MP4558

– 1A, 2MHz, 55V STEP-DOWN CONVERTER

MP4558 Rev. 1.01

www.MonolithicPower.com

11

10/28/2013

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Current Comparator and Current Limit

A current-sense MOSFET accurately senses the

current in the power MOSFET. This signal is then

fed to the high speed current comparator for

current-mode

–control purposes, which uses it as

one of its inputs with the COMP voltage. When

the power MOSFET turns on, the comparator is

first blanked until the end of the turn-on transition

to avoid noise issues. When the sensed current

exceeds the COMP voltage, the comparator

output is low and the power MOSFET turns off.

The cycle-by-cycle maximum current of the

internal power MOSFET is internally limited.

Short-Circuit Protection

When the output is shorted to the ground, the

switching frequency folds back and the current

limit falls to reduce the short circuit current. When

the FB voltage equals 0V, the current limit falls to

about 50% of its full current limit. The FB voltage

reaches its 100% of its current limit when it

exceeds 0.4V

When the short-circuit FB voltage is low, the SS

circuit is removed, the output voltage recovers at

the SS rate. When FB is high enough, the

frequency and current limit return to normal

values.

Startup and Shutdown

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 blanks the power MOSFET OFF for about

50µs to avoid start-up glitches. When the internal

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

output low to ensure the other circuits are ready

and then slowly ramps up.

low and thermal shutdown. In shutdown, the

power MOSFET turns off first to avoid any fault

triggering. The COMP voltage and the internal

supply rail are then pulled down.

Programmable Oscillator

An external resistor

—R

FREQ pin to GND

—sets the MP4558 oscillating

FREQ

S

100000

R

(k

Ω) =

- 5

f (kHz)