MP8768 – 8A, 28V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFET

MP8768 Rev. 1.0

www.MonolithicPower.com

8

12/12/2013

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Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered by

an external bootstrap capacitor. This floating

driver has its own UVLO protection. This UVLO’s

rising threshold is 2.2V with a hysteresis of

150mV. The bootstrap capacitor voltage is

regulated internally (Figure 2). In DCM mode at

will regulate M3 to maintain a 5V BST voltage

across C4.

SW

Figure 2—Internal Bootstrap Charging Circuit

Startup and Shutdown

If both VIN and EN are higher than their

appropriate thresholds, the chip starts. The

reference block starts first, generating stable

reference voltage and currents, and then the

internal regulator is enabled. The regulator

provides

stable

supply

for

the

remaining

circuitries.

Three events can shut down the chip: EN low,

VIN low and thermal shutdown. In the shutdown

procedure, the signaling path is first blocked to

avoid any fault triggering. The COMP voltage and

the internal supply rail are then pulled down. The

floating driver is not subject to this shutdown