DN06001/D Design Note – DN06001/D 9 V to 36 Vin dc, Constant Current LED Driver Device Application NCP3063 DC-DC Constant Current LED Driver Input Voltage Output Power Topology I/O Isolation 10 W Non Inverting Buck Boost None 9 Vdc to 36 Vdc Other Specifications Output Voltage Ripple Max Current Output 1 Output 2 Output 3 Output 4 12 Vdc 200 mV 1A N/A N/A N/A N/A N/A N/A N/A N/A N/A PFC (Yes/No) No Minimum Efficiency Cooling Method/Supply Orientation 70% Convection Circuit Description Key Features ON Semiconductor’s latest monolithic NCP3063 control circuit and family of e2PowerEdge bipolar transistors are combined to create a non inverting buck boost topology optimized to drive strings of LED’s at a constant current. A high side, low drop, current sensing scheme has been implemented, targeted for automotive and other high efficiency applications. The output from the current sense is used to vary the slope of the oscillator ramp and achieve duty cycle modulation, independent of the gated oscillator function provided on the IC. • Seamless buck or boost operation • Constant current output with voltage clamp • Low drop high side current sensing • Baker clamp drive on external transistor for enhanced efficiency • High frequency operation to enable cost effective magnetic and capacitive (e.g. MLCC) filter components • Minimal ripple current through LED string September 2006, Rev. 1 www.onsemi.com 1 DN06001/D Schematic Design Notes For low ripple current assume CCM operation. Switches Q1 and Q2 turn on for time D*Ts (D duty cycle, Ts switching period) charging inductor L1 from input Vin. When Q1 and Q2 turn off, diodes D1 and D2 deliver inductor energy to output Vout. For the inductor flux (volt microsecond) to remain in equilibrium after each switching cycle, Vin*D*Ts must equal Vout*(1-D)*Ts, neglecting circuit losses. Hence voltage gain of buck boost is given by Vout = Vin*D/(1-D). When D=0.5, Vout = Vin. Ripple current in inductor obtained from expression ΔI(L1) = Vin*D*Ts/L1. A value for L1 of 150 µH will maintain a +/-15% ripple current in a 700 mA application (3 watt LED’s) assuming Vin =12 V and 200 kHz operation (D*Ts = 0.5*5 µs). The base current for Q1 and Q2 is determined by Vin/R3. (100:1 base drive good starting point but remember to include Vbe/R2 contribution). Q1 and Q2 are turned off passively via base resistors R2 and R4. To improve switch off times, switch Q2 may be held out of saturation by the Baker clamp created by diodes D3a and D3b. Several points of efficiency improvement may be achieved using this technique. Low drop current sensing is achieved across current sense resistor R8. The constant output current is determined from the equation Io*R8 = (Vbe(Q3a)/R6)*R7 where Vo/R9 = 2*Vbe(Q3a)/R6. Vclamp equals Vz(D4) + 1.25 V. Alternatively, if D4 is replaced with a resistor R11 then Vclamp = (1+R11/R1)*1.25 volts. September 2006, Rev. 1 www.onsemi.com 2 DN06001/D Converter Waveforms Boost Mode from 8 Vin to 16 Vout Buck Mode from 16 Vin to 8 Vout September 2006, Rev. 1 www.onsemi.com 3 DN06001/D Efficiency data of Buck-Boost converter between 8 Vin dc and 26 Vin dc driving LED strings at 350 mA and 700 mA 1 OSTAR4 driven at 350mA 90 85 Buck/Boost Efficiency 80 Vo(9V@125C) Vo(12V@typC) Vo(14.5V@25C) 75 70 65 60 5 10 15 20 25 30 Input Voltage Range OSTAR4 driven at 700mA 90 85 Buck/Boost Efficiency 80 Vo(11.4V@125C) Vo(14V@typC) Vo(17.2V@25C) 75 70 65 60 5 10 15 20 25 30 Input Voltage Range 1 © 2006 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Dennis Solley, e-mail: [email protected] September 2006, Rev. 1 www.onsemi.com 4