MP8868 – SYNCHRONOUS STEP-DOWN CONVERTER

MP8868 Rev.1.1

www.MonolithicPower.com

16

5/15/2015

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100us delay.

The PG won’t response to output over voltage

condition.

indication as the external PG pin.

Output over Voltage Protection (OVP)

MP8868 monitors both FB pin and VOUT pin to

detect over voltage event. When “V_BOOT”

bit=1, internal comparator monitors FB pin while

“V_BOOT” bit=0, it monitors VOUT pin. When

the feedback voltage becomes higher than

120% of the internal reference voltage, the

controller will enter linear discharge mode.

During this period, there is a 35Ω resistor

connected between VOUT pin and ground, this

will then discharge the output and try to keep it

within the normal range. Once output voltage

falls lower than 105% of reference, controller

exits linear discharge mode.

Over-Current-Protection and Hiccup

The MP8868 has a cycle-by-cycle over-current

limit. When the inductor current peak value

exceeds the set current limit threshold, the HS-

FET will turn off and the LS-FET will turn 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 MP8868 enters hiccup mode to

periodically restart the part. 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 to protect the regulator. The

MP8868 exits the 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, it shuts down the whole chip.

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

exceeds 5V, U1 will regulate M1 to maintain a

5V BST voltage across C5.

Figure 5: Internal Bootstrap Charging Circuit

Startup and Shutdown

thresholds, the chip starts. 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,

procedure, the signaling path is first blocked 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 MP8868 is enabled which means

EN=high and VIN>UVLO, the chip starts up to

an output voltage which is set by FB feedback

resistors with programmed soft-start time. After

signal all the time, it works well through FB pin

feedback and performs the similar behavior as