MP8861 – 2.85V - 18V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8861 Rev. 1.1

www.MonolithicPower.com

17

5/28/2020

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

© 2020 MPS. All Rights Reserved.

Soft Start (SS) and Pre-Bias Start-Up

Soft start (SS) prevents the converter output

voltage from overshooting during start-up.

When the chip starts up, the internal circuitry

generates a soft-start voltage 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 approximate typical soft-start time can be

calculated with Equation (2):

()

(

)

()

7

ref

ss

ss

VV

C nF

tms

A







(2)

Where Vref is reference voltage.

If the output of the MP8861 is pre-biased to a

certain voltage during start-up, the IC disables

the switching of both the HS-FET and LS-FET

until the voltage on the internal SS capacitor

exceeds the sensed output voltage at FB.

The MP8861 also provides a selectable soft-

stop

function,

which

defines

the

output

discharge behavior after EN shutdown. By

default, the output is not controlled after EN

shutdown. If setting the Soft Stop control bit D[3]

to 1 in register 02 via the I2C, the output is

discharged linearly to zero in a quarter of the

soft-start time.

Over-Current Protection (OCP)

The MP8861 has a default, hiccup, cycle-by-

cycle, over-current limiting control. The current-

limit circuit employs both high-side current limit

and

a

low-side

"valley"

current-sensing

algorithm. The MP8861 uses the RDS(ON) of the

LS-FET as a current-sensing element for the

valley current limit. If the magnitude of the high-

side current-sense signal is above the current-

limit threshold, the PWM on pulse is terminated,

and the LS-FET is turned on. Afterward, the

inductor current is monitored by the voltage

between GND and SW. GND is used as the

positive current sensing node, so GND should

be connected to the source terminal of the

bottom MOSFET. PWM is not allowed to initiate

a new cycle before the inductor current falls to

the valley threshold.

After the cycle-by-cycle over-current limit

occurs, the output voltage drops until VOUT is

below

the

under-voltage

(UV)

threshold

(typically 60% below the reference). Once UV is

triggered, the MP8861 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 reduced greatly to alleviate

thermal issues and protect the regulator. The

MP8861 exits hiccup mode once the over-

current condition is removed.

Short the output to ground first, and then power

on the part. The MP8861’s I2C is disabled in

this condition. The I2C resumes operation after

the short circuit is removed. When the Hiccup

OCP bit D[1] in register 01 is set to 0 by the I2C,

a latch-off occurs if OCP is triggered, and FB

UVP is triggered.

Power Good (PG)

Power good (PG) indicates whether the output

voltage is in the normal range compared to the

internal reference voltage. PG is an open-drain

structure. An external pull-up supply is required.

During power-up, the PG output is pulled low.

This indicates to the system to remain off and

keep the load on the output to a minimum. This

helps reduce inrush current at start-up.

When the output voltage is higher than 90%

and lower than 115% of the internal reference

voltage and the soft start is finished, then the

PG signal is pulled high. When the output

voltage is lower than 85% after the soft start

finishes, the PG signal remains low. When the

output voltage is higher than 115% of the

internal reference, PG is switched low. The PG

signal rises high again after the output voltage

drops below 105% of the internal reference

voltage.

PG implements an adjustable deglitch time via

the I2C whenever VOUT crosses the UV/OV

rising and falling threshold. This guarantees the

correct indication when the output voltage is

scaled through the I2C.

The PG output is pulled low immediately when

EN UVLO, input UVLO, OCP, or over-

temperature protection (OTP) is triggered.