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MP3209DGU Datasheet(PDF) 7 Page - Monolithic Power Systems |
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MP3209DGU Datasheet(HTML) 7 Page - Monolithic Power Systems |
7 / 11 page MP3209 – 1.4MHZ, 350mA BOOST CONVERTER MP3209 Rev.0.941 www.MonolithicPower.com 7 11/30/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2011 MPS. All Rights Reserved. APPLICATIONS INFORMATION COMPONENT SELECTION Setting the Output Voltage Set the output voltage by selecting the resistive voltage divider ratio. Use 510kΩ for the high- side resistor R1 of the voltage divider. Determine the low-side resistor R2 by the equation: FB OUT FB V V V R1 R2 − × = where VOUT is the output voltage. (It is recommended to add feed-forward resistor R3 and C3 to improve the transient performance.) (Figure 2) Selecting the Input Capacitor An input capacitor is required to supply the AC ripple current to the inductor, while limiting noise at the input source. This capacitor must have low ESR, so ceramic is the best choice. Use an input capacitor value of 4.7μF or greater. This capacitor must be placed physically close to the IN pin. Since it reduces the voltage ripple seen at IN, it also reduces the amount of EMI passed back along that line to the other circuitry. Selecting the Output Capacitor A single 4.7 μF to 10μF ceramic capacitor usually provides sufficient output capacitance for most applications. If larger amount of capacitance is desired for improved line support and transient response, tantalum capacitors can be used in parallel with the ceramic. The impedance of the ceramic capacitor at the switching frequency is dominated by the capacitance, and so the output voltage ripple is mostly independent of the ESR. The output voltage ripple VRIPPLE is calculated as: ( ) SW UT O IN UT O LOAD RIPPLE f 2 C V V V I V × × − = Where VIN is the input voltage, ILOAD is the load current, C2 is the capacitance of the output capacitor, and fSW is the 1.4MHz switching frequency. Selecting the Inductor The inductor is required to force the output voltage higher while being driven by the lower input voltage. Choose an inductor that does not saturate at the SW current limit. A good rule for determining the inductance is to allow the peak- to-peak ripple current to be approximately 30%- 50% of the maximum input current. Make sure that the peak inductor current is below 75% of the typical current limit at the duty cycle used to prevent loss of regulation due to the current limit variation. Calculate the required inductance value L using the equations: I f V ) V - (V V L SW OUT IN OUT IN Δ × × = η × × = IN ) MAX ( LOAD OUT ) MAX ( IN V I V I ( ) ) MAX ( IN I % 50 % 30 I − = Δ Where ILOAD(MAX) is the maximum load current, ΔI is the peak-to-peak inductor ripple current, and η is efficiency. For the MP3209, 10µH is recommended for input voltages less than 3.3V and 22µH for inputs greater than 3.3V. Selecting the Diode The output rectifier diode supplies current to the inductor when the internal MOSFET is off. To reduce losses due to diode forward voltage and reverse recovery, use a Schottky diode. Choose a diode whose maximum reverse voltage rating is greater than the maximum output voltage. For output voltage less than 20V, it is recommended to choose the CMH5H-3 for most applications. This diode is used for load currents less than 300mA. For ultra-low-profile applications, choose BAT54-02 Schottky diode. Use two BAT54-02 in parallel for high load current. |
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