Issue 1 - August 2007 1 www.zetex.com
© Zetex Semiconductors plc 2007
DN84
ZXSC400 Driving 3W high power LEDs
Ray Liu, Applications Engineer, Zetex Semiconductors
Description
This design note shows the ZXSC400 driving a single 3W LED. The input voltage ranges from 1.8V
to 3.6V with constant output current of 700mA down to 2.6V with an overall 80% of efficiency.
Figure 1 shows typical constant current solution with ZXSC400 driving one 3W LED. The input
voltage range allows the use of two alkaline batteries or one Lithium Ion cell (CR123A) for
portable flashlight applications.
Q1 and Q2 forms a pseudo Darlington pair which provide enough current gain for a switching
current up to 1.5A. In order to provide better switch off performance, a Schottky diode, D2, is used
to drain the base current from the base of Q1 directly. In order to achieve higher efficiency, current
monitor U2 is used to provide a low voltage drop LED current sensing through the low ohmic
resistor, R2. The LED current is converted to 300mV feedback voltage through R3.
Figure 1 Schematic diagram
Q2
D2
Q1
D1
V
IN
= 1.8V to 3V
D1
C3
C2
L1
R1
R5
R4
U1
V
CC
Stdn
GND
V
DRIVE
I
SENSE
V
FB
ZXSC400
ZXCT1009
V
SENSE-
V
SENSE+
I
OUT
R6
R3
R2
U2
C1
C1
