICAL CHARACTERISTICS

The ● indicates specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. VDD = 8V, unless otherwise noted. (Note 4)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V

AUX

V

AUX

Current Limit Amplifier

I

ISNSGNDISNSGND

I

ISNS

V

ILIMTH

I

ICOMPICOMP

g

mILIM

G

ICOMP

PWRGD and OVP Comparators

V

PWRGD

I

PWRGD

V

OL

V

OVPREF

I

OVPIN

t

PWRGD

t

OVP

SYNC and Drivers

V

PT

V

NT

I

SYNC

f

SYNC

t

d

t

SYNC

tr, t

t

DDIS

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired. All voltages refer to GND.

Note 2: The LTC1698 incorporates a 5V linear regulator to power internal
circuitry. Driving these pins above 5.3V may cause excessive current flow.
Guaranteed by design and not subject to test.

Note 3: The LTC1698E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. For guaranteed performance to
specifications over the –40°C to 85°C range, the LTC1698I is available.

Note 4: All currents into device pins are positive; all currents out of the
device pins are negative. All voltages are referenced to ground unless
otherwise specified. For applications with V
Performance Characteristics.

Auxiliary Supply Voltage C

Input Current V

I

Input Current V

SNS

Current Limit Threshold V
(V

ISNS

– V

) ● –27.5 –25 –22.5 mV

ISNSGND

Source Current V

I

Sink Current V

COMP

Current Limit Amplifier V

= 0.1µF, I

VAUX

= 0V ● 0.05 1 µA

ISNSGND

= 0V ● 0.05 1 µA

ISNS

= 2.5V, V

ICOMP

= 0V, V

ISNSGND

= 0V, V

ISNSGND

= 0V, V

ISNSGND

= 0mA to 10mA, VDD = 7V to 12.6V ● 3.135 3.320 3.465 V

LOAD

= 0V –27.0 –25 –23.0 mV

ISNSGND

= –0.3V, V

ISNS

= 0.3V, V

ISNS

ICOMP

= 2.5V, I

= 2.5V (Note 8) –280 –200 – 120 µA

ICOMP

= 2.5V (Note 8) 120 200 280 µA

ICOMP

= ±10µA ● 2.2 3.5 5 millimho

ICOMP

● –370 – 200 – 80 µA

● 80 200 370 µA

Transconductance
Current Limit Amplifier V

= 2.5V, No Load ● 48 60 dB

ICOMP

Open-Loop DC Gain

Percent Below V

FB

V

↓, MARGIN = Open (Note 9) ● –9 –6 –3 %

FB

Power Good Sink Current VFB = 2V ● 10 µA

VFB = 0V ● 10 mA
Power Good Output Low Voltage I
OVPIN Threshold VFB = V
OVPIN Input Bias Current V

= 3mA, VFB = 0V ● 0.4 V

PWRGD

= V

ISNS

= 1.233V ● 0.1 1 µA

OVPIN

= 0V, OVPIN ↑ (Note 9) ● 1.18 1.233 1.28 V

ISNSGND

Power Good Response Time VFB ↑ ● 125 ms
Power Bad Response Time VFB ↓ ● 0.5 1 2.5 ms

Overvoltage Response Time V

OVPIN

↑, C

= 0.1µF ● 520 µs

OPTODRV

SYNC Input Positive Threshold ● 1 1.6 2.2 V
SYNC Input Negative Threshold ● – 2.2 –1.6 –1 V
SYNC Input Current V
SYNC Frequency Range CFG = CCG = 1000pF, V
SYNC Input to Driver Output Delay CFG = CCG = 1000pF, f
Minimum SYNC Pulse Width f
Driver Rise and Fall Time CFG = CCG = 1000pF, f

f

= ±10V ● 150 µA

SYNC

= ±5V ● 50 400 kHz

SYNC

= 100kHz, V

SYNC

= 100kHz, V

SYNC

= ±10V (Note 6) ● 75 ns

SYNC

= 100kHz, V

SYNC

= ±5V ● 40 90 ns

SYNC

= ±5V, ● 10 40 ns

SYNC

10% to 90%
Driver Disable Time-Out CFG = CCG = 1000pF, f

= 100kHz, V

SYNC

SYNC

= ±5V

Measured from CG ↑ (Note 10) ● 10 15 20 µs

Note 5: Supply current in active operation is dominated by the current
needed to charge and discharge the external FET gates. This will vary with
the LTC1698 operating frequency, supply voltage and the external FETs
used.

