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MAX747ESD Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX747ESD Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page gate capacitance, because that energy is dissipated by the gate-drive circuitry, not the P-FET. The Standard Application Circuit (Figure 1a, 1b) uses an 8-pin Si9405DY surface-mount P-FET that has 0.1 Ω on resistance with a 10V VGS. Optimum efficiency is obtained when the voltage at the drain swings between the supply rails (within a few hundred mV). Diode Selection The MAX747’s high switching frequency demands a high-speed rectifier. Schottky diodes are recommended. Ensure that the Schottky diode average current rating exceeds the load current level. Capacitor Selection Output Filter Capacitor The output filter capacitor C1 should have a low effective series resistance (ESR), and its capacitance should remain fairly constant over temperature. This is especially true when in CCM, since the output filter capacitor and the load form the dominant pole that stabilizes the loop. 430µF is adequate for load currents up to 2.3A in Figure 1a. At low input/output differentials, it may be necessary to use much larger output filter capacitors to maintain adequate load- transient response. See the AC Stability with Low Input/Output Differentials section. Sprague 595D surface-mount solid tantalum capacitors and Sanyo OS-CON through-hole capacitors are recommended due to their extremely low ESR. OS-CON capacitors are particularly useful at low temperatures. For best results when using other capacitors, increase the output filter capacitor’s size or use capacitors in parallel to reduce ESR. Input Bypass Capacitor The input bypass capacitor C2 reduces peak currents drawn from the voltage source, and also reduces noise at the voltage source caused by the MAX747’s fast switching action (this is especially important when other circuitry is operated from the same source). The input capacitor ripple current rating must exceed the RMS input current. For load currents up to 2.5A, 100µF (C2) in parallel with a 0.1µF (C3) is adequate. Smaller bypass capacitors may be acceptable for lighter loads. The input voltage source impedance determines the capacitor size required at the V+ input. As with the output filter capacitor, a low-ESR capacitor (Sanyo OS-CON, Sprague 595D, or equivalent) is recommended for input bypassing. Soft-Start and Reference Capacitors A typical value for the soft-start capacitor C4 is 0.1µF, which provides a 380ms ramp to full current limit. Use values in the 0.001µF and 1µF range. The nominal time for C4 to reach its steady-state value is given by: Note that tSS does not equal the time it takes for the MAX747 to power up, although it does affect start-up time. Start-up time is also a function of the input voltage and load current. With a 2.5A load current, a 7V input voltage, and a 0.1µF soft-start capacitor, power-up takes typically 360ms. Bypass REF with a 0.22µF capacitor (C5). Compensation Capacitor With a fixed +5V output, connect the compensation capacitor (C6) between CC and GND to optimize transient response. Appropriate compensation is determined by the ESR of the output filter capacitor (C1) and the feedback voltage-sense resistor network. 270pF is adequate for applications where V+ ≤ 9V. Over the full input voltage range, increase C6 to 470pF. C6 also depends on the load current, so for light loads, C6’s value can be reduced. If appropriate compensation is not obtained using 470pF, use the following equations to determine C6: For fixed 5V output operation, For adjustable-output operation, FB becomes the compensation input pin and CC is left unconnected. Connect C6 between FB and GND in parallel with R4 (Figure 5). C6 is determined by: For example, with a fixed 5V output, C1 = 330µF and an ESRC1 of 0.04Ω (at a 100kHz frequency), C6 (C1) (ESR ) 24k 783pF C1 = Ω = C6 (C1) (ESR ) R4 II R5 C1 = C6 (C1) (ESR ) 24k C1 = Ω t (sec) (C4) (3.8 10 ) SS 6 =× I RMS AC input current I V(V V) V RMS LOAD OUT IN OUT IN = = − High-Efficiency PWM, Step-Down P-Channel DC-DC Controller 10 ______________________________________________________________________________________ |
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