MP8869 datasheet(19/37 Pages) MPS

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MP8869

– 17V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

PRELIMINARY SPECIFICATIONS SUBJECT TO CHANGE

MP8869 Rev.1.0

www.MonolithicPower.com

6/8/2016

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

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The MP8869 UVLO comparator monitors the

input voltage, VIN, and output voltage of the

VCC regulator. The MP8869 is active when the

voltages exceed the UVLO rising threshold.

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

The 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):

( )

(

)

()

ref







(2)

Where Vref is reference voltage of the FB loop or

2C loop.

If the output of the MP8869 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 or

VOUT

(7).

NOTE:

7) V_BOOT = 1, sense FB voltage. V_BOOT = 0, sense VOUT

voltage.

The MP8869 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 I

2C, the output is

discharged linearly to zero in a quarter of the

soft-start time.

Over-Current-Protection (OCP)

The MP8869 has a default, hiccup, cycle-by-

cycle, over-current limiting control. The current-

limit circuit employs both high-side current limit

and

low-side

"valley"

current-sensing

algorithm. The MP8869 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 MP8869 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 greatly reduced to alleviate

thermal issues and protect the regulator. The

MP8869 exits hiccup mode once the over-

current condition is removed.

Short the output to ground first, and then power

on the part. The MP8869

’s I2C is disabled in

this condition. The I

2C resumes operation after

the short circuit is removed. When hiccup OCP

bit D[1] in register 01 is set to 0 by the I

2C, a

latch-off occurs if OCP is triggered, and FB

UVP is triggered.

Power Good (PG)

The 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 in-rush 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 the after output voltage

drops below 105% of the internal reference

voltage.

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