Note 6: This parameter is guaranteed by correlation and is not tested.
Note 7: VFB is tested in an op amp feedback loop which servos VFB to the

internal bandgap voltage.
Note 8: The current comparator output current varies linearly with

temperature.
Note 9: The PWRGD and OVP comparators incorporate 10mV of

hysteresis.

< 7V, refer to the Typical

DD

Note 10: The driver disable time-out is proportional to the SYNC period
within the frequency synchronization range.

1698f

3

LTC1698

TEMPERATURE (°C)

–50

g

mILIM

(millimho)

3.8

4.2

4.6

100 125

1698 G06

3.4

3.0

–25 250 50 75 150

2.6

2.2

5.0
VDD = 8V

UW

TYPICAL PERFOR A CE CHARACTERISTICS

VFB vs Temperature

1.248
VDD = 8V

1.242

1.236

(V)

FB

V

1.230

1.224

1.218

–50

–25 0

I

Threshold vs Temperature I

SNS

–22.5

VDD = 8V

–23.0
–23.5
–24.0

–24.5

–25.0
–25.5

THRESHOLD (mV)

–26.0

SNS

I

–26.5
–27.0

–27.5

–50

–25 25

50

25 75 150

TEMPERATURE (°C)

0

50

TEMPERATURE (°C)

75

100 125

100

1698 G01

125

1698 G04

150

VFB vs V

1.248
TA = 25°C

1.242

1.236

(V)

FB

V

1.230

1.224

1.218
5

–22.5
–23.0
–23.5
–24.0
–24.5
–25.0
–25.5

THRESHOLD (mV)

–26.0

SNS

I

–26.5
–27.0
–27.5

DD

68

7

Threshold vs V

SNS

TA = 25°C

5

69

10 14

9

VDD (V)

87

VDD (V)

VFB vs V

1.295

1.282

1.270

1.258

1.245

(V)

1.233

FB

V

1.221

1.208

1.196

1.184

13

1698 G05

1.171
0

14

12

11

13

1698 G02

DD

12

10

11

MARGIN

VDD = 8V

= 25°C

T

A

0.660.33

1.320.99
V

MARGIN

1.65

1.98 2.31 2.97
(V)

Current Limit Amplifier g
vs Temperature

2.64

1698 G03

m

3.3

5
4
3
2

∆V

1

FB

(%)

0
–1
–2
–3
–4
–5

OVPIN Threshold vs Temperature OVPIN Threshold vs V

1.28
VDD = 8V

1.26

1.24

1.22

OVPIN THRESHOLD (V)

1.20

1.18

–50

–25 0

4

50

25 75 150

TEMPERATURE (°C)

100 125

1698 G07

1.28
TA = 25°C

1.26

1.24

1.22

OVPIN THRESHOLD (V)

1.20

1.18

68

7

5

DD

12

10 14

11

9

VDD (V)

13

1698 G08

Power Good Threshold
vs Temperature

1.196
VDD = 8V

1.181

1.166

1.152

1.137

POWER GOOD THRESHOLD (V)

1.122

–50

–25 0

25 75 150

TEMPERATURE (°C)

–3.0

–4.2

∆V

–5.4

FB

(%)

–6.6

–7.8

50

100 125

–9.0

1698 G09

1698f

UW

TYPICAL PERFOR A CE CHARACTERISTICS

V

vs Temperature V

AUX

3.465
VDD = 8V

= 0mA

I

LOAD

3.424

3.383

3.341

(V)

3.300

AUX

V

3.259

3.218

3.176

3.135

–25

–50

V

0

Short-Circuit Current

AUX

50

25

TEMPERATURE (°C)

vs Temperature

0

VDD = 8V

–10

–20

–30

SHORT-CIRCUIT CURRENT (mA)

