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MP1542DK Datasheet(PDF) 7 Page - Monolithic Power Systems |
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MP1542DK Datasheet(HTML) 7 Page - Monolithic Power Systems |
7 / 10 page MP1542 – 700KHz/1.3MHz BOOST CONVERTER WITH A 2A SWITCH MP1542 Rev. 1.3 www.MonolithicPower.com 7 9/7/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. Selecting the Output Capacitor The output capacitor is required to maintain the DC output voltage. Low ESR capacitors are preferred to keep the output voltage ripple to a minimum. The characteristic of the output capacitor also affects the stability of the regulation control system. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. In the case of ceramic capacitors, the impedance of the 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 is estimated to be: SW LOAD OUT IN RIPPLE f 2 C I V V - 1 V × × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ≈ Where VRIPPLE is the output ripple voltage, VIN and VOUT are the DC input and output voltages respectively, ILOAD is the load current, fSW is the switching frequency, and C2 is the capacitance of the output capacitor. In the case of tantalum or low-ESR electrolytic capacitors, the ESR dominates the impedance at the switching frequency, and so the output ripple is calculated as: IN OUT ESR LOAD SW LOAD OUT IN RIPPLE V V R I f 2 C I ) V V 1 ( V × × + × × − ≈ Where RESR is the equivalent series resistance of the output capacitors. Choose an output capacitor to satisfy the output ripple and load transient requirements of the design. A 4.7µF-22µF ceramic capacitor is suitable for most applications. Selecting the Inductor The inductor is required to force the higher output voltage while being driven by the input voltage. A larger value inductor results in less ripple current that results in lower peak inductor current, reducing stress on the internal N-Channel.switch. However, the larger value inductor has a larger physical size, higher series resistance, and/or lower saturation current. A 4.7µH inductor is recommended for most 1.3MHz applications and a 10µH inductor is recommended for most 700KHz applications. However, a more exact inductance value can be calculated. A good rule of thumb 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 current limit at the operating duty cycle to prevent loss of regulation due to the current limit. Also make sure that the inductor does not saturate under the worst-case load transient and startup conditions. Calculate the required inductance value by the equation: I f V ) V - (V V L SW OUT IN OUT IN ∆ × × = × η × × = IN ) MAX ( LOAD OUT ) MAX ( IN V I V I ( ) ) MAX ( IN I % 50 % 30 I − = ∆ Where ILOAD(MAX) is the maximum load current, ∆I is the peak-to-peak inductor ripple current, and η is efficiency. 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 recovery time, use a Schottky diode with the MP1542. The diode should be rated for a reverse voltage equal to or greater than the output voltage used. The average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. |
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