AN48 Getting more out of the ZXLD1350 - high output current Ray Liu, Systems Engineer, Zetex Semiconductors Introduction The ZXLD1350 is a continuous mode inductive step-down converter, designed for driving single or multiple series connected LEDs efficiently from a voltage source higher than the LED voltage. The device operates from an input supply between 7V and 30V and provides an externally adjustable output current of up to 350mA. In order to obtain higher output current to drive LEDs with higher power, a high current externally connected output stage is required. 700mA driver for multiple 3W LEDs in series This driver is designed to drive up to six 3W LEDs in series which could deliver total output power of 15W with an overall efficiency of around 90%. RS2 LED1 LEDN L1 RS1 D1 R3 Q3 V6 6 VIN + 4 ISENSE U1 C1 R1 LX GND ADJ 1 2 3 R2 D2 C2 Q1 Q2 0 Figure 1 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 Schematic of 700mA driver 1 www.zetex.com AN48 Part list Table 1 Part ref. Part no. Remark U1 ZXLD1350 Q1 FCX619 Q2 FMMT619 Q3 FMMT619 D1 ZLLS1000 D 25.6V Zener diode L1 68H 1A RS1 150m⍀ RS2 2.2⍀ R1 2.2K⍀ R2 470⍀ R3 15K⍀ C1 3.3F 50V X5/7R or other low ESR cap C2 0.1F Optional Circuit description The output driver consists of two NPN transistors (Q1 and Q2). Transistor Q2 acts as a small signal inverter which inverts the original LX switch signal. The collector of Q2 is connected to the base of transistor Q1 which acts as the power output switch. Transistor Q3 and Zener diode D2 form a simple regulator to supply a constant voltage to the driver stage. The voltage at emitter of Q3 is around 5V. This helps to provide a stable driving current to both Q1 and Q2. The driving currents are around 2mA and 9mA respectively. Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time of the output switch are less than 70ns when input supply voltage is 30V. www.zetex.com 2 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 AN48 Typical performance graphs 5 LEDs in series with total VF = 17.9V 100 800 Output Current (mA) Efficiency (%) 90 80 70 60 700 600 500 400 50 21 22 23 24 25 26 27 28 29 21 30 22 23 24 25 26 27 28 29 30 Input Voltage (V) Input Voltage (V) Efficiency vs Input Voltage (5 LEDs) Output Current vs Input Voltage (5 LEDs) 4 LEDs in series with total VF = 14.9V 100 800 Output Current (mA) Efficiency (%) 90 80 70 60 700 600 500 400 50 18 20 22 24 26 28 18 30 20 22 24 26 28 30 Input Voltage (V) Input Voltage (V) Efficiency vs Input Voltage (4 LEDs) Output Current vs Input Voltage (4 LEDs) 3 LEDs in series with total VF = 11.1V 100 800 Output Current (mA) Efficiency (%) 90 80 70 60 700 600 500 400 50 15 20 25 15 30 Efficiency vs Input Voltage (3 LEDs) Issue 3 - May 2007 © Zetex Semiconductors plc 2007 20 25 30 Input Voltage (V) Input Voltage (V) Output Current vs Input Voltage (3 LEDs) 3 www.zetex.com AN48 Typical performance graphs (cont.) 2 LEDs in series with total VF = 7.7V 100 800 Output Current (mA) Efficiency (%) 90 80 70 60 700 600 500 400 50 12 15 18 21 24 27 12 30 15 18 21 24 27 30 Input Voltage (V) Input Voltage (V) Efficiency vs Input Voltage (2 LEDs) Output Current vs Input Voltage (2 LEDs) 1 LED in series with VF = 3.8V 100 800 Efficiency (%) Output Current (mA) 90 80 70 60 700 600 500 400 50 10 15 20 25 10 30 Efficiency vs Input Voltage (1 LED) www.zetex.com 15 20 25 30 Input Voltage (V) Input Voltage (V) Output Current vs Input Voltage (1 LED) 4 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 AN48 A driver for supply voltage up to 16V This driver is a simplified version to the 700mA driver described above. The driver is designed to drive up to 3 Luxeon® K2 LEDs in series which could deliver a total output power of 10W with a maximum input supply voltage of 16V. LED1 LEDN RS1 L1 D1 V6 + 6 4 VIN ISENSE R1 R2 U1 C1 Q1 LX GND ADJ 1 2 3 C2 Q2 0 Figure 2 Schematic of 1A driver Part List Table 2 Part ref. Part no. U1 ZXLD1350 Q1 ZXTN25020DFH Q2 ZXTN25020DFH D1 ZLLS2000 L1 47H 1.5A RS 100m⍀ R1 4.7K⍀ R2 1.5K⍀ C1 4.7F 25V X5/7R or other low ESR cap C2 0.1F Optional Issue 3 - May 2007 © Zetex Semiconductors plc 2007 Remark 5 www.zetex.com AN48 Circuit description This circuit is similar to the 700mA driver described above. The output driver consists of two NPN transistors (Q1 and Q2). Transistor Q2 acts as a small signal inverter which inverts the original LX switch signal. The collector of Q2 is connected to the base of transistor Q1 which act as the power output switch. Unlike the 700mA driver, the driving current to both Q1 and Q2 varies with the input supply voltage. Hence, the maximum input supply voltage is limited to 16V. The driving current to Q1 is between 5mA and 10mA with input supply voltage between 8V and 16V. Lowering the maximum supply voltage to 16V enables us to use a lower voltage BJT with better switching performance. Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time of the output switch are less than 60ns when input supply voltage is 16V. www.zetex.com 6 Issue 3 - May 2007 © Zetex Semiconductors plc 2007 AN48 Typical performance graphs 2 LEDs in series with total VF = 7.1V 1100 Output Current (mA) 100 Efficiency (%) 90 80 70 60 50 900 700 500 10 11 12 13 14 15 16 10 11 12 13 14 15 16 Input Voltage (V) Output Current vs Input Voltage (2 LEDs) Input Voltage (V) Efficiency vs Input Voltage (2 LEDs) 1 LED in series with total VF = 3.5V 1100 Output Current (mA) 100 Efficiency (%) 90 80 70 60 50 900 700 500 8 10 12 14 16 8 Input Voltage (V) Efficiency vs Input Voltage (1 LED) Issue 3 - May 2007 © Zetex Semiconductors plc 2007 10 12 14 16 Input Voltage (V) Output Current vs Input Voltage (1 LED) 7 www.zetex.com AN48 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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