–40

AUX

V

–50

–50

–25 0

50

25 75 150

TEMPERATURE (°C)

75

100

125

100 125

1698 G10

1698 G13

3.465

3.424

3.383

3.341

(V)

3.300

AUX

V

3.259

3.218

3.176

150

3.135

V

AUX

vs V

0

TA = 25°C

–10

–20

–30

SHORT-CIRCUIT CURRENT (mA)

–40

AUX

V

–50

5

vs Line Voltage V

AUX

VDD = 8V

= 0mA

I

LOAD

10

6

5

78

9

VDD (V)

11

12

Short-Circuit Current

DD

9

67

810 14

11 12 13

VDD (V)

1698 G14

AUX

3.465
VDD = 8V

= 25°C

T

A

3.424

3.383

3.341

(V)

3.300

AUX

V

3.259

3.218

3.176

13

1698 G11

14

3.135

0

Opto Driver Load Regulation

3.030
VDD = 8V

3.024

= 25°C

T

A

3.018

3.012

3.006

3.000

2.994

2.988

2.982

OPTO DRIVER OUTPUT VOLTAGE (V)

2.976

2.970

21

0

vs Load Current

1

23

4

LOAD CURRENT (mA)

67 9

43

5

LOAD CURRENT (mA)

LTC1698

5

6

7

8

1698 G12

1.0

0.8

0.6

0.4

0.2
0
–0.2
–0.4
–0.6
–0.8

1698 G15

–1.0

10

8

109

PERCENT (%)

Maximum OPTO Driver Output
Voltage vs Load Current

8

6

4

2

TA = 25°C
V

= 0V

COMP

MAXIMUM OPTO DRIVER OUTPUT VOLTAGE (V)

0

2

1

0

5

4

3

LOAD CURRENT (mA)

6

VDD = 10V

VDD = 8V

VDD = 7V

VDD = 6V
VDD = 5V

7

8

9

1698 G22

Maximum OPTO Driver Output
Voltage vs Temperature

8

6

4

2

= 0V

V

COMP

I

= –10mA

OPTODRV

MAXIMUM OPTO DRIVER OUTPUT VOLTAGE (V)

0

–50

10

–25 0 25 50

TEMPERATURE (°C)

VDD = 10V

VDD = 8V

VDD = 7V

VDD = 6V

VDD = 5V

75 100 125 150

1698 G23

Opto Driver Short-Circuit Current
vs Temperature

–10

VDD = 8V

= 1.233V

V

OPTODRV

–15

–20

–25

–30

–35

–40

–45

OPTO DRIVER SHORT-CIRCUIT CURRENT (mA)

–50

–50

–25

0

50

25

TEMPERATURE (°C)

75

100

125

1698 G16

150

1698f

5

LTC1698

VDD (V)

5

1.00

SYNC POSITIVE THRESHOLD (V)

1.24

1.72

1.96

2.20

7

9

10 14

1698 G21

1.48

68

11

12

13

TA = 25°C

UW

TYPICAL PERFOR A CE CHARACTERISTICS

I

vs SYNC Frequency

VDD

50

VDD = 8V

45

T

= 25°C

(mA)

VDD

I

A

40
35
30
25
20
15
10

5
0

50

CFG = CCG = 2200pF

150100

CFG = CCG = 4700pF

CFG = CCG = 3300pF

CFG = CCG = 1000pF

350 400

250200

300

f

(kHz)

SYNC

Opto Driver Short-Circuit Current
vs V

DD

0

TA = 25°C

= 1.233V

V

OPTODRV

–10

–20

–30

–40

OPTO DRIVER SHORT-CIRCUIT CURRENT (mA)

–50

5

67

9

810 14

11 12 13

VDD (V)

450

1698 G19

1698 G17

500

SYNC Positive Threshold
vs Temperature

2.25
VDD = 8V

2.00

1.75

1.50

1.25

SYNC POSITIVE THRESHOLD (V)

1.00

(mA)

VDD

I

20
18
16
14
12
10

8
6
4
2
0

–50

I

VDD

TA = 25°C
f

SYNC

5

–25 0

vs V

= 100kHz

CFG = CCG = 3300pF

CFG = CCG = 1000pF

76

50

25 75 150

TEMPERATURE (°C)

