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MP3221 Datasheet(PDF) 11 Page  Monolithic Power Systems 

MP3221 Datasheet(HTML) 11 Page  Monolithic Power Systems 
11 / 14 page MP3221 –2.7A, 1.2MHZ, HIGHEFFICIENCY STEP UP CONVERTER WITH INPUT DISCONNECT MP3221 Rev. 1.0 www.MonolithicPower.com 11 9/23/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2013 MPS. All Rights Reserved. APPLICATION INFORMATION COMPONENT SELECTION Input Capacitor Selection Low ESR input capacitors reduce input switching noise and reduce the peak current drawn from the battery. Ceramic capacitors are good choice for input decoupling and it should be located as close as possible to the IN and GND pin. Add a ceramic capacitor larger than 10μF close to the IC. Selecting the Output Capacitor The output capacitor requires a minimum capacitance value of 22μF at the programmed output voltage to ensure stability over the full operating range. A higher capacitance value may be required to lower the output ripple and also the transient response. Low ESR capacitors, such as X5R or X7Rtype ceramic capacitors, are recommended. Assuming that the ESR is zero, estimate the minimum output capacitance to support the ripple in the CCM mode as: O OUT F IN(MIN) O S OUT F I (V V V ) C f (V V ) V Where: VOUT = output voltage VIN(MIN) = Minimum Input voltage VF= Diode Forward voltage IO= Output current fS = Switching frequency ΔV= Acceptable output ripple 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 2.2μH inductor is recommended for most applications. However, a more exact inductance value can be calculated. A good rule of thumb is to allow the peaktopeak ripple current to be approximately 3050% 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 worstcase load transient and startup conditions. Calculate the required inductance value by the equation: IN OUT F IN OUT F SW V (V V V ) L (V V ) f I OUT LOAD(MAX) IN(MAX) IN VI I V IN(MAX) I (30% 50%)I Where ILOAD(MAX) is the maximum load current, ΔI is the peaktopeak 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 power loss due to diode forward voltage and recovery current, use a Schottky diode with the MP3221. The diode should be rated for a reverse voltage equal to or higher than the output voltage. The average current rating must be higher than the maximum load current, and the peak current rating must be higher than the peak inductor current. Selecting the PMOS The MP3221 is capable of driving PChannel power MOSFETS to disconnect input and output. The critical parameter selection of a MOSFET are: 1. Maximum drain to source voltage, VDS(MAX) 2. Maximum current ID(MAX) 3. Onresistance RDS(ON) 4. Total gate charge, QG. Ideally, the offstate voltage across the MOSFET is equal to the input voltage. VDS(MAX) 
