MP4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER

MP4560 Rev. 1.0

www.MonolithicPower.com

9

11/5/2012

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© 2012 MPS. All Rights Reserved.

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.

Short Circuit Protection

When the output is shorted to the ground, the

switching frequency is folded back and the

current limit is reduced to lower the short circuit

current. When the voltage of FB is at zero, the

current limit is reduced to about 50% of its full

current limit. When FB voltage is higher than

0.4V, current limit reaches 100%.

In short circuit FB voltage is low, the SS is pulled

down by FB and SS is about 100mV above FB.

In case the short circuit is removed, the output

voltage will recover at the SS pace. When FB is

high enough, the frequency and current limit

return to normal values.

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 power MOSFET 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 remaining 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, power MOSFET is turned off first to

avoid any fault triggering. The COMP voltage and

the internal supply rail are then pulled down.

Programmable Oscillator

The MP4560 oscillating frequency is set by an

from:

FREQ

S

100000

R(kΩ)=

- 5

f(kHz)