AL9910EV6
Issue 4 – 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 C1, C2, D1,
D2, D3 and R1. This design gives power factors of greater than 0.9.
Constant Current Flyback Converter section
The isolated Flyback converter section consists of LED controller AL9910A (U1), ultra-fast free-wheeling
rectifier ES2G (D6), transformer (T8) and a high voltage MOSFET switch (Q1). On this evaluation board, the
AL9910A is configured to operate in the constant frequency mode to provide the best LED current regulation.
The switching frequency of the AL9910A is set to 100kHz using resistor R2. Please refer to the product
datasheet for more details on how to configure the constant frequency mode.
The Flyback converter have been designed to provide an total output current of 300mA. The LED current is
regulated with an isolated secondary side feedback, consisting of an opto coupler (U2), sense resistors (R1,
R2) and voltage reference (U3). The LED current level is set by resistors R1 and R2.
The voltage regulator network consisting of Q2, R9 and Z2 provides stable voltage to the AL9910A from the
transformer auxillary winding. Zener diode Z4 and resistor R13 provides protection to the power stage under
open LED condition.
Multi-channel LED Current Balancing section
In this section of the evaluation board, bipolar transistors Q4-Q7 are connected as current mirror to provide an
effective LED current balancing function to drive up to 4 strings of LEDs with minimal losses. The transistors
are mounted as close to one another on the PCB to achieve the best possible thermal coupling. R16, R17,
R22 and R23 are the transistor’s emitter resistors added to improve the LED current balancing.
