AL9910EV5

Issue 3 – June 2011

www.diodes.com

© Diodes Incorporated 2011

3

OPERATION

The AL9910A device is a PWM high-efficiency LED driver control IC. It allows efficient operation of High

Brightness (HB) LEDs from voltage sources ranging from 85VAC up to 265VAC. The AL9910A can be

configured to work in either fixed frequency or constant OFF-time mode. In fixed frequency mode, the

AL9910A controls an external MOSFET at a fixed switching frequency up to 300kHz, which can be

programmed using a single resistor. The LED string is driven at a constant current rather than at a constant

voltage, thus providing constant light output and enhanced reliability. The output current can be programmed

between a few milliamps and more than 1.0A.

Input filter and Rectifier section

In order to meet the EMI requirement, Cx1, Cx2, CX3, CX4, L1 and L2 are used to provide sufficient filtering

for both differential-mode and common-mode noise generated from the switching converter circuit after the

bridge rectifier.

The rectified line voltage from the bridge rectifier DB1 is fed into a valley-filled passive power factor correction

stage to provide a high power factor for the AC line input. The passive PFC stage comprises of C11, C2, D1,

D2, D3 and R1. This design gives power factors of greater than 0.9 at low line voltage.

Constant Current Buck Converter section

The buck converter section consists of LED controller AL9910A (U1), Ultra-fast free-wheeling rectifier SF30JG

(D4), Inductor (L3 and L4) and a high voltage MOSFET switch (Q1). On this evaluation board, the AL9910A is

configured to operate in the constant OFF-time mode to provide the best LED current regulation. The constant

OFF-time of the AL9910A’s gate output is set to approximately 14µs using resistor R9. Please refer to the

product datasheet for more details on how to configure the constant OFF-time mode.

The circuit values have been designed to provide an output current of 240mA. The LED current level is set by

resistors R5-R7. For detailed applications information please refer to the AL9910/A datasheet.

There is also provision for an optional gate transistor pair (Q2) to drive a MOSFET with a higher gate charge if

a higher LED current is required. Q2 is a high current gate driver in a SOT23-6 package.