MIC2172/3172 SEPIC Evaluation Board Micrel MIC2172/3172 SEPIC Evaluation Board 100kHz Switching Regulator by Steve Chenetz The maximum input voltage of the board is 16V, limited by the input capacitor’s voltage rating. Exceeding this voltage for an extended period of time will damage the capacitors. The tantalum output capacitors are rated at 10V. Do not exceed the 10V rating of these capacitors. Circuit Operation The schematic of the MIC3172/MIC2172 SEPIC evaluation board is shown in Figure 1. The MIC3172 switching regulator IC combines a switching transistor, control circuitry, and reference in a single package, which provides switching and output voltage regulation. When the MIC3172 switching transistor turns on, the current flowing into the switch is equal to the sum of the currents in L1 and L2 (IL1 + IL2). Energy is stored in both inductors. The voltage across L1 is equal to VIN (neglecting the switch voltage) and the voltage at the anode of D1 is –VIN. During this on time the diode, D1, sees a reverse voltage of VIN + VOUT. When the switching transistor turns off, the currents flowing in the inductors forces the voltage across each inductor to reverse polarity. The voltage across the inductors rise until D1 conducts and clamps the inductor voltage at VOUT + VD1. The energy in the inductors is discharged into the output. The peak diode current is equal to IL1 + IL2. R7, C5 and C6 provide compensation to stabilize the voltage control loop. R7 and C5 create a zero in the feedback loop to compensate for the pole formed by the output capacitance (C3, C4) and the output resistance. C6 creates a pole that compensates for the zero caused by the output capacitance and the output capacitor ESR. General Description This evaluation board demonstrates how the MIC3172 or MIC2172 may be used in a SEPIC converter topology. The SEPIC (single-ended primary inductance converter) topology provides a regulated output voltage from an input that can range from greater than to less than the output voltage. It also capacitively decouples the input from the output, completely shutting down the output (zero volts) when the converter stops switching. The entire circuit takes up less than 0.6 in2 of board space while producing up to 1.5W of output power. The switching frequency is 100kHz. Its efficiency at full load is better than 65%. The MIC3172 features a low quiescent current shutdown pin. The MIC2172 features a sync pin for external synchronization. Requirements The SEPIC evaluation board requires an input power source, which should be able to supply a minimum of 0.5A. The output may be loaded with a resistive or active load. The active load may be set for constant current or constant resistance mode. Precautions The evaluation board does not have input reverse polarity protection. Applying a negative voltage to the VIN terminal (with respect to ground) may permanently damage the components on the board. C2 10µF, 16V L1a 33µH VIN 1 ON S1 Enable C1 10µF 16V 4 2 SYNC IN C7 33pF R8 open R7 4.75k C5 0.01µF VOUT 3 L1b 33µH MIC3172 5 OFF 2 D1 MBRM140 C3 220µF 10V C4 220µF 10V C8 0.1µF 4 IN SW EN FB COMP 7 3 R2* SGND PGND1 PGND2 1 8 R1 3.01k R3 1.82k R4 1.62k R5 1k R6 787 6 J1a* C6 3300pF J1b 3.3V J1c 3.6V J1d 5.0V J1e 6.0V GND GND D2 CMPD6263 * for other voltages Note 1: Remove switch S1 when using the MIC2172 (sync input). Note 2: C7, R8 and D2 are not used with the MIC3172 (enable input). Single-Ended Primary Inductance Converter Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com August 1998 1 MIC2172/3172 SEPIC Evaluation Board MIC2172/3172 SEPIC Evaluation Board Micrel MIC3172 (Enable Input) The board comes assembled for use with the MIC3172 converter IC which has the enable function. Switch S1 is used to enable (pin 4 high) or disable (pin 4 low) the converter. MIC2172 (Sync Input) If a sync function is required, the control IC, U1, is replaced with the MIC2172BM. The enable switch, S1, must be removed for proper operation. C7 capacitively couples the sync signal to the IC. Diode D2 prevents the sync signal from going more than a diode drop below ground. An 0805-size resistor may be added in the R8 location to adjust the switching frequency. Refer to the MIC2172 data sheet for additional information on using the sync input. Output Voltage Configuration One of four standard voltages may be selected by placing a shorting jumper across the corresponding pins of header J1. Open resistor location R2 accepts an 0805-size resistor for a nonstandard output voltage. For a desired output voltage, calculate the value of resistor R2 using: R2 = 1.24R1 VOUT − 1.24 Output Capacitors Both the output capacitance and the capacitor’s ESR (equivalent series resistance) affect output ripple. ESR is the primary contributor to output ripple. Two low impedance tantalum capacitors are provided to minimize the effective ESR and reduce the output ripple voltage. While two capacitors may be necessary at higher output currents, only one capacitor may be necessary when the output current is less than 100mA or when ripple voltage is not critical. Bill of Materials Item Part Number Manufacturer U1 MIC3172BM Micrel Description 100kHz 1.25A switching reg. C1, C2 C23Y5V1C106ZP Tokin ceramic C3, C4 TPSE227M010R0060 AVX C5 08053C103MATMA AVX C6 08053C332MATMA C7 08055A330MATMA C8 08055A104MATMA D1 MBRM140 Motorola D2 CMPD6263 Central Semiconductor L1 CTX33-2P Coiltronics Value Quantity 1 10µF, 16V 2 tantalum 220µF, 10V 2 ceramic 0.01µF 1 AVX ceramic 3300pF 1 AVX ceramic 33pF 1 ceramic, X7R 0.1µF 1 Schottky 1A, 40V 1 AVX signal Schottky 1 inductor 1 R1 0805-size resistor R2 0805-size resistor open resistor location 1 R3 0805-size resistor 1.82k, 1% 1 R4 0805-size resistor 1.62k, 1% 1 R5 0805-size resistor 1.00k, 1% 1 R6 0805-size resistor 787Ω, 1% 1 R7 0805-size resistor 4.75k, 1% 1 R8 0805-size resistor open resistor location 1 S1 JP1A-E 2551-2-00-01-00-00-07-0 Mill-Max MIC2172/3172 SEPIC Evaluation Board 3.01k, 1% 1 toggle switch SPDT 1 jumper turret pins 1 2 August 1998 MIC2172/3172 SEPIC Evaluation Board August 1998 Micrel Component Side Solder Side Component-Side Silk Screen Solder-Side Silk Screen 3 MIC2172/3172 SEPIC Evaluation Board MIC2172/3172 SEPIC Evaluation Board Micrel MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. © 1998 Micrel Incorporated MIC2172/3172 SEPIC Evaluation Board 4 August 1998