FC9931

2012. 10. 31

4/6

Revision No : 0

Input and Output Current Feedback

Two current sense comparators are included in the FC9931.

Both comparators have their non-inverting inputs internally

connected to ground (GND). The CS1 and CS2 inputs are

inverting inputs of the comparators. Connecting a resistor

divider into either of these inputs from a positive reference

voltage and a negative current sense signal programs the

current sense threshold of the comparator. The V

of the FC9931 can be used as the reference voltage. If more

accuracy is needed, an external reference voltage can be

applied. When either the CS1 or the CS2 pin voltage falls

below GND, the GATE pulse is terminated. A leading edge

blanking delay of 215ns (typ) is added. The GATE voltage

becomes high again upon receiving the next clock pulse of

the oscillator circuit.

Referring to the Functional Circuit Diagram, the CS2

comparator is responsible for regulating output current. The

output LED current can be programmed using the following

equation:

where ∆I

comparator limits the current in the input inductor L1. There

is no charge in the capacitor C1 upon the start-up of the

converter. Therefore, L2 cannot develop the output current,

and the FC9931 starts-up in the input current limiting mode.

The CS1 current threshold must be programmed such that no

input current limiting occurs in normal steady-state operation.

The CS1 threshold can be programmed in accordance with

a similar equation:

where I

MOSFET Gate Driver

Typically, the GATE driving capability of the FC9931 is limited

by the amount of power dissipation in its linear regulator.

Thus, care must be taken selecting a switching MOSFET

to be used in the circuit. An optimal trade-off must be found

between the GATE charge and the on-resistance of the

MOSFET to minimize the input regulator current.