MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER

MP4570 Rev. 1.01

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16

1/4/2017

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the inductor valley current is lower than a certain

current threshold which is called the valley

current limit. It is very useful to avoid the inductor

current to run away. Also, both the peak current

limit and the valley current limit value are

dependent on the FB voltage. If the feedback

output voltage is higher than the 50%xREF, the

current limit value is as the normal value. If the

feedback output voltage is lower than 50%xRFF,

the current limit will decrease and drop to the half

normal value when the feedback output voltage

is zero. This feature is very helpful to reduce the

OCP thermal dissipation which may especially

get worse when the output voltage is shorted.

Also, it is very helpful to reduce the high inrush

current during the startup.

UVLO Protection

The MP4570 has input under voltage lockout

protection (UVLO). Assuming the EN is active,

the MP4570 is powered on when the input

voltage is higher than the UVLO rising threshold

and is powered off when input voltage drops

below the UVLO falling threshold.

Thermal Shutdown Protection

The thermal shutdown is employed in MP4570 by

monitoring the IC temperature internally. If the

junction temperature exceeds the threshold

turns on again when the temperature drops

Power Good

MP4570 has one power good (PG) pin out to

indicate the normal operation after soft start time.

The PG pin is the open drain of an internal

MOSFET. It should be connected to VDD or

external voltage source through a resistor (i.e.

100kohm). After the input voltage is applied, the

MOSFET is turned on and the PG pin is pulled to

GND before SS is ready. After the FB voltage

reaches 90% REF voltage, the MOSFET turns off

and PG pin is pulled to high by external voltage

source. When the FB voltage drops to 85% REF

voltage, the PG voltage is pulled to GND to

indicate a failure output status.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor (0.1µF typically)

between BST pin and SW pin 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 driver’s UVLO is soft-start related: When the

bootstrap voltage hits its UVLO threshold, the

soft-start circuit resets. When bootstrap UVLO is

gone, the reset is off and then 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, BST and then

to SW.

bootstrap capacitor can be charged. When the

bootstrap capacitor can’t be charged. When low

period to charge. When there is no current in the

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 the

bootstrap voltage is within the normal operational

region.