MP8794

– 16V, 20A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8794 Rev. 1.0

www.MonolithicPower.com

15

12/2/2019

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© 2019 MPS. All Rights Reserved.

The feedback voltage (VFB) follows this external

voltage signal, and the soft-start settings are

ignored. The REF/TRK input signal can range

from 0.3V to 1.4V. During initial start-up, the

REF/TRK pin must reach at least 600mV to

ensure proper operation. After that, it can be set

to any value between 0.3V and 1.4V.

Pre-Bias Start-Up

The MP8794 is designed for monotonic start-up

into pre-biased loads. If the output is pre-biased

to a certain voltage during start-up, the IC

disables switching for both the HS-FET and LS-

FET until the voltage on the SS capacitor

exceeds the sensed output voltage at FB. If the

BST voltage (from BST to SW) is below 2.3V

before the REF/TRK voltage reaches the pre-

biased FB level, the LS-FET turns on to allow

the BST voltage to be charged through VCC.

The LS-FET turns on for narrow pulses, so the

drop in the pre-biased level is negligible.

Output Voltage Discharge

When the MP8794 is disabled through EN,

output voltage discharge mode is enabled. This

causes both the HS-FET and LS-FET to latch

off. A discharge FET connected between SW

and PGND is turned on to discharge the output

voltage. The typical switch on resistance of this

FET is about 80Ω. Once VFB drops below 10%

of VREF, the discharge FET turns off.

Current Sense and Over-Current Protection

(OCP)

The MP8794 features an on-die current sense

and

a

configurable

positive

current

limit

threshold.

The current limit is active when the MP8794 is

enabled. When the LS-FET is on, the SW

current

(inductor

current)

is

sensed

and

mirrored to CS with the ratio of GCS. By using a

resistor (RCS) from CS to AGND, VCS is

proportional to the SW current cycle by cycle.

The HS-FET turns on only when VCS drops

below the internal over-current protection (OCP)

voltage threshold (VOCP) while the LS-FET is on

to limit the SW valley current cycle by cycle.

Calculate the current limit threshold setting from

RCS with Equation (3):

OCP

CS

IN

O

O

CS

LIM

IN

SW

V

R ( )

(V

V ) V

1

G

(I

)

V

2 L f













(3)

Where VOCP = 1.2V, GCS = 10µA/A, and ILIM is

the desired output current limit (in A).

OCP hiccup becomes active 3ms after the

MP8794 is enabled. Once OCP hiccup is active,

if the MP8794 detects an over-current condition

for 31 consecutive cycles, or if VFB drops below

the under-voltage protection (UVP) threshold,

the device enters hiccup mode. In hiccup mode,

the

MP8794

latches

off

the

HS-FET

immediately and latches off the LS-FET after

zero-current cross detection (ZCD). Meanwhile,

the SS capacitor is also discharged.

After about 11ms, the MP8794 attempts to soft

start automatically. If the over-current condition

still remains after 3ms, the MP8794 repeats this

operation cycle until the over-current condition

is removed. Then the output voltage smoothly

rises back to the regulation level.

Negative Inductor Current Limit

When the LS-FET detects a -18A current, the

MP8794 turns off the LS-FET for 200ns to limit

the negative current.

Output Sinking Mode (OSM)

The MP8794 employs output sinking mode

(OSM) to regulate the output voltage to the

targeted value. When VFB exceeds 104% of

VREF but is below the OVP threshold, it triggers

OSM. During OSM operation, the LSFET

remains on until it reaches the -5.5A negative

current limit. Upon reaching -5.5A, the LS-FET

turns off for 200ns; the HS-FET turns on during

this period. After 200ns, the LS-FET turns on

again. The MP8794 maintains this operation

until VFB drops below 102% of VREF. Once it

does,

the

MP8794

exits

OSM

after

15

consecutive cycles of forced CCM.

Over-Voltage Protection (OVP)

The MP8794 monitors the output voltage by

connecting FB to the tap of the output voltage

feedback resistor divider to detect an over-

voltage condition. This provides hiccup over-

voltage protection (OVP).