MPQ9840 – 36V, 3.5A, LOW IQ, SYNC STEP-DOWN CONVERTER, AEC-Q100

MPQ9840 Rev. 1.02

www.MonolithicPower.com

23

7/1/2020

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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 is a little different with VSS due to

a 0.5V offset and some delay. When VSS is

lower than 0.5V, VSSI is 0V. VSSI rises from 0V to

0.8V during the period of VSS rising from 0.5V to

1.6V. At this time the error amplifier uses VSSI

as the reference, so 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):







SS

SS

SS

C(nF) 1.1V

t(ms)

I( A)

(2)

Where CSS is the external SS capacitor, and ISS

is the internal 10μA SS charge current.

There is also an internal fixed 700us soft start.

The final SS time is determined by the longer

time between 700us and the external SS setting

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 chip 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

is

shut

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 any start-up glitches. When the

soft-start block is enabled, the SS output is held

low to ensure that the rest of the circuitries are

ready before slowly ramping up.

Three events can shut down the chip: EN low,

VIN low, and thermal shutdown. In the

shutdown procedure, the signaling path is

blocked first to avoid any fault triggering. VCOMP

and the internal supply rail are then pulled down.