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LTC1698I

Isolated Secondary Synchronous Rectifier Controller

Linear Technology 

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OPERATIO (Refer to Block Diagram)

catch MOSFETs Q3 and Q4 are off. As soon as transistors

Q1 and Q2 turn on, the flux in the power transformer T1

forces the body diodes of Q3 and Q4 to conduct, and the

whole circuit starts like a conventional forward converter.

At the same time, the LTC1698 VDD potential ramps up

quickly through the VDD bias circuitry. Once the VDD

voltage exceeds 4.0V, the LTC1698 enables its drivers and

enters synchronous operation.

The pulse transformer T2 synchronizes the primary and

secondary MOSFET drivers. In a typical conversion cycle,

the primary MOSFETs Q1 and Q2 turn on simultaneously.

SG goes low and generates a negative spike at the LTC1698

SYNC input through the pulse transformer. The LTC1698

forces FG to turn on and CG to turn off. Power is delivered

to the load through the transformer T1 and the inductor L1.

At the beginning of the next phase in which Q1 and Q2 turn

off, SG goes high, SYNC sees a positive spike, the MOSFET

Q3 shuts off, Q4 conducts and allows continuous current

to flow through the inductor L1. The capacitor COUT filters

the switching waveform to provide a steady DC output

voltage for the load.

The LTC1698 error amplifier ERR senses the output volt-

age through an external resistor divider and regulates the

VFB pin potential to the 1.233V internal bandgap voltage.

An external RC network across the VFB and VCOMP pins

frequency compensates the error amplifier feedback. The

opto driver amplifies the voltage difference between the

VCOMP pin and the bandgap potential, driving the external

optocoupler diode with an inverting gain of 5. The

optocoupler feeds the amplified output error signal to the

primary controller and closes the forward converter volt-

age feedback loop. Under start-up conditions, the internal

diode across the LTC1698 error amplifier clamps the

VCOMP pin. This speeds up the opto driver recovery time by

reducing the negative slew rate excursion at the COMP pin.

The forward converter output voltage can be easily ad-

justed. The potential at the MARGIN pin is capable of

LTC1698

forcing the error amplifier reference voltage to move

linearly by ±5%. The internal RMARGIN resistor converts

the MARGIN voltage to a current and linearly controls the

offset of the error amplifier. Connecting the MARGIN pin

to 3.3V increases the VFB voltage by 5%, and connecting

the MARGIN pin to 0V reduces VFB by 5%. With the

MARGIN pin floating, the VFB voltage is regulated to the

internal bandgap voltage.

The current limit transconductance amplifier ILIM provides

the secondary side average current limit function. The

average voltage drops across the RSECSEN resistor is

sensed and compared to the – 25mV threshold set by the

internal ILIM amplifier. Once ILIM detects high output

current, the current amplifier output pulls high, overrides

the error amplifier, injects more current into the photo

diode and forces a lower duty cycle. An RC network

connected to the ICOMP pin is used to stabilize the second-

ary current limit loop. Alternatively, if only overcurrent

fault protection is required, ICOMP can float.

If under abnormal conditions the feedback path is broken,

OVPIN provides another route for overvoltage fault pro-

tection. If the voltage at OVPIN is higher than the bandgap

voltage, the OVP comparator forces OPTODRV high im-

mediately. A simple external RC filter prevents a momen-

tary overshoot at OVPIN from triggering the OVP

comparator. Short OVPIN to ground if this pin is not used.

The LTC1698 provides an open-drain PWRGD output. If

VFB is less than 94% of its nominal value for more than

1ms, the PWRGD comparator pulls the PWRGD pin low.

If VFB is higher than 94% of its nominal value for more than

2ms, the transistor MPWRGD shuts off, and an external

resistor pulls the PWRGD pin high.

The LTC1698 provides an auxiliary 3.3V logic power

supply. This auxiliary power supply is externally compen-

sated with a minimum 0.1µF bypass capacitor. It supplies

up to 10mA of current to any external devices.

1698f

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