Issue 1 - August 2007

1

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© 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

D1

C3

C2

L1

R1

R5

R4

U1

Stdn

GND

ZXSC400

ZXCT1009

R6

R3

R2

U2

C1

C1