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MP28367DJ Datasheet(PDF) 7 Page - Monolithic Power Systems |
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MP28367DJ Datasheet(HTML) 7 Page - Monolithic Power Systems |
7 / 9 page MP28367 – 1A, 21V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6 MP28367 Rev. 0.9 www.MonolithicPower.com 7 12/13/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2007 MPS. All Rights Reserved. TM APPLICATION INFORMATION Setting Output Voltage The external resistor divider is used to set the output voltage (see the schematic on front page). The feedback resistor R1 also sets the feedback loop bandwidth with the internal compensation capacitor (see Figure 1). R2 can be determined by: 1 V 81 . 0 V 1 R 2 R OUT − = Table 1—Resistor Selection for Common Output Voltages VOUT (V) R1 (kΩ) R2 (kΩ) 1.8 80.6 (1%) 64.9 (1%) 2.5 49.9 (1%) 23.7 (1%) 3.3 49.9 (1%) 16.2 (1%) 5 49.9 (1%) 9.53 (1%) Selecting the Inductor A 1µH to 10µH inductor with a DC current rating of at least 25% percent higher than the maximum load current is recommended for most applications. For highest efficiency, the inductor’s DC resistance should be less than 200mΩ. For most designs, the required inductance value can be derived from the following equation. OSC L IN OUT IN OUT f I V ) V V ( V L × Δ × − × = Where ΔIL is the inductor ripple current. Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor peak current is calculated from: 2 I I I L LOAD ) MAX ( L Δ + = Under light load conditions below 100mA, a larger inductance is recommended for improved efficiency. See Table 2 for suggested inductors. Also note that the maximum recommended load current is 1A if the duty cycle exceeds 35%. Selecting the Input Capacitor The input capacitor reduces the surge current drawn from the input and the switching noise from the device. The input capacitor impedance at the switching frequency should be less than the input source impedance to prevent high frequency switching current from passing through the input. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. For most applications, a 4.7µF capacitor is sufficient. Selecting the Output Capacitor The output capacitor keeps the output voltage ripple small and ensures loop stability. The output capacitor impedance should be low at the switching frequency. Ceramic capacitors with X5R or X7R dielectrics are recommended for their low ESR characteristics. For most applications, a 22µF ceramic capacitor will be sufficient. PC Board Layout The high current paths (GND, IN and SW) should be placed very close to the device with short, direct and wide traces. The input capacitor needs to be as close as possible to the IN and GND pins. The external feedback resistors should be placed next to the FB pin. Keep the switch node traces short and away from the feedback network. External Bootstrap Diode It is recommended that an external bootstrap diode be added when a 5V rail is available in the system. This helps improve the efficiency of the regulator. The bootstrap diode can be a low cost one such as IN4148 or BAT54. MP28367 SW BS 10nF 5V (External) Figure 2—External Bootstrap Diode This diode is also recommended for high duty cycle operation (when IN OUT V V >65%) applications. |
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