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LTM4612IV-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTM4612IV-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTM4612 11 4612f Figure 4. Conducted Emission Scan with 24VIN to 12VOUT at 5A (3 × 10μF Ceramic Capacitors on VIN Pads and 1 × 10μF Ceramic Capacitor on VD Pads). APPLICATIONS INFORMATION If lower output ripple is required, the operating frequency f can be increased by adding a resistor RfSET between fSET pin and SGND, as shown in Figure 19. f V Rk OUT fSET = () − 1 5 10 93 1 10 .• || . For output voltages more than 12V, the frequency can be higher than 1MHz, thus reducing the efficiency significantly. Additionally, the minimum off time 400ns normally limits the operation when the input voltage is close to the output voltage. Therefore, it is recommended to lower the fre- quency in these conditions by connecting a resistor (RfSET) from the fSET pin to VIN, as shown in Figure 20. f V Rk Rk OUT fSET fSET = − ⎛ ⎝⎜ ⎞ − 510 393 1 393 1 11 • •• . •. ⎠⎠⎟ The load current can affect the frequency due to its constant on-time control. If constant frequency is a necessity, the PLLIN pin can be used to synchronize the frequency of the LTM4612 to an external clock, as shown in Figures 21 to 23. Input Capacitors LTM4612 is designed to achieve the low input conducted EMI noise due to the fast switching of turn-on and turn-off. In the LTM4612, a high-frequency inductor is integrated into the input line for noise attenuation. VD and VIN pins are available for external input capacitors to form a high frequency π filter. As shown in Figure 18, the ceramic capacitor C1 on the VD pins is used to handle most of the RMS current into the converter, so careful attention is needed for capacitor C1 selection. For a buck converter, the switching duty cycle can be estimated as: D V V OUT IN = Without considering the inductor current ripple, the RMS current of the input capacitor can be estimated as: I I DD CIN RMS OUT MAX () () •• – = () η 1 In this equation, η is the estimated efficiency of the power module. Note the capacitor ripple current ratings are often based on temperature and hours of life. This makes it advisable to properly derate the input capacitor, or choose a capacitor rated at a higher temperature than required. Always contact the capacitor manufacturer for derating requirements. In a typical 5A output application, one very low ESR, X5R or X7R, 10μF ceramic capacitor is recommended for C1. This decoupling capacitor should be placed directly adja- cent to the module VD pins in the PCB layout to minimize the trace inductance and high frequency AC noise. Each 10μF ceramic is typically good for 2A to 3A of RMS ripple current. Refer to your ceramics capacitor catalog for the RMS current ratings. To attenuate the high frequency noise, extra input capacitors should be connected to the VIN pads and placed before the high frequency inductor to form the π filter. One of these low ESR ceramic input capacitors is recommended to be close to the connection into the system board. A large bulk 100μF capacitor is only needed if the input source impedance is compromised by long inductive leads or traces. Figure 4 shows the conducted EMI testing results to meet the Level 5 of the CISPR 25 limit. For different applications, input capacitance may be varied to meet different conducted EMI limits. FREQUENCY (MHz) 0.15 0 20 40 60 80 10 30 50 70 130 10 4612 F04 CIS25QP |
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