MP8792 – 16V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8792 Rev. 1.0

www.MonolithicPower.com

15

12/2/2019

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input signal ranges from 0.3V to 1.4V. During

initial start-up, TRK/REF must reach 600mV to

ensure proper operation. After that, it can be set

to any value between 0.3V and 1.4V.

Pre-Biased Start-Up

The

MP8792

has

been

designed

for

a

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

high-side and low-side MOSFETs until the

voltage on the TRK/REF capacitor exceeds the

sensed output voltage at FB. If the BST voltage

(from BST to SW) is below 2.3V before the

TRK/REF 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 MP8792 is disabled through EN, it

enables output voltage discharge mode. This

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

off. A discharge FET connected between SW

and PGND turns on to discharge the output

voltage. The typical switch on resistance of this

FET is about 80

Ω. Once VFB drops below 10%

of VREF, the discharge FET turns off.

Current Sense and Over-Current Protection

(OCP)

The MP8792 features an on-die current sense

(CS) and a configurable positive current limit

threshold.

The current limit is active when the MP8792 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 (VLIM) 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):

)

2

1

)

(

(

)

(

SW

IN

O

O

IN

LIM

CS

LIM

CS

f

L

V

V

V

V

I

G

V

R



















(3)

Where, VLIM = 1.2V, GCS = 20µA/A, and ILIM is

the desired output current limit (in A).

OCP hiccup mode is active 3ms after the

MP8792 is enabled. Once OCP hiccup is active,

if the MP8792 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

MP8792

latches

off

the

HS-FET

immediately, and latches off the LS-FET after

zero-current cross detection (ZCD). Meanwhile,

the TRK/REF capacitor is also discharged. After

about 11ms, the MP8792 automatically tries to

soft start.

If the over-current condition remains after 3ms

of running, the MP8792 repeats this operation

cycle until the over-current condition disappears.

Then the output voltage smoothly rises back to

the regulation level.

Negative Inductor Current limit

When the LS-FET detects a -9A current, the

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

negative current.

Output Sinking Mode (OSM)

The MP8792 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, the LS-FET 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

MP8792 maintains this operation until VFB drops

below 102% of VREF. Once it does, the MP8792

exits OSM after 15 consecutive cycles of forced

CCM.

Over-Voltage Protection (OVP)

The MP8792 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).

If VFB exceeds 116% of VREF, it triggers OVP.

The LS-FET remains on until it reaches the low-

side negative current limit (NOCP). Once it

reaches NOCP, the LS-FET turns off for 200ns;

the HS-FET turns on during this period.