MP8846 – 6A, 8V SYNCHRONOUS STEP-DOWN CONVERTER

MP8846 Rev. 1.0

www.MonolithicPower.com

16

5/19/2015

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Output Over-Voltage Protection

(9)

(9)

becomes

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

FET remains on until the LS current drops to

2.5A. This discharges the output and tries to

keep it within the normal range. The part will

below 105% of the reference voltage.

Over-Current Protection and Hiccup

The MP8846 has a 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 is employed to

decrease the operation frequency after the

“peak” current-limit threshold is triggered.

(9)

is below the under-voltage (UV) threshold (50%

below the reference, typically). Once UV is

triggered, the MP8846 enters hiccup mode to

re-start the part periodically. This protection

mode is useful when the output is dead-shorted

to ground. The average short-circuit current is

reduced greatly to alleviate thermal issues and

protect the regulator. The MP8846 exits hiccup

mode once the over-current condition is

removed.

Thermal Shutdown (TSD)

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the die temperature exceeds 160°C, it

shuts down the whole chip. 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. This floating

driver has its own UVLO protection. The

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

exceeds 5V, U1 regulates M1 to maintain a 5V

BST voltage across C4. A 10Ω resistor

placed between SW and the BST capacitor is

recommended to reduce SW spike voltage.

FIGURE 5. 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 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 this shutdown command.

When the MP8846 is enabled (which means

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

an output voltage set by FB feedback resistors

(with a programmed soft-start time). After the

MP8865

is

enabled,

the

bus

can

communicate with the master. If the chip does

constantly, it works efficiently through FB

feedback and performs behavior similar to a

reference and voltage scaling instruction (if

V_BOOT=“1”), the output voltage is determined

0.6V):

)

R2

R1

(1

V

V

REF

OUT

+

×

=