LTC1698E View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1698E

Isolated Secondary Synchronous Rectifier Controller

Linear Technology 

LTC1698

PI FU CTIO S

VDD (Pin 1): Power Supply Input. For isolated applica-

tions, a simple rectifier from the power transformer is

used to power the chip. This pin powers the opto driver,

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

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 VDD

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 di-

rectly at the LTC1698.

OPTODRV (Pin 5): Optocoupler Driver Output. This pin

drives a ground referenced optocoupler through an exter-

nal 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 condi-

tions, 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.

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

is able to drive more than 2kΩ and 100pF of load. The

internal diode connected from VFB to VCOMP reduces

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 VCOMP

compensates the feedback loop. If VFB goes low, VCOMP

pulls high and OPTODRV goes low.

OVPIN (Pin 9): Overvoltage Input. OVPIN is a high imped-

ance 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 open-

drain output. PWRGD floats if VFB is above 94% of the

nominal value for more than 2ms. PWRGD pulls low if VFB

is below 94% of the nominal value for more than 1ms. The

PWRGD threshold is independent of the MARGIN pin

potential.

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

positive side of the sense resistor, normally grounded.

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

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.

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

at this pin compensates the current limit feedback loop.

Referencing the RC to VOUT controls output voltage over-

shoot on start-up. This pin can float if current limit loop

compensation is not required.

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

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 mini-

mum 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.

1698f

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