MP24971 –1.5A, 50V, 100kHz, 5V FIXED OUTPUT STEP-DOWN CONVERTER

MP24971 Rev. 1.01

www.MonolithicPower.com

9

1/6/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

Main Control Loop

The MP24971 is a current-mode buck regulator

where the output voltage of the error amplifier

(EA) is proportional to the peak inductor

current.

When a cycle starts, SW is disconnected and

the EA output voltage is higher than the current

sense amplifier (CSA) output. The rising edge

of the 100kHz CLK signal sets the RS flip-flop,

which connects SW and the inductor to the

input supply.

The CSA amplifies the rising inductor current.

sum of the CSA output and the ramp

compensation (positive input) against the EA

output (negative input). When the positive

input, the RS flip-flop resets and the MP24971

reverts to its initial SW off state: Otherwise, the

falling edge of the CLK resets the flip-flop.

The EA amplifies the voltage difference

connects to the negative input of the EA, when

Since the EA output voltage is proportional to

the peak inductor current, an increase in

inductor voltage increases current delivered to

the

output.

An

external

Schottky

Diode

dissipates the inductor current when SW is off.

Hiccup-Mode Current Limit Protection

The ISP and ISN pins sense the output current

information for current limit protection. Once the

threshold, the current limit loop will turn off the

high-side MOSFET immediately. The internal

soft-start circuit resets after the VOS voltage

this point, the high side switch turns on and

MP24971 restarts with a full soft start. This

hiccup process repeats until the device stops

detecting a fault.

The current limit value can be lowered by an

internal current source and external resistors

connected to ISN and ISP pins when the output

voltage drops below 200mV. This feature can

greatly reduce the average short circuit current.

Output Over-Voltage Protection

The

MP24971

has

output

over-voltage

protection to prevent the output voltage from

rising above a threshold of 6V—such as if the

output sense is left open—and prevents any

voltage damage to the circuit load. If the output

voltage—as sensed by the ISN pin—rises

above 6V, the high-side MOSFET will turn off

immediately and part will be lathed off after a

timer delay.

Output-Line–Drop Compensation

MP24971 is capable of compensating an output

voltage drop—such as high impedance caused

by a very long trace—to keep a constant 5V

load-side voltage. Compensation feedback

resistor. Program the load line compensation

gain using a resistor connected to VOC pin,

values.