MP2224

–18V/4A SYNCHRONOUS STEP-DOWN CONVERTER

MP2224 Rev. 1.0

www.MonolithicPower.com

12

12/18/2015

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To use external clock synchronization, connect

a clock with a frequency range between 200kHz

and 2MHz. The internal clock rising edge will

synchronize with the external clock rising edge.

Meanwhile the width of the high level should be

longer than 250ns, and the width of low level

should be longer than 100ns.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The MP2224 UVLO comparator monitors the

output voltage of the internal regulator (VCC).

The UVLO rising threshold is about 4.1V while

its falling threshold is 3.4V.

Soft Start

The soft start prevents the converter output

voltage

from

overshooting

during

start-up.

When the chip starts up, the internal circuitry

generates a soft-start voltage (SS) that ramps

up from 0V to VCC. When SS is lower than

REF, the error amplifier uses SS as the

reference. When SS is higher than REF, the

error amplifier uses REF as the reference.

The SS time is set to 2.4ms internally.

Over-Current Protection and Hiccup

The MP2224 has a cycle-by-cycle over-current

limit, which limits the inductor current in case of

an output over load or short circuit (SC). If the

over load or SC events last long, the FB voltage

can

drop

below

the

under-voltage

(UV)

threshold

—typically 30% of the reference. Once

UV is triggered, the MP2224 enters hiccup

mode to restart the part periodically. This

protection mode is especially useful when the

output is dead shorted to ground. The average

short-circuit

current

is

greatly

reduced

to

alleviate

thermal

issues

and

protect

the

regulator. The MP2224 exits hiccup mode once

the over-current condition is removed.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the temperature of the silicon reaches

150

temperature drops below its lower threshold,

(typically 130°C), the chip is enabled again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.6V with a

hysteresis of 350mV. The bootstrap capacitor

maintain a 5V BST voltage across C4.

Figure 4: Internal Bootstrap Charging Circuit

Start-Up and Shutdown

thresholds, the chip starts up. The reference

block starts up first, generating stable reference

voltage, and then the internal regulator is

enabled. The regulator provides a stable supply

for the remaining circuitries.

Three events can shut down the chip: EN low,

shutdown procedure, the signaling path is

blocked first 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.