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MIC38C42 Datasheet(PDF) 7 Page - Micrel Semiconductor |
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MIC38C42 Datasheet(HTML) 7 Page - Micrel Semiconductor |
7 / 11 page MIC38C42/3/4/5 Micrel, Inc. September 2007 7 M9999-091107 When designing high-frequency converters, avoid capacitive and inductive coupling of the switching waveform into high- impedance circuitry such as the error amplifier, oscillator, and current sense amplifier. Avoid long printed-circuit traces and componentleads.Locateoscillatorandcompensationcircuitry near the IC. Use high frequency decoupling capacitors on VREF, and if necessary, on VDD. Return high di/dt currents directly to their source and use large area ground planes. Buck Converter Refer to figure 1. When at least 26V is applied to the input, C5 is charged through R2 until the voltage VDD is greater than14.5V(theundervoltagelockoutvalueoftheMIC38C42). Output switching begins when Q1 is turned on by the gate drive transformer T1, charging the output filter capacitor C3 through L1. D5 supplies a regulated +12V to VDD once the circuit is running. Current sense transformer CT1 provides current feedback to ISNS for current-mode operation and cycle-by-cycle current limiting. Thisismoreefficientthanahigh-powersenseresistor and provides the required ground-referenced level shift. WhenQ1turnsoff,currentflowcontinuesfromgroundthrough D1 and L1 until Q1 is turned on again. The100VSchottkydiodeD1reducestheforwardvoltagedrop in the main current path, resulting in higher efficiency than could be accomplished using an ultra-fast-recovery diode. R1 and C2 suppress parasitic oscillations from D1. Using a high-value inductance for L1 and a low-ESR capaci- tor for C3 permits small capacitance with minimum output ripple. This inductance value also improves circuit efficiency by reducing the flux swing in L1. Magnetic components are carefully chosen for minimal loss at 500kHz. CT1 and T1 are wound on Magnetics, Inc. P- type material toroids. L1 is wound on a Siemens N49 EFD core. Application Information Familiarity with 384x converter designs is assumed. The MIC38C4x has been designed to be compatible with 384xA series controllers. MIC38C4x Advantages Start-up Current Start-up current has been reduced to an ultra-low 50µA (typi- cal)permittinghigher-valued,lower-wattage,start-upresistors (powerscontrollerduringpowersupplystart-up). Thereduced resistor wattage reduces cost and printed circuit space. Operating Current Operating current has been reduced to 4mA compared to 11mA for a typical bipolar controller. The controller runs cooler and the VDD hold-up capacitance required during start-up may be reduced. Output Driver Complementary internal P- and N-channel MOSFETs pro- duce rail-to-rail output voltages for better performance driving external power MOSFETs. The driver transistor’s low on- resistance and high peak current capability can drive gate capacitances of greater than 1000pF. The value of output capacitance which can be driven is determined only by the rise/fall time requirements. Within the restrictions of output capacityandcontrollerpowerdissipation,maximumswitching frequency can approach 500kHz. Design Precautions When operating near 20V, circuit transients can easily exceed the20Vabsolutemaximumrating,permanentlydamagingthe controller’s CMOS construction. To reduce transients, use a 0.1µF low-ESR capacitor to next to the controller’s supply VDD (or VD for ‘-1’ versions) and ground connections. Film type capacitors, such as Wima MKS2, are recommended. Figure 1. 500kHz, 25W, Buck Converter VOUT 12V, 2A COM P F B I S N S RT/CT GND OUT VDD V R E F C2 1000pF R1 10 1/2W 31DQ10 D1 L1 48µH C3 3.3µF C4 0.1µF 6.19k 1% 1.62k 1% R4 18 C7 200pF R5 16k 0.1µF D3 MBR030 C8 0.1µF T1 6.8k 0.22µF 100k Q1 IRF820 0.1µF C5 4.7µF R2 68k D2 M17Z105 1/4W D5 1N4001 D4 1N765B VIN 26V to 40V CT1 4.7Ω MIC38C42 1 2 3 4 8 7 6 5 0.1µF* *Locate near MIC38C42 supply pins MKS2 |
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