DN84

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. VIN = 1.8V to 3V L1 D1 R2 Q2 R4 U1 VCC VDRIVE Stdn ISENSE C1 GND VFB ZXSC400 D2 R5 U2 C1 Q1 VSENSE+ VSENSEIOUT ZXCT1009 C2 R6 D1 C3 R3 R1 Figure 1 Schematic diagram Issue 1 - August 2007 © Zetex Semiconductors plc 2007 1 www.zetex.com DN84 Ref. U1 U2 Q1 Q2 D1 D2 L1 R1 R2 R3 R4 R5 R6 C1 C2 C3 10% C4 Value 15␮H 20m⍀ 1% 50m⍀ 1% 820⍀ 1% 82⍀ 5% 4.7⍀ 5% 10⍀ 5% 22␮F 10V 10% 4.7␮F 10V 10% 0.22␮F 16V Generic 330pF/10V Part number ZXSC400E6 ZXCT1009 ZXTN25012EFH ZXTN25012EFL ZHCS2000 ZHCS400 744 561...