MP38874 – 8A, 21V, 600KHz STEP-DOWN WITH SYNCHRONOUS GATE DRIVER

MP38874 Rev. 1.1

www.MonolithicPower.com

8

10/13/2010

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© 2010 MPS. All Rights Reserved.

Enable/Synch Control

The MP38874 has a dedicated Enable/Synch

control pin (EN/SYNC). 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 MP38874 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

MP38874

UVLO

comparator monitors the output voltage of the

internal regulator, VCC. The UVLO rising

threshold is about 4.0V 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 and Latch off

The MP8652 has cycle-by-cycle over

current limit. If the soft start Voltage is

greater than 1.2V, and inductor current

exceeds the current limit threshold, and FB

voltage drops below 50% of reference

Voltage, then the MP8652 goes into latch

off until En or IN is recycled. This protection

mode is especially useful when the output

is dead-short to ground.

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

voltage is regulated internally (Figure 2). Even

to maintain a 5V BST voltage across C4.

--

+

--

+

5V

U2

D1

M1

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.