MP4458 – 1A, 4MHz, 36V STEP-DOWN CONVERTER

MP4458 Rev. 1.02

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8

3/4/2013

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from VIN, BST and then to SW. External circuit

should provide enough voltage headroom to

facilitate the charging.

As long as VIN is sufficiently higher than SW, the

bootstrap capacitor can be charged. When the

power MOSFET is ON, VIN is about equal to SW

so the bootstrap capacitor cannot be charged.

When the external diode is on, the difference

between VIN and SW is largest, thus making it

the best period to charge. When there is no

current in the inductor, SW equals the output

capacitor.

At higher duty cycle operation condition, the time

period available to the bootstrap charging is less

so the bootstrap capacitor may not be sufficiently

charged.

In case the internal circuit does not have

sufficient voltage and the bootstrap capacitor is

not charged, extra external circuitry can be used

to ensure the bootstrap voltage is in the normal

operational region. Refer to External Bootstrap

Diode

in Application section.

The DC quiescent current of the floating driver is

about 20µA. Make sure the bleeding current

at the SW node is higher than this value, such

that:

A

20

)

2

R

1

R

(

V

I

O

O

μ

>

+

+

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 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 MP4458 oscillating frequency is set by an

refer to table1 in Application section.