MP9840 – 36V, 3.5A, LOW IQ, SYNC STEP-DOWN CONVERTER

PHASE is used when two or more MP9840

devices are in parallel with the same SYNC

clock. Pulling PHASE high forces the MP9840

to operate in-phase of the SYNC clock. Pulling

PHASE low forces the device to be 180° out-of-

phase of the SYNC clock. By setting different

voltages for PHASE, two devices can operate

180° out-of-phase to reduce the total input

current ripple, so a smaller input bypass

capacitor can be used (see Figure 4). The

PHASE rising threshold is about 2.5V with a

400mV hysteresis.

SYNC

CLK

SW1

SW2

t

SW1, 2 has a 180o phase shift

SW1: Phase high

SW2: Phase low

Figure 4: 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 soft-start period

begins, an internal current source charges the

external soft-start capacitor. When the soft-start

voltage (SS) is lower than the internal reference

(REF), SS overrides REF, so the error amplifier

uses SS as the reference. When SS is higher

than REF, REF regains control. CSS can be

calculated with Equation (2):

×

=

µ

SS

REF

SS

SS

C (nF) V

(V)

t (ms)

I ( A)

(2)

Where CSS is the external SS capacitor, VREF is

0.8V, and ISS is the internal 10

μA SS charge

current.

SS can be used for tracking and sequencing.

Pre-Bias Start-Up

During start-up, if FB is greater than SS -

150mV, which means the output has a pre-bias

voltage, then neither the HS-FET nor LS-FET

turn on until SS 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.

MP9840 Rev. 1.01

www.MonolithicPower.com

19

1/23/2018

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