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AMS2596M Datasheet(PDF) 9 Page - Advanced Monolithic Systems |
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AMS2596M Datasheet(HTML) 9 Page - Advanced Monolithic Systems |
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9 / 15 page  AMS2596 3A 30V Step-Down Converter 3/4/2010 www.advanced-monolithic.com Phone (925) 443-0722 9 Fax (925) 443-0723 There are a wide range 2 and 3 Amp, shielded and non-shielded inductors available. Table 1 lists a few. Table 1. Inductor Selection Guide Series Type Dimensions (mm) W L H Sumida CDRH127 Shielded 12.3 12.3 8 CDRH127/LD Shielded 12.3 12.3 8 CDRH105R Shielded 10.3 10.5 5.1 Coilcraft MSS1246 Shielded 12 12 4.6 MSS1246T Shielded 12 12 4.6 DO5022P Non- Shielded 18 15 7.1 DO5010H Non- Shielded 18 15.3 7.6 Output Capacitor The optimum solution for the switching regulator is to use a large bulk capacitor for large load transients in parallel with a smaller, low ESR, X5R or X7R ceramic capacitor to minimize the switching frequency ripple. High Frequency Ripple The following equation determines the required low ESR ceramic output capacitance for a given inductor current ripple (Ipp). C= Ipp Fs·8·dV = 0.88A 150kHz·8·20mV =36μF Large Signal Transient For applications with large load transients an additional capacitor may be required to keep the output voltage within the limits required during large load transients. In this case the required capacitance can be examined for the load application and load removal. For full load to no load transient the required capacitance is Cbulk= L·Io2 Vos2-Vo2 = 22μH·(3A) 2 (5.2V) 2-(5V)2 =97μF For the application of a load pulse the capacitance required form hold up depends on the time it takes for the power supply loop to build up the inductor current to match the load current. For the AMS2596 this can be estimated to be less than 20 µsec or about three clock cycles. Cbulk= Io·t dV = 3A·20μsec 0.2V =300μF For applications that do not have any significant load transient requirements a ceramic capacitor alone is typically sufficient. Input Capacitor The low esr ceramic capacitor required at the input to filter out high frequency noise as well as switching frequency ripple. Placement of the capacitor is critical for good high frequency noise rejection. See the PCB layout guidelines section for details. Switching frequency ripple is also filtered by the ceramic bypass input capacitor. Given a desired input voltage ripple (Vripple) limit, the required input capacitor can be estimated with: Dmax= Vo+Vfwd Vinmin-V ce+Vfwd Vce is the forward voltage drop of the switching transistor and Vfwd is the external Schottky forward voltage. C= Dmax·Io·(1-Dmax) Fs·Vripple = 5V +0.2V 10V-0.3V+0.2V ·3A· 1- 5V 0.2V 10V-0.3V+0.2V 150kHz·0.2V =25μF . Vce is the forward voltage drop of the switching transistor and Vfwd is the external Schottky forward voltage. For high voltage input converters the duty cycle is always less than 50% so the maximum ripple is at the minimum input voltage. The ripple will increase as the duty cycle approaches 50% where it is a maximum. Feedback Resistor Selection The step down converter and LDO both use a 0.6V reference voltage at the positive terminal of the error amplifier. To set the output voltage a programming resistor form the feedback node to ground must first be selected (R2,R3 of figure 4). A 10kΩ resistor is a good selection for a programming resistor. A higher value could result in an excessively sensitive feedback node while a lower value will draw more |
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