DD

CFG = CCG = 4700pF

98

VDD (V)

100 125

1698 G20

CFG = CCG = 2200pF

11 12

10

13

1698 G18

SYNC Positive Threshold
vs V

DD

Undervoltage Lockout Threshold
vs Temperature

5

4

3

(V)

UVLO

V

2

1

0

–50

14

–25 0

50

25 75 150

TEMPERATURE (°C)

100 125

1698 G24

Driver Rise, Fall and Propagation
Delay vs Driver Load

90

VDD = 8V

= 25°C

T

80

A

70

60

50

40

TIME (ns)

30

20

10

0

0

6

CG, FG t

2000 6000

CG, FG t

PLH

t

f

PHL

4000

DRIVER LOAD (pF)

t

SYNC Input to Driver Output Delay
vs Temperature

90

VDD = 8V
C

= CFG = 1000pF

80

CG

= 100kHz

f

SYNC

70

60

CG, FG t

r

10000

8000

1698 G25

50

(ns)

d

t

40

30

20

10

0

–50 –25 0 25 50 75 100 125 150

PLH

CG, FG t

TEMPERATURE (°C)

PHL

1698 G26

Driver Disable Time-Out vs SYNC
Frequency

30

VDD = 8V

= 25°C

T

A

25

(µs)

DISS

20

15

10

5

DRIVER DISABLE TIME-OUT t

0

100 200

150

50

t

DISS

t

DISS

300 500

350

250

f

(kHz)

SYNC

2.2

NORMALIZED DRIVER DISABLE TIME-OUT

2.0

× f

SYNC

400

450

1698 G27

1.8

1.6

1.4

1.2

1.0

t

DISS

× f

SYNC

1698f

LTC1698

U

UU

PI FU CTIO S

VDD (Pin 1): Power Supply Input. For isolated applica­tions, a simple rectifier from the power transformer is
used to power the chip. This pin powers the opto driver,
the V
regulator powers the remaining circuitry. VDD requires an
external 4.7µF bypass capacitor.

CG (Pin 2): Catch Gate Driver. If SYNC slews positive, CG
pulls high to drive an external N-channel MOSFET. CG
draws power from the VDD pin and swings between V
and PGND.

PGND (Pin 3): Power Ground. Connect PGND to a low
impedance ground plane in close proximity to the ground
terminal of the external current sensing resistor.

GND (Pin 4): Logic and Signal Ground. GND is referenced
to the internal low power circuitry. Careful board layout
techniques must be used to prevent corruption of signal
ground reference. Connect GND and PGND together di­rectly at the LTC1698.

OPTODRV (Pin 5): Optocoupler Driver Output. This pin
drives a ground referenced optocoupler through an exter­nal resistor. If VFB is low, OPTODRV pulls low. If VFB is
high, OPTODRV pulls high. This optocoupler driver has a
DC gain of 5. During overvoltage or overcurrent condi­tions, OPTODRV pulls high. The output is capable of
sourcing 10mA of current and will drive an external 0.1µF
capacitive load and is short-circuit protected.

V

COMP

is able to drive more than 2kΩ and 100pF of load. The
internal diode connected from VFB to V
OPTODRV recovery time under start-up conditions.

MARGIN (Pin 7): Current Input to Adjust the Output
Voltage Linearly. The MARGIN pin connects to an internal

16.5k resistor. The other end of this resistor is regulated
to 1.65V. Connecting MARGIN to a 3.3V logic supply
sources 100µA of current into the chip and moves the
output voltage 5% higher. Connecting MARGIN to 0V
sinks 100µA out of the pin and moves the regulated output
voltage 5% lower. The MARGIN pin voltage does not affect
the PWRGD and OVPIN trip points.

VFB (Pin 8): Feedback Voltage. VFB senses the regulated
output voltage through an external resistor divider. The
VFB pin is servoed to the reference voltage of 1.233V under
closed-loop conditions. An RC network from VFB to V

supply and the FG and CG drivers. An internal 5V

AUX

DD

(Pin 6): Error Amplifier Output. This error amplifier

reduces

COMP

COMP

compensates the feedback loop. If VFB goes low, V
pulls high and OPTODRV goes low.

