Fairchild FEB157-001 - Manual (Page 3)

© 2007 Fairchild Semiconductor Page 3 of 17 Rev 1.1 April 2007
www.fairchildsemi.com
1. Introduction
1.1 Product Description
A high brightness LED evaluation board has been developed using the Fairchild Semiconductor
FAN7554D PWM controller. The board has the capability of driving one, two or three Lumiled,
or similar LEDs. The output current is user selectable at 350mA, 700mA, or 1A. The current is
selectable by inserting or removing jumpers JP1 and JP2 on the evaluation board. The board is
designed to operate over the universal AC line range of 90Vac to 270Vac. A picture of the board is
shown in Figure 1.
Figure 1: Offline High Brightness LED Driver Board
1.2 Circuit Description
The FEB157 evaluation board is an offline flyback converter utilizing the PWM controller,
FAN7554D. Rather than controlling the output voltage, it is designed to control the LED load
current. This is the preferred technique to drive high brightness LEDs. Controlling the LED current
allows for a constant brightness as well as long life of the LEDs. The schematic of the FEB157 is
shown in Figure 3 below.
Referring to Figure 3, the input AC line voltage comes in through J1. Capacitor C1, and common
mode line choke, LF1, form the conducted EMI filter. The dc resistance of LF1 and R1 limit the
inrush current at turn on. Diode bridge BD1 and filter capacitor C2 convert the ac voltage to an
unregulated dc voltage. Resistors R5 and R6 provide the initial startup current for the FAN7554D.
Capacitor C8 off of pin 2 of the FAN7554D provides for soft start in that the pulse width will
gradually increase slowly at turn on thereby decreasing the stress on Q1. Once the converter is up and
running the operating bias voltage will come from the auxiliary winding (pins 4–5) on transformer T1.
The oscillator frequency of the FAN7554D is set by an RC network, R10 and C9. In this board the
frequency is set to 100kHz. However, the FAN7554D can be operated as high as 500kHz.
The FAN7554D is a current mode PWM controller. Current mode operation implies that there are
two control loops. An inside loop that controls the current through the primary winding of the
transformer and an outer loop that controls the output voltage. However, as previously stated, in this
application the outer loop will actually provide feedback information regarding the LED load current
instead of the output voltage.