MP4433 - 36V, 3A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MP4433 Rev. 1.01

www.MonolithicPower.com

21

1/12/2018

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BST Refresh

To improve drop out, the MP4433 is designed

to operate at close to 100% duty cycle for as

long as the BST to SW voltage is greater than

2.5V. When the voltage from BST to SW drops

below 2.5V, the HS-FET is turned off using a

UVLO circuit, which forces the LS-FET on to

refresh the charge on the BST capacitor.

Since the supply current sourced from the BST

capacitor is low, the HS-FET can remain on for

more switching cycles than are required to

refresh the capacitor, thus making the effective

duty cycle of the switching regulator high.

The effective duty cycle during dropout of the

regulator is mainly influenced by the voltage

drops across the HS-FET, LS-FET, inductor

resistance, and printed circuit board resistance.

Thermal Shutdown

Thermal shutdown is implemented to prevent

the chip from running away thermally. When the

silicon die temperature exceeds its upper

threshold, the power MOSFETs shut down.

When the temperature drops below its lower

threshold, the chip is enabled again.

Start-Up and Shutdown

If both VIN and EN exceed their appropriate

thresholds, the chip starts up. The reference

block starts first, generating a stable reference

voltage and current, and then the internal

regulator is enabled. The regulator provides a

stable supply for the rest of the circuitries.

While the internal supply rail is up, an internal

timer holds the power MOSFET off for about

50µs to blank the start-up glitches. When the

soft-start block is enabled, it first holds its SS

output low to ensure that the rest of the

circuitries are ready, and then slowly ramps up.

Three events can shut down the chip: VIN low,

EN low, and thermal shutdown. During the

shutdown procedure, the signaling path is

and the internal supply rail are then pulled down.

The floating driver is not subject to this

shutdown command, but its charging path is

disabled.