MP8796B datasheet(21/52 Pages) MPS

21 / 52 page

MP8796B – 16V, 30A, SYNCHRONOUS STEP-DOWN CONVERTER WITH PMBUS

MP8796B Rev 1.0

www.MonolithicPower.com

1/19/2020

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

transient step size and slew rate. The SET

signal can be generated with a minimum 50ns

interval (i.e. the next phase can turn on 50ns

after the turn-on of the previous phase to

provide fast load transient response). Figure 5

on page 19 shows this operation.

Ramp Compensation

The

MP8796B

provides

internal

ramp

compensation to support all types of output

capacitors. Only the master phase utilizes ramp

compensation. When a SET signal is generated,

the ramp increases with a certain amplitude in a

fixed period. The ramp is then discharged with

an adaptive slew rate. This ramp signal is

superimposed onto the FB signal. When the

superimposed ramp and FB signal reaches the

REF signal, a new SET signal is generated.

The ramp is selectable through PMBus

command D0h[3:1] to support a wide range of

operation configurations. Larger ramps improve

jitter but result in slower load transient response.

It is recommended to choose an optimal ramp

that meets the application’s load transient target

design.

In single-phase operation, the ramp does not

need to be reset by the SET signal and can be

reset by pulse-width modulation (PWM) instead.

This option can be selected through PMBus

command EAh[3]. When EAh[3] = 0, the ramp

is reset by PWM. This is only for single-phase

operation. When EAh[3] = 1b’1, the ramp is

reset by the SET signal. This is optimal for both

single-phase and multi-phase operation.

Mode Selection

The MP8796B provides both forced continuous

conduction mode (CCM) operation and pulse

skip operation under light-load conditions. The

operation mode is selected through PMBus

command D2h[0]. When D2h[0] = 1b’1, the

device operates in CCM. When D2h[0] = 0, the

device operates in pulse skip mode.

Phase-Shedding Operation (Slave)

For multi-phase operation, the slave phases

can be enabled or disabled through the PMBus

or PS# pin. The phase-shedding function is

disabled in the master phase to ensure proper

operation.

Phase shedding is controlled through the E5h[0]

command on the PMBus. When E5h[0] = 1b’0,

the slave phases are enabled. When E5h[0] =

1b’1, the slave phases are disabled.

If phase shedding is controlled through the PS#

pin, set the E5h[1] command to 1b’1. The slave

phases are enabled when PS# is pulled high,

and disabled when PS# is pulled low.

Soft Start (SS)

The soft-start (SS) time can be programmed

through PMBus command TON_RISE (61h).

The minimum SS time is 1ms when 61h =

0x0000. Selectable SS time options include

1ms, 2ms, 4ms, 8ms, and 16ms.

Pre-Biased Start-Up

The MP8796B is designed for monotonic start-

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

pre-biased to a certain voltage during start-up,

the IC disables switching for both the high-side

and

low-side switches until

the

internal

reference voltage exceeds the sensed output

voltage at the FB pin (VFB). If the BST voltage is

below the 2.4V threshold before the reference

voltage (VREF) reaches the pre-biased FB level,

the low-side MOSFET (LS-FET) is forced on for

about 200ns to charge up the BST voltage.

Output Voltage Discharge

When the MP8796B is disabled through CTRL

or the PMBus OPERATION command, output

voltage discharge mode is enabled if this

function is selected. Both the HS-FET and LS-

FET are latched off. A discharge MOSFET

connected between SW and GND turns on to

discharge the output voltage. The typical switch

on resistance for this MOSFET is about 50Ω.

Once VFB drops below 10% of VREF, the

discharge MOSFET turns off. This feature can

be enabled or disabled through PMBus

command MFR_CTRL_01 (D1h[6]).

Current Sense and Over-Current Protection

(OCP)

The MP8796B features on-die current sensing

and a programmable, positive, current-limit

threshold. The MP8796B provides inductor

valley current limiting (set by PMBus command

D7h).

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