DN0007 Design note STBB1 buck-boost converter used as a 500mA LED driver with 1.8VDC-5.5VDC Vin Designs from our labs describe tested circuit designs from ST labs which provide optimized solutions for specific applications. For more information or support, visit www.st.com By Patrick Jankowiak Main components STBB1-APUR 1 A, high efficiency adjustable single inductor dual mode buckboost DC-DC converter Specification • Output current = 500mA (1.2A max) • Input Voltage Range 2.1VDC-5.5VDC • 300mA LED current @ 1.8VDC Input Voltage. • 800mA LED current @ 3.3VDC Input Voltage. Circuit description A buck-boost converter with a wide input voltage range is useful for current-limited or LED power applications where it is desired to take advantage of low input voltages, to ensure start-up under battery end-of-life conditions, or in a battery-powered application where maintaining illumination in an emergency is more important than other considerations. The STBB1 was designed as a buck-boost voltage regulator, but it also lends itself very well to a current regulated voltage source. The circuit shown in Figure 1 was developed to operate a 500mA white LED from a single Li-ion or LiPo cell, two alkaline or NiCd cells, or any other power source within the acceptable voltage range. At higher voltages, where the STBB1 operates in buck mode, the LED current may be up to 1A, limited only by thermal factors and the switch current rating. When Vin drops to 1.8V, the circuit still supplies 300mA. A 100uF capacitor, C1, helps the circuit start at low voltages by applying voltage to the FB pin. A 1K resistor, R1, serves to discharge C1 when the circuit is not operating. A 3.9V Zener diode, D2, limits the output voltage to a safe value in case the LED opens or is removed. The circuit shown in Figure 2 was developed to operate a 200mA LED and uses a slightly different starting and protection scheme. When power is applied to Vin, a 100nF capacitor, C2, provides an initial FB voltage of about 3.7V until sufficient output voltage is generated to forward bias the LED. Resistor R4 prevents the FB voltage pulse from C2 from being absorbed by current programming resistor R2. Once the LED is conducting, a feedback voltage of 0.5V is provided by the voltage drop across R3. Diode D2 prevents the resistor June 2012 DN0007 Rev 1 1/5 www.st.com network R1-R2 from influencing the FB voltage provided by R3. Should the LED open, D2 conducts and FB voltage is provided through the R1-R2 voltage divider and the output voltage is limited to a safe value until the LED can be replaced. Figure 1. Circuit diagram – 500mA Table 1. Reference BOM for Figure 1 Manufacturer Part Number/ Description Value Size U1 STMicroelectronics STBB1-APUR CIN Murata GRM21BR71A106KE51L 10 µF 0805 COUT1 Murata GRM21BR60J226ME39L 22 µF 0805 COUT2 Murata GRM21BR71A106KE51L 22 µF 0805 TDK VLCF4020T-2R2N1R7 Coilcraft XFL4020-222ME C1 Any Capacitor, 6VDC 100uF R1 Any Resistor, Chip, 1/16W, 1% 1K 0603 R2 Any Resistor, Chip, 1W, 1% 1Ω 2512 D1 Any LED White D2 Any Zener diode, 2W Battery Any 2 x Battery Lithium AA L1 June 2012 DFN10 (3 x 3 mm) 2.2 µH 4 x 4 x 2 mm DN0007 Rev 1 3.9V 2/5 www.st.com Figure 2. Circuit diagram – 200mA Table 2. Reference BOM for Figure 1 Manufacturer Part Number/ Description Value Size U1 STMicroelectronics STBB1-APUR CIN Murata GRM21BR71A106KE51L 10 µF 0805 COUT Murata GRM21BR60J226ME39L 22 µF 0805 TDK VLCF4020T-2R2N1R7 Coilcraft XFL4020-222ME C1,C2 Murata GRM188R71C104KA01D 100nF 0603 R1 Any Resistor, Chip, 1/16W, 1% 470K 0603 R2 Any Resistor, Chip, 1/16W, 1% 100K 0603 R3 Any Resistor, Chip, 1/2W, 1% 2.2 Ω 2010 R4 Any Resistor, Chip, 1/16W, 1% 1M 0603 R5 Any Resistor, Chip, 1/16W, 1% 150K 0603 D1 Any LED White D2 FAIRCHILD 1N4148WS Battery Any 2 x Battery Lithium AA L1 DFN10 (3 x 3 mm) 2.2 µH 4 x 4 x 2 mm SOD-323F Measurement results Measurements were made of the input voltage and LED current in the basic circuit of Figure 1, using a high power white LED with Vf of about 3.7V. These figures should be considered guidelines and not absolute. Slightly more or less current may be available depending on the individual circuit and layout, but ultimately the device will limit the current June 2012 DN0007 Rev 1 3/5 www.st.com and internal temperature to a safe value while providing as much as possible under the circumstances. • Vin=1.8V: ILED =300mA • Vin = 3.3V: ILED =800mA • Vin = 5.5V: ILED=1000mA Current output capability of the STBB1 was also measured with a load requiring 2.5V. The graph is the maximum output current at VIN = 1.8 V with VOUT = 2.5V. The maximum output current at VIN = 1.8 V with VOUT = 3.3 V is around 400 mA. Figure 3. Output current vs. Temperature at Vin=1.8VDC Support material Documentation Datasheet STBB1-AXX, 1 A, high efficiency single inductor dual mode buck-boost DC-DC converter Revision history Date 6-Jun-2012 June 2012 Version 1 Changes Initial release DN0007 Rev 1 4/5 www.st.com Please Read Carefully Information in this document is provided solely in connection with ST products. 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