MP4473GL datasheet(15/21 Pages) MPS

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MP4473–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4473 Rev. 1.0

www.MonolithicPower.com

12/15/2014

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

A VFB drop below 50%xVREF triggers UVP. Also, it

triggers a current limit that initiates SCP.

Under-Voltage Lock-Out Protection (UVLO)

The

MP4473

has

under-voltage

lock-out

protection (UVLO). When the input voltage is

higher than the UVLO rising threshold voltage,

the MP4473 begins to power 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 powers 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 travels from VIN to BST

and then to SW. The external circuit should

provide enough voltage headroom to facilitate

charging.

The bootstrap capacitor remains charged if the

VIN is significantly higher than the SW. When the

HS-FET is ON, VIN≈VS. This prevents the

bootstrap capacitor from charging.

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

maximum for fast charging. When there is no

inductor

current,

VSW=VOUT; the

difference

between VIN and VOUT can charge the bootstrap

capacitor.

At higher duty cycles, the bootstrap-charging

time is shorter. Therefore, the bootstrap capacitor

may not charge sufficiently. If the internal circuit

does not have sufficient time and voltage to

charge the bootstrap capacitor, the bootstrap

capacitor voltage drops 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 the bootstrap capacitor voltage is charged,

the HS-FET

turns ON again and the part

resumes

normal

switching.

The

bootstrap

refreshing function allows the MP4473 to work on

low drop-out mode.

Thermal Shutdown (TSD)

The MP4473 uses thermal shutdown (TSD). 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, with about 45°C hysteresis. Once the

junction temperature drops to 130°C, it initiates a

SS.

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