OVPIN (Pin 9): Overvoltage Input. OVPIN is a high imped­ance input to an internal comparator. The threshold of this
comparator is set to 1.233V. If the OVPIN potential is
higher than the threshold voltage, OPTODRV pulls high
immediately. Use an external RC lowpass filter to prevent
noisy signals from triggering this comparator.

PWRGD (Pin 10): Power Good Output. This is an open­drain output. PWRGD floats if VFB is above 94% of the
nominal value for more than 2ms. PWRGD pulls low if V
is below 94% of the nominal value for more than 1ms. The
PWRGD threshold is independent of the MARGIN pin
potential.

I

SNSGND

positive side of the sense resistor, normally grounded.

I

SNS

tive side of the sense resistor through an external RC
lowpass filter. This pin normally sees a negative voltage,
which is proportional to the average load current. If
current limit is exceeded, OPTODRV pulls high.

I

COMP

at this pin compensates the current limit feedback loop.
Referencing the RC to V
shoot on start-up. This pin can float if current limit loop
compensation is not required.

V

AUX

requires a 0.1µF or greater bypass capacitor. This auxiliary
power supply can power external devices and sources
10mA of current. Internal current limiting is provided.

SYNC (Pin 15): Drivers Synchronization Input. A negative
voltage slew at SYNC forces FG to pull high and CG to pull
low. A positive voltage slew at SYNC resets the FG pin and
CG pulls high. If SYNC loses its synchronization signal for
more than the driver disable time-out interval, both the
forward and catch drivers output are forced low. The SYNC
circuit accepts pulse and square wave signals. The mini­mum pulse width is 75ns. The synchronization frequency
range is between 50kHz to 400kHz.

FG (Pin 16): Forward Gate Driver. If SYNC slews negative,
FG goes high. FG draws power from VDD and swings
between VDD and PGND.

(Pin 11): Current Sense Ground. Connect to the

(Pin 12): Current Sense Input. Connect to the nega-

(Pin 13): Current Amplifier Output. An RC network

controls output voltage over-

OUT

(Pin 14): Auxiliary 3.3V Logic Supply. This pin

COMP

FB

1698f

7

LTC1698

BLOCK DIAGRA

V

14

SYNC

15

MARGIN

7

W

AUX

1

V

DD

AUX GEN VCC GEN

V

SYNC IN

R

MARGIN

I-TO-V CONVERTER

CC

CG

FG

16

2

OPERATIO

OPTODRV

5

PWRGD

10

–

V

U

(Refer to Block Diagram)

M

PWRGD

PWRGD

FB

BANDGAP

100k

+

0.94V

REF

OPTO

V

REF

±5% V

REF

+

20k

–

M

ILIM

R

ILIM

3k

R

OVP

3k

+

ERR

–

+

I

LIM 25mV

+

–

+

OVP

–

V

FB

8

V

COMP

6

I

SNSGND

11

I

SNS

12

I

COMP

13

V

REF

OVPIN

9

1698 BD

The LTC1698 is a secondary-side synchronous rectifier
controller designed to work with the LT3781 primary-side
synchronous controller chip to form an isolated synchro­nous forward converter. This chip set uses a dual transis­tor forward topology that is predominantly used in distrib­uted power supply systems where isolated low voltages
are needed to power complex electronic equipment. The
primary stage is a current mode, fixed frequency forward
converter and provides the typical PWM operation. A
power transformer is used to provide the functions of
input/output isolation and voltage step-down to achieve
the required low output voltage. Instead of using typical

8

Schottky diodes, synchronous rectification on the sec­ondary offers isolation with high efficiency. It supplies
high power without the need of bulky heat sinks, which is
often a problem in any space constrained application.

The LTC1698 not only provides synchronous drivers for
the external MOSFETs, it comes with other housekeeping
functions performed on the secondary side of the power
supply, all within a single integrated controller. Figure 1
shows the typical chip-set application. Upon power up, the
LTC1698’s VDD input is low, the gate drivers TG and BG are
both at the ground potential. The secondary forward and

1698f