MP2467 – 2.5A, 36V, 500KHz STEP-DOWN CONVERTER

MP2467 Rev. 1.1

www.MonolithicPower.com

9

1/25/2010

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Current Comparator and Current Limit

The power MOSFET current is accurately

sensed via a current sense MOSFET. It is then

fed to the high speed current comparator for the

current mode control purpose. The current

comparator takes this sensed current as one of

its inputs. When the power MOSFET is turned

on, the comparator is first blanked till the end of

the turn-on transition to avoid noise issues. The

comparator then compares the power switch

current with the COMP voltage. When the

sensed current is higher than the COMP

voltage, the comparator output is low, turning

off the power MOSFET. The cycle-by-cycle

maximum

current

of

the

internal

power

MOSFET is internally limited.

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.

While the internal supply rail is up, an internal

timer holds the signaling path of the power

MOSFET turn-on at OFF for about 50µs to

blank the startup glitches. When the internal

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

output low to ensure the reamining circuitries

are ready and then slowly ramps up.

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.