MP2420 datasheet(13/22 Pages) MPS

13 / 22 page

MP2420―75V, 0.3A, SYNCHRONOUS STEP-DOWN CONVERTER WITH WATCHDOG

MP2420 Rev. 1.0

www.MonolithicPower.com

9/6/2015

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

Soft Start (SS)

The reference-type soft start prevents the

converter output voltage from overshooting

during start-up. When the chip starts, the internal

circuitry generates a constant current to charge

the external soft-start capacitor. The SS voltage

ramps up slowly from 0V at a pace set by the SS

time. When VSS is less than VREF, VSS overrides

VREF, and the FB comparator uses VSS as the

reference instead of VREF. When VSS is higher

than VREF, VREF resumes control.

VSS can be much smaller than VFB, but it can only

barely exceed VFB. If VFB drops, VSS tracks VFB.

This

function

prevents

output

voltage

overshoot during short-circuit recovery. When the

short circuit is removed, a new SS process

ramps up.

Thermal Shutdown

Thermal shutdown prevents a thermal runaway

on the chip. When the silicon die reaches

temperatures exceeding its upper threshold, the

entire chip shuts down. When the temperature

falls below its lower threshold, the chip is enabled

again.

Floating Driver and Bootstrap Charging

The external bootstrap capacitor powers the

floating HS-FET driver. This floating driver has

its own UVLO protection. This UVLO’s rising

threshold is about 2.4V with a hysteresis of about

300mV. During the UVLO, the SS voltage resets

to zero. When the UVLO is disabled, the

regulator follows the soft-start process.

The

dedicated

internal

bootstrap

regulator

charges and regulates the bootstrap capacitor to

about 5V. When the voltage difference between

BST and SW falls below its working parameters,

a PMOS pass transistor connected from VIN to

BST turns on to charge the bootstrap capacitor.

The current path runs from VIN to BST to SW.

The external circuit must have sufficient voltage

headroom to accommodate charging.

As long as VIN is sufficiently higher than SW, the

bootstrap capacitor can charge. When the HS-

FET is on, VIN is about equal to SW, and the

bootstrap capacitor cannot charge. The optimal

charging period occurs when the LS-FET is on,

and VIN - VSW is at its largest. VSW is equal to VOUT

when there is no current in the inductor. The

difference between VIN and VOUT charges the

bootstrap capacitor.

If the internal circuit does not have sufficient

voltage and time to charge the bootstrap

capacitor, extra external circuitry can be used to

ensure that the bootstrap voltage operates in its

normal region.

Start-Up and Shutdown

If both VIN and VEN are higher than their

appropriate thresholds, the chip starts up. The

reference block starts first, generating stable

reference voltages and currents, and then it

enables the internal regulator. The regulator

provides a stable supply for the rest of the device.

If the internal supply rail is high, an internal timer

holds the power MOSFET off for about 50µs to

clear up any start-up glitches. When the soft-start

block is enabled, the SS output voltage is first

held low and then slowly ramps up.

Three events can shut down the chip: VEN low,

VIN low, and thermal shutdown. During the

shutdown procedure, the signaling path is

blocked first to avoid any fault triggering. The

internal supply rails are then pulled down. The

floating driver is not subject to this shutdown

command, but its charging path is disabled.

Power OK (POK)

POK is an open-drain power good output. A high

output indicates that VOUT is higher than 90% of

its nominal value. POK is pulled down in

shutdown mode.

Reference Voltage Output (VREF)

VREF has an output reference voltage of 1V. It has

up to 500µA of source current capability.

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