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. D1 L1 VIN = 1.8V to 3V R2 U2 VSENSE+ Q2 C1 R4 VSENSE- IOUT ZXCT1009 Q1 C2 U1 C1 VCC VDRIVE Stdn ISENSE GND VFB D2 R6 D1 R5 ZXSC400 C3 R3 R1 Figure 1 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 Schematic diagram 1 www.zetex.com DN84 Ref. Value U1 Part number Manufacturer Comments ZXSC400E6 Zetex LED driver in SOT23-6 U2 ZXCT1009 Zetex Current monitor in SOT23 Q1 ZXTN25012EFH Zetex Low sat NPN in SOT23 Q2 ZXTN25012EFL Zetex Low sat NPN in SOT23 D1 ZHCS2000 Zetex 2A Schottky in SOT23 D2 ZHCS400 Zetex 400mA Schottky L1 15H 744 561 15 Wurth Electronik ISAT = 3A DCR=60m⍀ R1 20m⍀ 1% Generic Generic 0805 size low ohmic R2 50m⍀ 1% Generic Generic 0805 size low ohmic R3 820⍀ 1% Generic Generic 0805 size R4 82⍀ 5% Generic Generic 0805 size R5 4.7⍀ 5% Generic Generic 0805 size R6 10⍀ 5% Generic Generic 0805 size C1 22F 10V 10% Generic Generic 1206 size X7R/X5R C2 4.7F 10V 10% Generic Generic 1206 size X7R/X5R C3 0.22F 16V 10% Generic Generic C4 330pF/10V Generic Generic 0805 size Table 1 www.zetex.com Bill of materials 2 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 DN84 Typical operating characteristics (For typical application circuit where Tamb = 25°C unless otherwise stated) 90 0.8 Output Current (A) 1.0 Efficiency (%) 100 80 70 60 50 0.6 0.4 0.2 0.0 3 2.8 2.6 2.4 2.2 2 1.8 3 2.8 2.6 Input Voltage (V) Input Voltage vs Efficiency 2.2 2 1.8 Input Voltage vs Output Current 1.6 4 1.2 3 Output Voltage (V) Input Current (A) 2.4 Input Voltage (V) 0.8 0.4 2 1 0 0.0 3 2.8 2.6 2.4 2.2 2 3 1.8 2.8 2.6 Input Voltage vs Input Current Figure 2 Issue 1 - August 2007 © Zetex Semiconductors plc 2007 2.4 2.2 2 1.8 Input Voltage (V) Input Voltage (V) Input Voltage vs Output Voltage Performance graphs 3 www.zetex.com DN84 Definitions Product change Zetex Semiconductors reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or service. Customers are solely responsible for obtaining the latest relevant information before placing orders. Applications disclaimer The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is assumed by Zetex with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Zetex does not assume any legal responsibility or will not be held legally liable (whether in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business, contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances. 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Samples may be available “Active” Product status recommended for new designs “Last time buy (LTB)” Device will be discontinued and last time buy period and delivery is in effect “Not recommended for new designs” Device is still in production to support existing designs and production “Obsolete” Production has been discontinued Datasheet status key: “Draft version” This term denotes a very early datasheet version and contains highly provisional information, which may change in any manner without notice. “Provisional version” This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance. However, changes to the test conditions and specifications may occur, at any time and without notice. “Issue” This term denotes an issued datasheet containing finalized specifications. However, changes to specifications may occur, at any time and without notice. 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