Datasheet MP1542 (Monolithic Power Systems) - 8

MP1542 – 700KHz/1.3MHz BOOST CONVERTER WITH A 2A SWITCH Selecting the Output Capacitor Selecting the Inductor
The output capacitor is required to maintain the The inductor is required to force the higher DC output voltage. Low ESR capacitors are output voltage while being driven by the input preferred to keep the output voltage ripple to a voltage. A larger value inductor results in less minimum. The characteristic of the output ripple current that results in lower peak inductor capacitor also affects the stability of the current, reducing stress on the internal regulation control system. Ceramic, tantalum, or N-Channel.switch. However, the larger value low ESR electrolytic capacitors are inductor has a larger physical size, higher recommended. In the case of ceramic series resistance, and/or lower saturation capacitors, the impedance of the capacitor at current. the switching frequency is dominated by the A 4.7µH inductor is recommended for most capacitance, and so the output voltage ripple is 1.3MHz applications and a 10µH inductor is mostly independent of the ESR. The output recommended for most 700kHz applications. voltage ripple is estimated to be: However, a more exact inductance value can ⎛ ⎞ be calculated. A good rule of thumb is to allow IN V - 1 × ILOAD ⎜⎜ ⎟⎟ the peak-to-peak ripple current to be ⎝ OUT V ⎠ ≈ RIPPLE V approximately 30-50% of the maximum input 2 C × fSW current. Make sure that the peak inductor Where V current is below 75% of the current limit at the RIPPLE is the output ripple voltage, VIN and V operating duty cycle to prevent loss of OUT are the DC input and output voltages respectively, I regulation due to the current limit. Also make LOAD is the load current, fSW is the switching frequency, and C2 is the capacitance sure that the inductor does not saturate under of the output capacitor. the worst-case load transient and startup conditions. Calculate the required inductance In the case of tantalum or low-ESR electrolytic value by the equation: capacitors, the ESR dominates the impedance at the switching frequency, and so the output V × (V V - ) L IN OUT IN = ripple is calculated as: V × f × I Δ OUT SW IN V V ×I 1 ( − ) × I OUT LOAD(MAX) LOAD × × I = ( IN MAX) OUT V ILOAD RESR OUT V ≈ + V ×η RIPPLE V IN 2 C × f SW IN V I Δ = (30% − 50%) ( IN I MAX) Where RESR is the equivalent series resistance of the output capacitors. Where ILOAD(MAX) is the maximum load current, ΔI is the peak-to-peak inductor ripple current, and η Choose an output capacitor to satisfy the output is efficiency. ripple and load transient requirements of the design. A 4.7μF-22μF ceramic capacitor is
Selecting the Diode
suitable for most applications. 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. MP1542 Rev. 1.5 www.MonolithicPower.com
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6/22/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2011 MPS. All Rights Reserved.