MPQ9842GL datasheet(23/37 Pages) MPSIND

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MPQ9842

– 36V, 2A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MPQ9842 Rev. 1.1

www.MonolithicPower.com

7/29/2022

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

SYNC

CLK

SW1

SW2

SW1, 2 has a 180o phase shift

SW1: Phase high

SW2: Phase low

Figure 3: In-Phase and 180° Out-of-Phase

Soft Start (SS)

Soft start (SS) is implemented to prevent the

converter output voltage from overshooting

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

internal current source begins charging the

external soft-start capacitor. The internal SS

voltage (VSSI) rises with the soft-start voltage

(VSS), but VSSI differs from VSS in that it has a

0.5V offset and some delay. When VSS is below

0.5V, VSSI is 0V. VSSI rises from 0V to 0.8V while

VSS rises from 0.5V to 1.6V. At this time, the

error amplifier uses VSSI as the reference, and

the output voltage ramps up from 0V to the

regulated value following VSSI rising. When VSS

reaches 1.6V, VSSI is 0.8V and overrides the

internal VREF, so the error amplifier uses the

internal VREF as the reference.

The soft-start time (tSS) set by the external SS

capacitor can be calculated with Equation (2):





C (nF) 1.1V

t (ms)

I ( A)

(2)

Where CSS is the external SS capacitor, and ISS

is the internal 10

μA SS charge current. There is

also an internal 0.7ms soft start when the SS

pin is floating. The final SS time is determined

by the longer time between 0.7ms and the

external SS set time.

SS can be used for tracking and sequencing.

Pre-Bias Start-Up

During start-up, if VFB > VSSI-150mV, then the

output has a pre-bias voltage, and neither the

HS-FET or LS-FET turn on until VSSI-150mV is

higher than FB.

Thermal Shutdown

Thermal shutdown is implemented to prevent

the chip from running away thermally. When the

silicon die temperature is higher than its upper

threshold, the power MOSFETs are shut down.

When the temperature is lower than its lower

threshold, thermal shutdown is removed, and

the MPQ9842 is enabled again.

Current Comparator and Current Limit

The power MOSFET current is accurately

sensed via a current sense MOSFET. The

current is then fed to the high-speed current

comparator for current-mode control purposes.

The current comparator takes this sensed

current as one of its inputs. When the HS-FET

is turned on, the comparator is first blanked

until the end of the turn-on transition to avoid

noise. Then the comparator compares the

power switch current with VCOMP. When the

sensed current is higher than VCOMP, the

comparator outputs low to turn off the HS-FET.

The maximum current of the internal power

MOSFET is limited cycle-by-cycle internally.

Hiccup Protection

When the output is shorted to ground, causing

the output voltage to drop below 55% of its

nominal output, the IC shuts down momentarily

and begins discharging the soft-start capacitor.

The IC restarts with a full soft start when the

soft-start capacitor is fully discharged. This

hiccup process is repeated until the fault is

removed.

Start-Up and Shutdown

If both VIN and EN are higher than their

appropriate thresholds, the chip starts up. The

reference block starts first, generating a stable

reference voltage and current, and then the

internal regulator is enabled. The regulator

provides a stable supply for the rest of the

circuitries.

While the internal supply rail is up, an internal

timer holds the power MOSFET off for about

50µs to blank the start-up glitches. When the

soft-start block is enabled, it first holds its SS

output low to ensure that the rest of the

circuitries are ready before ramping up slowly.

Three events can shut down the chip: EN low,

VIN

low,

and

thermal

shutdown.

the

shutdown procedure, the signaling path is

blocked first to avoid any fault triggering. VCOMP

and the internal supply rail are then pulled down.

The floating driver is not subject to this

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