MP9447–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP9447 Rev. 1.2

www.MonolithicPower.com

13

1/5/2016

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UVLO Protection

The

MP9447

has

under-voltage

lock-out

protection (UVLO). When the input voltage is

higher than the UVLO rising threshold voltage,

the MP9447 will be powered up. It shuts off when

the input voltage is lower than the UVLO falling

threshold voltage. This is non-latch protection.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor power the

floating-power-MOSFET

driver.

A

dedicated

internal regulator charges and regulates the

bootstrap capacitor voltage to ~5V. When the

voltage between the BST and SW nodes drops

below regulation, a PMOS pass transistor

connected from VIN to BST turns on. The

charging current path is from VIN, BST and then

to SW. The external circuit should provide

enough voltage headroom to facilitate charging.

bootstrap capacitor remains charged. When the

capacitor cannot charge.

maximum for fast charging. When there is no

capacitor.

At higher duty cycles, the bootstrap-charging

time is shorter so the bootstrap capacitor may not

charge sufficiently. In case the internal circuit has

insufficient voltage and time to charge the

bootstrap capacitor, the bootstrap capacitor

below 2.3V, the HS-FET turns OFF. A UVLO

circuit allows the LS-FET to conduct and refresh

the charge on the bootstrap capacitor. Once

bootstrap capacitor voltage is charged, the HS-

FET can turn on again and the part resumes

normal switching. With this bootstrap refreshing

function, MP9447 is able to work on the low drop-

out mode.

Thermal Shutdown

The MP9447 uses thermal shutdown. The

junction temperature of the IC is internally

monitored. If the junction temperature exceeds

the threshold value (typically 165°C), the

converter shuts off. This is a non-latched

protection. There is about 25°C hysteresis. Once

the junction temperature drops to about 140°C, it

initiates a SS.