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MP1570DN Datasheet(PDF) 5 Page - Monolithic Power Systems |
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MP1570DN Datasheet(HTML) 5 Page - Monolithic Power Systems |
5 / 11 page MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER MP1570 Rev. 1.5 www.MonolithicPower.com 5 1/31/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. TM OPERATION FUNCTIONAL DESCRIPTION The MP1570 is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages from 4.75V to 23V down to an output voltage as low as 1.230V, and supplies up to 3A of load current. The MP1570 uses current-mode control to regulate the output voltage. The output voltage is measured at FB through a resistive voltage divider and amplified through the internal transconductance error amplifier. The voltage at COMP pin is compared to the switch current measured internally to control the output voltage. The converter uses internal N-Channel MOSFET switches to step-down the input voltage to the regulated output voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor connected between SW and BS is needed to drive the high side gate. The boost capacitor is charged from the internal 5V rail when SW is low. When the MP1570 FB pin exceeds 20% of the nominal regulation voltage of 1.230V, the over voltage comparator is tripped and latched; the COMP pin and the SS pin are discharged to GND, forcing the high-side switch off. Latch cannot be cleared unless the EN or IN pin is reset. Following discharge, the MP1570 actively recycles the energy stored in the output capacitor. Initially the low-side synchronous rectifier turns on. Once the internal, negative 900mA current limit is reached, the low-side switch turns off, forcing inductor current to flow through the high-side switch body diode. The inductor current is recycled back into the input as an energy saving feature. This cycle continues until the output voltage is discharged below 10% of the initial regulation voltage (0.123V at FB), at which time the low-side switch turns off. APPLICATIONS INFORMATION COMPONENT SELECTION Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio: 2 R 1 R 2 R V V OUT FB + = Thus the output voltage is: 2 R 2 R 1 R 23 . 1 VOUT + × = Where VFB is the feedback voltage and VOUT is the output voltage. A typical value for R2 can be as high as 100kΩ, but a typical value is 10kΩ. Using that value, R1 is determined by: ) k )( 23 . 1 V ( 18 . 8 1 R OUT Ω − × = For example, for a 3.3V output voltage, R2 is 10kΩ, and R1 is 17kΩ. Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the |
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