MP8642 — DUAL 3A, 23V, 600kHz STEP-DOWN CONVERTER

MP8642 Rev. 1.0

www.MonolithicPower.com

8

9/13/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

Enable/Synch Control

The MP8642 has a dedicated Enable/Synch

control pin (EN/SYNC) on each channel. By

pulling it high or low, the IC can be enabled and

disabled by EN. Tie EN to VIN for automatic

start up. To disable the part, EN must be pulled

low for at least 5µs.

The MP8642 can be synchronized to external

clock range from 300kHz up to 1.4MHz through

the EN/SYNC pin. The internal clock rising

edge is synchronized to the external clock rising

edge.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented

to protect the chip from operating at insufficient

supply voltage. The MP8642 UVLO comparator

monitors the output voltage of the internal

regulator, VCC. The UVLO rising threshold is

about 4.1V while its falling threshold is a

consistent 3.2V.

Internal Soft-Start

The soft-start is implemented to prevent the

converter output voltage from overshooting

during startup. When the chip starts, the

internal circuitry generates a soft-start voltage

(SS) ramping up from 0V to 1.2V. When it is

lower than the internal reference (REF), SS

overrides REF so the error amplifier uses SS as

the reference. When SS is higher than REF,

REF regains control.

Over-Current-Protection (OCP)

The MP8642 has cycle-by-cycle over current

limit when the inductor current peak value

exceeds the set current limit threshold.

Meanwhile, output voltage starts to drop until

FB is below the Under-Voltage (UV) threshold,

typically 30% below the reference. Once a

output UV is triggered, the MP8642 enters latch

off mode. Mode is especially useful to ensure

system safety under fault condition. The

MP8642 exits the latch off mode once the EN or

input power is re-cycled.

Thermal Shutdown

Thermal shutdown is implemented to prevent

the chip from operating at exceedingly high

temperatures. When the silicon die temperature

is higher than 150

C, it shuts down the whole

chip. When the temperature is lower than its

lower threshold, typically 140

C, the chip is

enabled again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered

by an external bootstrap capacitor. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

more than 5V, U2 will regulate M3 to maintain a

5V BST voltage across C4.

--

+

--

+

5V

U2

D1

M3

BST

SW

C4

C2

L1

Figure 2—Internal Bootstrap Charging

Circuit

Startup and Shutdown

If both VIN and EN are higher than their

appropriate 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 stable supply for the remaining

circuitries.

Three events can shut down the chip: EN low,

VIN low and thermal shutdown. In the shutdown

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.