DN63 ZXSC310 Solution to drive 8 LEDs connected in series Khagendra Thapa, Principal Systems Engineer, Zetex Semiconductors Description Low cost, small simple and low power multi-LED drive solutions are important in applications including LCD backlight, key illuminations and effects for handheld devices (e.g. cell phones), signage and indicators. The LED current is generally between 10mA to 30mA and is powered from a single cell Li-Ion or three cell alkaline/NiMH/NiCad batteries. For battery powered applications low shutdown quiescent current is important to conserve battery life. Figure 1 shows a simple low cost boost convertor, ZXSC310, driving eight series connected LEDs. ZXSC310 is in a small SOT23-5 package. The design solution is for an application with an input voltage range of 4.5V to 2.5V (e.g. a single cell Li-Ion can have a voltage range of 4.3V to 2.6V) with LED current optimized at 20mA typical, at 4.0V supply. The LED current at 4V is chosen to match the 20mA typical forward current of the LED used. D1 ZHCS1000 VIN 4.5V to 2.5V L1 68H LED1 U1 Q1 ZXTN25040DFH VCC LED2 Drv Stdn Stdn L1 68H ISENSE GND C1 100nF LED7 ZXSC310 R1 100k⍀ RSENSE 200m⍀ LED8 GND Figure 1 Schematic diagram With a single cell Li-Ion battery, the circuit in Figure 1 can drive 3 or more series connected LEDs, the maximum number of LEDs limited by the breakdown voltage of the bipolar transistor Q1. Depending on the number of LEDs connected in series, the sense resistor, RSENSE, will have to be adjusted to obtain the required LED current at a certain supply voltage. The ZXSC310 can be shutdown by pulling the Stdn pin low. The quiescent current in the shutdown mode is typically 5A. If shutdown feature is not required tie the Stdn pin to the VCC pin. Figure 2 shows the efficiency and the LED current against supply voltage. The LED current decreases with the supply voltage. This helps to draw less current from a discharged battery. The bill of materials for the circuit in Figure 1 is shown in Table 1. Issue 5 - August 2007 © Zetex Semiconductors plc 2007 1 www.zetex.com DN63 Efficiency vs input voltage LED current vs supply voltage 100 20 90 LED Current (mA) Efficiency (%) 80 70 60 50 40 30 20 3 LEDs in Series 10 0 4.5 4 3.5 3 15 10 5 3 LEDs in Series 0 4.5 2.5 4 Input voltage (V) 3.5 3 2.5 Input voltage (V) Figure 2 Performance graphs Ref. Part no. Value Manufacturer Contact details U1 Q1 ZXSC310E5 NA ZXTN25040DFH NPN, VCEO = 40V D1 ZHCS1000 Zetex Zetex www.zetex.com www.zetex.com 1A, low forward voltage VF Zetex www.zetex.com RSENSE Generic 200m⍀ Generic NA R1 C1 C2 L1 LED1-8 100k⍀ 100nF, 6.3V, X7R 2.2F, 35V 68H White LED Generic Generic Generic Coilcraft Nichia NA NA NA www.coilcraft.com www.nichia.com Generic Generic Generic DO1608P-683 NSPMW500BS Table 1 www.zetex.com Bill of materials 2 Issue 5 - August 2007 © Zetex Semiconductors plc 2007 DN63 Intentionally left blank Issue 5 - August 2007 © Zetex Semiconductors plc 2007 3 www.zetex.com DN63 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. 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