MP4470GL datasheet(12/18 Pages) MPS

12 / 18 page

MP4470/4470A–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4470/4470A Rev.0.8

www.MonolithicPower.com

8/26/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

Preliminary Specifications Subject to Change

discharge VO until the inductor current drops to

zero while the HS-FET remains off. The MP4470

needs to power cycle to restart. Note that

MP4470A has no this OVP function.

The MP4470/4470A also monitors FB pin voltage

to detect output under-voltage condition. A VFB

drop below 50% ×VREF triggers UVP as well as a

current-limit that triggers SCP.

UVLO Protection

The MP4470/4470A has under-voltage lock-out

protection (UVLO). When the input voltage is

higher than the UVLO rising threshold voltage,

the MP4470/4470A 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.

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.

As long as VIN is significantly higher than SW, the

bootstrap capacitor remains charged. When the

HS-FET is ON, VIN≈VSW so the bootstrap

capacitor cannot charge.

When the LS-FET is ON, VIN−VSW reaches its

maximum for fast charging. When there is no

inductor current, VSW=VOUT so the difference

between VIN and VOUT can charge the bootstrap

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

voltage will drop low. When VBST−VSW drops

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, MP4470/4470A is able to work on the

low drop-out mode.

Thermal Shutdown

The MP4470/4470A uses thermal shutdown. The

junction temperature of the IC is internally

monitored. If the junction temperature exceeds

the threshold value (typically 175°C), the

converter shuts off. This is a non-latched

protection. There is about 45°C hysteresis. Once

the junction temperature drops to about 130°C, it

initiates a SS.

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