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LM2655 데이터시트(PDF) 11 Page - Texas Instruments |
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LM2655 데이터시트(HTML) 11 Page - Texas Instruments |
11 / 18 page Design Procedure (Continued) ment. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple current in- creases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss equal 2% of the output power. OUTPUT CAPACITOR The selection of C OUT is primarily determined by the maxi- mum allowable output voltage ripple. The output ripple in the constant frequency, PWM mode is approximated by: The ESR term usually plays the dominant role in determining the voltage ripple. A low ESR aluminum electrolytic or tanta- lum capacitor (such as Nichicon PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX TPS, and CDE polymer alumi- num) is recommended. An electrolytic capacitor is not rec- ommended for temperatures below −25˚C since its ESR rises dramatically at cold temperature. A tantalum capacitor has a much better ESR specification at cold temperature and is preferred for low temperature applications. The output voltage ripple in constant frequency mode has to be less than the sleep mode voltage hysteresis to avoid entering the sleep mode at full load: V RIPPLE < 20mV * VOUT /VFB TABLE 1. MOSFET Manufacturers Manufacturer Model Number Package Type www Address Phone Fax Fairchild Semiconductor FDC653N SuperSOT-6 www.fairchildsemi.com 888-522-5372 207-761-6020 General Semiconductor GF4420 SO-8 www.gensemi.com 631-847-3000 631-847-3236 International Rectifier IRF7807 SO-8 www.irf.com 310-322-3331 310-322-3332 Vishay Siliconix Si4812DY SO-8 www.vishay.com 800-554-5565 408-567-8995 Si4874DY SO-8 Zetex ZXM64N03X SO-8 www.zetex.com (44) 161-622-4422 (44) 161-622-4420 LOW-SIDE MOSFET SELECTION When operating in synchronous mode, special attention should be given to the selection of the low-side MOSFET. Besides choosing a MOSFET with minimal size and on resistance, it is critical that the MOSFET meet certain rise and fall time specifications. A 30ns deadtime between the low-side and high-side MOSFET switching transitions is pro- grammed into the LM2655, as shown in Figure 1. The pre- vent shoot-through current, the low-side MOSFET must turn off before the high-side MOSFET turns on. Hence, the low- side MOSFET has 30ns to turn off from the time the low-side driver goes low. The fall time of the low-side MOSFET is governed by the equation: I C =CIN*dVC/dt. where I C is the LDR sink current capability, CIN is the equiva- lent capacitance seen at the LDR pin, and V C is the gate-to- source voltage of the MOSFET. I C is limited by the low-side driver of the LM2655, but C IN is fixed by the MOSFET. Therefore, it is important that the chosen MOSFET has a suitable C IN so that the LM2655 will be able to turn it off 10128421 FIGURE 1. Low-side/high-side driver timing diagram. www.national.com 10 |
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