FET (Q

remove the fault (Q

The disconnect switch will help to disconnect the shorted

load from the input.

Over Voltage Protection

Over voltage protection is achieved by connecting the output

voltage to the OVP pin through a resistive divider. The voltage

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When the voltage at this pin exceeds 1.25V, the IC is turned

off and FAULT goes low.

Output Short Circuit Protection

The output short circuit condition is indicated by FAULT. At

startup, a monoshot circuit, (triggered by the POR circuit),

resets an internal flip-flop, which causes FAULT to go high,

and remains high during normal operation. This also allows

the GATE drive to function normally.

The steady state current is reflected in the reference

voltage connected to the transconductance amplifier. The

instantaneous output current is sensed from the FDBK

terminal of the amplifier. The short circuit threshold current is

internally set to 200% of the steady state current.

During short circuit condition, when the current exceeds the

internally set threshold, the SR flip-flop is set and FAULT

goes low. At the same time, the GATE driver of the power

FET is inhibited, providing a latching protection. The system

can be reset by cycling the input voltage to the IC.

Note:

The short circuit FET should be connected before the

current sense resistor as reversing R

the accuracy of the output current (due to the additional

voltage drop across Q

Synchronization

The SYNC pin is an input/output (I/O) port to a fault tolerant

peer-to-peer and/or master clock synchronization circuit.

For synchronization, the SYNC pins of multiple FC9911

based converters can be connected together, and may also

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When connected in this manner, the oscillators will lock

to the device with the highest operating frequency. When

synchronizing multiple ICs, it is recommended that the same

timing resistor, corresponding to the switching frequency, be

used in all the FC9911 circuits.

On rare occasions, given the length of the connecting lines for

the SYNC pins, a resistor between SYNC and GND may be

required to damp any ringing due to parasitic capacitances.

It is recommended that the resistor chosen be greater than

300kΩ.

When synchronized in this manner, a permanent HIGH

or LOW condition on the SYNC pin will result in a loss of

synchronization, but the FC9911 based converters will

continue to operate at their individually set operating

frequency. Since loss of synchronization will not result in total

system failure, the SYNC pin is considered fault tolerant.

Note:

The FC9911 is designed to SYNC up to four ICs at a time

without the use of an external buffer. To SYNC more than

four ICs, it is recommended that a buffered external clock

be used.

Internal 1MHz Transconductance Amplifier

FC9911 includes a built in 1MHz transconductance amplifier,

with tri-state output, which can be used to close the feedback

loop. The output current sense signal is connected to the

FDBK pin and the current reference is connected to the

IREF pin.

The output of the opamp is controlled by the signal applied

to the PWMD pin. When PWMD is high, the output of the

opamp is connected to the COMP pin. When PWMD is low,

the output is left open. This enables the integrating capacitor

to hold the charge when the PWMD signal has turned off

the GATE drive. When the IC is enabled, the voltage on the

integrating capacitor will force the converter into steady state

almost instantaneously.

The output of the opamp is buffered and connected to the

current sense comparator using a 15:1 divider. The buffer

helps to prevent the integrator capacitor from discharging

during the PWM dimming state.

Linear Dimming

Linear dimming can be accomplished by varying the voltage

at the IREF pin, as the output current is proportional to the

voltage at the IREF pin. This can be done either by using a

potentiometer from the REF pin or by applying an external

voltage source at the IREF pin.

Note:

Due to the offset voltage of the transconductance opamp,

pulling the IREFpin very close to GND will cause the internal

short circuit comparator to trigger and shut down the IC.

This limits the linear dimming range of the IC. However,

a 1:10 linear dimming range can be easily obtained. It is

recommended that the PWMD pin be used to get zero

output current rather than pull the I

FC9911

2012. 10. 31

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Revision No : 0