MP4575

– 5A, 55V, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4575 Rev. 1.0

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16

8/16/2016

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threshold (valley current limit). This is useful for

preventing an inductor current runaway. Both

the peak current limit and the valley current limit

values are dependent on the FB voltage. If the

feedback

output

voltage

is

higher

than

50%xREF, the current limit value is normal. If

the feedback output voltage is lower than

50%xREF, the current limit decreases to half

the normal value when the feedback output

voltage is zero. This feature is useful for

reducing OCP thermal dissipation, which may

worsen when the output voltage is shorted. It is

also useful for reducing high inrush current

during start-up.

Under-Voltage Lockout (UVLO) Protection

The

MP4575

has

an

input

under-voltage

lockout (UVLO) protection. When EN is active,

the MP4575 is powered on when the input

voltage is higher than the UVLO rising threshold,

and is powered off when the input voltage drops

below the UVLO falling threshold.

Thermal Shutdown Protection

The thermal shutdown is employed in the

MP4575 by monitoring the IC temperature

internally. If the junction temperature exceeds

the threshold (typically 170°C), the regulator

shuts

off

and turns on

again

when

the

temperature drops below 160°C. There is a

hysteresis of about 10°C.

Power Good (PG)

The MP4575 uses one power good (PG) pin out

to indicate normal operation after the soft-start

time.

PG is the open drain of the internal MOSFET.

PG should be connected to VDD or an external

voltage source through a resistor (i.e.: 100k

Ω).

After the input voltage is applied, the MOSFET

is turned on, and PG is pulled to GND before

SS is ready. After the FB voltage reaches 90%

of the REF voltage, the MOSFET turns off, and

PG is pulled high by an external voltage source.

When the FB voltage drops to 85% of the REF

voltage, the PG voltage is pulled to GND to

indicate a failure output status.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor (typically 0.1µF)

between BST and SW powers the floating

power MOSFET driver. This floating driver has

its own UVLO protection. This UVLO’s rising

threshold is 2.3V with a hysteresis of 300mV.

The driv

er’s UVLO is soft-start related. When

the

bootstrap

voltage

reaches

its

UVLO

threshold, the soft-start circuit resets. When the

bootstrap UVLO is removed, the soft-start reset

is off, and the soft-start process resumes.

The

dedicated

internal

bootstrap

regulator

regulates and charges the bootstrap capacitor

to 4.2V. When the voltage between the BST

and SW nodes is less than its regulation, a

PMOS pass transistor from VIN to BST turns

on. The charging current path is from VIN to

BST to SW.

the bootstrap capacitor can be charged. When

the HS-FET is on, VIN

capacitor cannot be charged. When the LS-FET

its largest, making this the best period to

charge. When there is no current in the

VIN

and

can

charge

the

bootstrap

capacitor.

At higher duty cycles, the time period available

for bootstrap charging is shorter,

so the

bootstrap capacitor may not be sufficiently

charged. If the internal circuit does not have

sufficient voltage, and the bootstrap capacitor is

not charged, extra external circuitry can be

used to ensure that the bootstrap voltage is

within the normal operating region.