Datasheet MP1542 (Monolithic Power Systems) - 7

MP1542 – 700KHz/1.3MHz BOOST CONVERTER WITH A 2A SWITCH
At the beginning of each cycle, the N-Channel voltages are equal, the PWM comparator turns MOSFET switch is turned on, forcing the off the switch forcing the inductor current to the inductor current to rise. The current at the output capacitor through the external rectifier. source of the switch is internally measured and This causes the inductor current to decrease. converted to a voltage by the current sense The peak inductor current is controlled by the amplifier. That voltage is compared to the error voltage at COMP, which in turn is controlled by voltage at COMP. The voltage at the output of the output voltage. Thus the output voltage the error amplifier is an amplified version of the controls the inductor current to satisfy the load. difference between the 1.25V reference voltage The use of current mode regulation improves and the feedback voltage. When these two transient response and control loop stability.
APPLICATION INFORMATION Components referenced below apply to Typical Application Circuit on page 1. Selecting the Soft-Start Capacitor
where VOUT is the output voltage. The MP1542 includes a soft-start timer that For R2 = 10kΩ and V limits the voltage at COMP during startup to FB = 1.25V, then prevent excessive current at the input. This R1 (kΩ) = 8kΩ (VOUT – 1.25V). prevents premature termination of the source
Selecting the Input Capacitor
voltage at startup due to input current overshoot. An input capacitor is required to supply the AC When power is applied to the MP1542, and ripple current to the inductor, while limiting enable is asserted, a 6μA internal current noise at the input source. A low ESR capacitor source charges the external capacitor at SS. As is required to keep the noise at the IC to a the SS capacitor is charged, the voltage at SS minimum. Ceramic capacitors are preferred, but rises. The MP1542 internally clamps the tantalum or low-ESR electrolytic capacitors may voltage at COMP to 700mV above the voltage also suffice. at SS. The soft-start ends when the voltage at SS reaches 0.45V. This limits the inductor Use an input capacitor value greater than 4.7μF. current at start-up, forcing the input current to The capacitor can be electrolytic, tantalum or rise slowly to the current required to regulate ceramic. However since it absorbs the input the output voltage. switching current it requires an adequate ripple current rating. Use a capacitor with RMS The soft-start period is determined by the current rating greater than the inductor ripple equation: current (see Selecting The Inductor to t = × determine the inductor ripple current). SS 7 5 C SS Where C To insure stable operation place the input SS (in nF) is the soft-start capacitor from SS to GND, and t capacitor as close to the IC as possible. SS (in µs) is the soft-start period. Alternately a smaller high quality ceramic 0.1μF capacitor may be placed closer to the IC with Determine the capacitor required for a given the larger capacitor placed further away. If soft-start period by the equation: using this technique, it is recommended that the C = × larger capacitor be a tantalum or electrolytic SS 0. 0133 t SS type. All ceramic capacitors should be placed
Setting the Output Voltage
close to the MP1542. Set the output voltage by selecting the resistive voltage divider ratio. Use 10kΩ for the low-side resistor R2 of the voltage divider. Determine the high-side resistor R1 by the equation: R ( 2 − OUT V FB V ) 1 R = FB V 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.