FC9961

2015. 03. 23

7/7

Revision No : 1

PWM Dimming

Due to the fast open-loop response of the average-current

control loop of the FC9961, its PWM dimming performance

nearly matches that of the FC9910B. The inductor current

waveform comparison is shown in Fig. 4.

Fig.4. Typical PWM dimming response of an FC9961

LED driver.

[CH2 (red): PWMD; CH4 (green): Inductor Current; CH3 (blue):

Same as FC9910B for comparison]

The rising and falling edges are limited by the current slew

rate in the inductor. The first switching cycle is terminated

upon reaching the 250mV (V

cuit is further reaching its steady-state within 3~4 switching

cycles regardless of the switching frequency.

Pin Description

Pin #

Function Description

8-Lead SOIC

16-Lead SOIC

1

1

VIN

This pin is the input of an 8.0 - 450V linear regulator.

2

4

CS

This pin is the current sense pin used to sense the FET current by means

of an external sense resistor.

3

5

GND

Ground return for all internal circuitry. This pin must be electrically con-

nected to the ground of the power train.

4

8

GATE

This pin is the output GATE driver for an external N-channel power

MOSFET.

5

9

PWMD

This is the PWM dimming input of the IC. When this pin is pulled to GND,

the gate driver is turned off. When the pin is pulled high, the gate driver

operates normally.

6

12

VDD

This is the power supply pin for all internal circuits. It must be bypassed

with a low ESR capacitor to GND (at least 0.1μF).

7

13

LD

This pin is the linear dimming input, and it sets the current sense thresh-

old as long as the voltage at this pin is less than 1.5V. If voltage at LD falls

below 150mV, the GATE output is disabled. The GATE signal recovers at

200mV at LD.

8

14

RT

A resistor connected between this pin and GND programs the GATE off-

time.

-

2, 3, 6, 7, 10,

11, 15, 16

NC

No connection