DESIGN IDEAS L Compact Controller is a Basic Building Block for Wide Array of DC/DC Conversion Solutions by Victor Khasiev and Hong Ren Introduction CIN 2.2µF x2 4 1• 221k D3 D1 5• 2 51Ω 4.7µF LTC3873 GND 15k Q1 NGATE RUN/SS 70 2000 60 50 POWER LOSS 40 VIN = 72V VIN = 60V 500 VIN = 48V VIN = 36V 0 10000 20 10 0 100 LOAD CURRENT (mA) Figure 2. Efficiency of the converter in Figure 1 peaks at 86%. • VOUT 3.3V 3A 7 8 COUT 100µF 6.3V x3 9 10 21.5k SECONDARY RTN 150pF 200V 220Ω SW FB 1500 1000 30 D2 VCC ITH 2.2nF 2500 T1 PA1861NL Q2 IPRG EFFICIENCY 80 48V Input, 3.3V, 3A Output Flyback Figure 1 shows a nonisolated stepdown converter for telecom and industrial applications with a 36V to 72V input range and a 3.3V, 3A output, impressive for such a compact converter. Efficiency is over 85%, resulting in low power loss. 221k PRIMARY RTN 3000 90 Applications OPTIONAL 36V TO 72V VIN 100 POWER LOSS (mW) One of interesting features of this IC is its programmable current limit. The current sense voltage can be set to 290mV, 110mV or 185mV. This feature allows flexibility in MOSFET selection. If a higher sensing threshold is selected, the circuit is less sensitive to noise and PCB layout. 48V Input, 3.3V, 3A Output Isolated Flyback Figure 3 shows an isolated application. In this case, feedback is provided by the LT4430 optocoupler driver, which controls the PWM via ITH pin of LTC3873. qNo RSENSE™ eliminates the need for current-sensing resistor. qProgrammable soft-start qAdjustable current limit enables a wide range of power MOSFETs qPulse-skipping mode maintains constant frequency operation at light loads. qExtremely small packages: 2.8mm × 2.9mm 8-lead SOT-23 or 3mm × 2mm QFN. EFFICIENCY (%) The LTC3873 is a compact PWM controller that can be used in boost, flyback and SEPIC DC/DC converters. Other features include: qWide input range, suitable for telecom and industrial applications 0.068Ω D1: PDZ6.8B D2: UPS840 D3: BAS516 Q1: FDC2512 Q2: MMBTA42 0.1µF 12.4k 100pF Figure 1. A nonisolated flyback converter Linear Technology Magazine • October 2007 39 L DESIGN IDEAS T1 PA1861NL ISOLATION BARRIER VIN 36V TO 72V 221k OPT 4.7MF 100V MMBTA42 OPT PDZ6.8B OPT 221k 4 1• BAS516 5• 2 517 • VOUT 3.3V 3A 7 8 100MF 6.3V s3 9 10 UPS840 FDC2512 1 2 3 4 LTC3873 IPRG SW ITH RUN/SS FB VCC GND GATE 2.27 2 8 7 1 BAT54CWT1G 0.0687 6 3 5 1MF OPT 4.7MF 1210 AND 0805 0.1MF 2747 6.8k BAT760 4 NEC PS2801-1 1 3 2 BAS516 1 2 3 LT4430 VIN OPTO GND COMP OC 0.6V FB 6 22nF 5 4 100k 330pF 3.01k 22.1k 0.47MF 2200pF 250V AC Figure 3. Isolated converter can be controlled by the LT4430 optoisolator driver, which also provides soft-start and overshoot control. 9V–15V Input, 12V, 2A Output SEPIC Figure 4 shows a SEPIC that converts input voltages that can be higher or lower than the output. The advantage of a SEPIC over a flyback converter is in the higher efficiency and lower EMI. VIN 9V TO 15V 10µF ×3 + A SEPIC converter does not provide isolation. Adjustable Current Limit One of interesting features of this IC is programmable current limit. The current sense voltage can be set to T1 4.56µH BH510-1009 BH ELECTRONICS 1 4 100µF 20V 2 • 3 301Ω 100k 1 2 3 10nF 33.2k 11k 4 LTC3873 IPRG SW ITH RUN/SS FB VCC GND NGATE Conclusion • 10µF 25V UPS840 + Si4840 8 47µF 16V ×3 10µF 16V VOUT 12V 2A The LTC3873 is a constant frequency, current mode controller. It requires no sense resistor and can be used in a wide variety of applications as a boost, flyback and SEPIC converter. L 7 6 5 4.7µF 0.1µF Figure 4. A SEPIC converter for applications with higher power levels and input voltages that can be higher or lower than the output voltage 40 290mV, 110mV or 185mV by tying the IPRG pin to VIN, tying the IPRG pin to GND or leaving it floating, respectively. This feature allows flexibility in MOSFET selection. If a higher sensing threshold is selected, the circuit is less sensitive to noise and PCB layout. Keep in mind that a higher sense voltage results in higher power dissipation in the MOSFET. Want to know more? Visit: www.linear.com or call 1-800-4-LINEAR Linear Technology Magazine • October 2007