MP8867 – SYNCHRONOUS STEP-DOWN CONVERTER

MP8867 Rev.1.01

www.MonolithicPower.com

16

2/21/2017

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Preliminary Specifications Subject to Change

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indication as the external PG.

Output Over-Voltage Protection (OVP)

to detect an over-voltage event. When the

“V_BOOT” bit = 1, the internal comparator

monitors the FB while the “V_BOOT” bit = 0

rises above 120% of the internal reference

voltage, the controller enters linear discharge

mode. During this period, a 35Ω resistor

connected

between

and

ground

discharges the output to keep it within the

normal range. Once the output voltage drops

below 105% of the reference, the controller

exits linear discharge mode.

Over-Current Protection and Hiccup Mode

The MP8867 has a cycle-by-cycle, over-current

limit. When the inductor current peak value

exceeds the set current limit threshold, The HS-

FET turns off, and the LS-FET turns on and

remains on until the inductor current falls below

the internal “valley” current limit threshold. The

“valley” current limit circuit is employed to

decrease the operation frequency after the

“peak” current limit threshold is triggered.

(9)

is below the under-voltage (UV) threshold,

typically 30% below the reference. Once UV is

triggered, the MP8867 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

MP8867 exits hiccup mode once the over-

current condition is removed.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die reaches temperatures that

exceed 160°C, the entire chip shuts down.

When the temperature is less than its lower

threshold, typically 140°C, the chip is enabled

again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

maintain a 5V BST voltage across C5.

Figure 5: Internal Bootstrap Charging Circuit

Start-Up and Shutdown

respective thresholds, the chip starts up. The

reference block starts first, generating stable

reference voltage and currents, and then the

internal regulator is enabled. The regulator

provides a stable supply for the remaining

circuitries.

Three events can shut down the chip: EN low,

shutdown procedure, the signaling path is

blocked first to avoid any fault triggering. The

COMP voltage and the internal supply rail are

then pulled down. The floating driver is not

subject to the shutdown command.

When the MP8867 is enabled (EN=high and

voltage set by the FB resistors with a

then communicate with the master. If it doesn’t

the chip works through the FB feedback and