MP9943 – 36V, 3A PEAK SYNCHRONOUS STEP-DOWN CONVERTER

MP9943 Rev.1.1

www.MonolithicPower.com

13

7/4/2017

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Under-Voltage Lockout

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The MP9943 UVLO comparator monitors the

output voltage of the internal regulator, VCC. The

UVLO rising threshold is about 3.5V while its

falling threshold is 3.3V.

Internal Soft-Start

The soft-start prevents the converter output

voltage from overshooting during startup. When

the chip starts, the internal circuitry generates a

soft-start voltage (SS) that ramps up from 0V to

1.2V. When SS is lower than REF, SS overrides

REF so the error amplifier uses SS as the

reference. When SS exceeds REF, the error

amplifier uses REF as the reference. The SS

time is internally set to 1.5ms.

Over-Current Protection and Hiccup

The MP9943 has cycle-by-cycle over current limit

when the inductor current peak value exceeds

the set current limit threshold. If the output

voltage starts to drop until FB is below the Under-

Voltage (UV) threshold—typically 84% below the

reference—the MP9943 enters hiccup mode to

periodically restart the part. This protection mode

is especially useful when the output is dead-

shorted to ground. The average short-circuit

current is greatly reduced to alleviate the thermal

issue and to protect the regulator. The MP9943

exits the hiccup mode once the over-current

condition is removed.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die temperature exceeds 170°C,

it shuts down the whole chip. When the

temperature drops below its lower threshold

(typically 140°C) the chip is enabled again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor power the

floating-power-MOSFET

driver.

A

dedicated

internal regulator (see Figure 4) charges and

regulates the bootstrap capacitor voltage to ~5V.

When the voltage between the BST and SW

nodes drops below regulation, a PMOS pass

then to SW. The external circuit should provide

enough voltage headroom to facilitate charging.

bootstrap capacitor remains charged. When the

capacitor can’t be charged. When the LS-FET is

charging. When there is no inductor current,

floating driver has its own UVLO protection, with

a rising threshold of 2.2V and hysteresis of

150mV. A 20Ω resistor placed between SW and

BST cap is strongly recommended to reduce SW

spike voltage.

Figure 4: Internal Bootstrap Charging Circuit

Startup and Shutdown

thresholds, the chip starts: The reference block

starts first, generating stable reference voltage

and currents, and then the internal regulator is

enabled. The regulator provides stable supply for

the remaining circuitries.

shutdown procedure, the signaling path is first

blocked to avoid any fault triggering. The COMP

voltage and the internal supply rail are then

pulled down. The floating driver is not subject to

this shutdown command.

Power Good

The MP9943 has power good (PG) output. The

PG pin is the open drain of a MOSFET. It should

be connected to VCC or some other voltage

source through a resistor (e.g. 100kΩ). In the

presence of an input voltage, the MOSFET turns

on so that the PG pin is pulled to low before SS