MP28256 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER

MP28256 Rev. 1.0

www.MonolithicPower.com

9

11/3/2011

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© 2011 MPS. All Rights Reserved.

Thermal Shutdown

Thermal shutdown is implemented to prevent the

chip

from

operating

at

exceedingly

high

temperatures. When the silicon die temperature

is higher than 150

C, it shuts down the whole

chip. When the temperature is lower than its

lower threshold, typically 140

C, the chip is

enabled again.

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 by VIN through D1, M1, C4,

L1 and C2 (Figure 2). If (VIN-VSW) is more than

5V, U1 will regulate M1 to maintain a 5V BST

voltage across C4.

D1

M1

BST

C4

L1

SW

U1

5V

C2

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

command.