MP8864

– SYNCHRONOUS STEP-DOWN CONVERTER

MP8864 Rev. 1.02

www.MonolithicPower.com

16

1/18/2018

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Output Over-Voltage Protection (OVP)

The

MP8864

monitors

a

resistor

divider

higher than 115% of the target voltage, the LS-

FET remains on until the LS current drops to

-2.5 A. This discharges the output and tries to

keep it within normal range. The part exits this

105% of the reference voltage.

Over-Current Protection and Hiccup (OCP)

The MP8864 has cycle-by-cycle over-current

limit control. 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 decreases the

operation frequency after the peak current-limit

threshold is triggered. Meanwhile, the output

voltage

(UV)

threshold

(50%

below

the

reference, typically). Once UV is triggered, the

MP8864 enters hiccup mode to re-start the part

periodically. This protection mode is especially

useful when the output is dead-shorted to

ground. The average short-circuit current is

reduced greatly to alleviate thermal issues and

to protect the regulator. The MP8864 exits

hiccup mode once the over-current condition is

removed.

Thermal Shutdown (TSD)

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures. If

the die temperature exceeds 160°C, the entire

chip shuts down. When the temperature is less

than its lower threshold (140°C, typically) the

chip is enabled again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. The floating

driver has its own UVLO protection. The

UVLO’s rising threshold is 2.2 V with a

hysteresis of 150 mV. The bootstrap capacitor

maintain a 5 V BST voltage across C4. A 10

Ω

resistor placed between SW and the BST

capacitor is recommended to reduce SW spike

voltage.

out

Figure 6

—Internal bootstrap charging circuit

Start-Up and Shutdown

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, 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 this

shutdown command.

When the MP8864 is enabled (EN=high and

VIN>UVLO), the chip starts up to an output

voltage that is set by the FB feedback resistors

bus can communicate with the master. If the

signal continuously, it works efficiently through

FB feedback and performs similar to traditional

voltage scaling instruction (if V_BOOT=

“1”), the

output voltage is determined by the resistor

value can be calculated using equation (2). The

)

R2

R1

(1

V

V

REF

OUT







(2)