MP8796B datasheet(24/52 Pages) MPS

24 / 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.

The

MP8796B

provides

output

voltage

monitoring through register READ_VOUT (8Bh).

To achieve correct output voltage setting and

monitoring, the registers VOUT_COMMAND

(21h),

VOUT_MARGIN_

HIGH

(25h),

VOUT_MARGIN_LOW

(26h),

and

VOUT_SCALE_LOOP (29h) should be set

correspondingly. Follow the steps below to set

the output voltage to 2.5V:

1. Determine the Vo_set source using the

OPERATION (01h) command. Assume that

VOUT_COMMAND (21h) is selected.

2. Choose the D1[1:0] value based on the

Vo_set value. The D1[1:0] value should pull

the FB voltage as close to 600mV as

possible. In this case, both D1[1:0] = 2’b10

and D1[1:0] = 2’b11 can provide VO = 2.5V.

However, D1[1:0] = 2’b10 yields a 625mV

FB voltage, while D1[1:0] = 2’b11 yields a

312.5mV FB voltage. Choose D1[1:0] =

2’b10.

3. Set VOUT_SCALE_LOOP (29h) to 0x00FA

to match D1[1:0] = 2’b10.

4. Set VOUT_COMMAND (21h) to 0x4E2

(LSB = 2mV).

5. VOUT_COMMAND (21h) must be sent after

VOUT_SCALE_LOOP

(29h)

and

D1h.

Otherwise, the change of VOUT_SCALE_

LOOP (29h) and D1h will not be effective.

6. VOUT_COMMAND (21h) cannot exceed

the minimum/maximum value specified in

Table 2.

Power Good (PG)

The MP8796B has a power good (PG) output.

PG is the open drain of a MOSFET. Connect

PG to VDRV or another external voltage source

below 3.6V through a pull-up resistor (typically

100kΩ). After applying the input voltage, the

MOSFET turns on, and PG is pulled to GND

before soft start is ready. After the FB voltage

(VFB) reaches the threshold set by PMBus

command POWER_GOOD_ON, PG is pulled

high after a delay set by PMBus. The delay can

be chosen through the PMBus command

MFR_CTRL_VOUT (D1h) [5:2].

When the converter encounters any fault (e.g.

UV, OV, OT, UVLO), PG is latched low and

cannot be pulled high again until a new soft

start initiates.

When the MP8796B is configured as the master

in single- or multi-phase operation, the PG pin

is used for fault indication. Therefore, PG must

be pulled high to ensure proper operation.

Otherwise,

the

MP8796B

may

enter

protection mode.

This slave fault detection feature can be

enabled or disabled through the PMBus bit

D0h[0].

If the input supply fails to power the MP8796B,

PG is clamped low even though it is tied to an

external DC source through a pull-up resistor.

Figure 8 shows the relationship between the

PG voltage and the pull-up current.

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

0.6

0.65

0.7

0.75

0.8

0.85

0.9

VPG (V)

Figure 8: PGOOD Clamped Voltage vs. Pull-Up

Current

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