MP4568

―60V, 100mA, SYNCHRONOUS STEP-DOWN CONVERTER

MP4568 Rev. 1.01

www.MonolithicPower.com

11

8/18/2013

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

© 2013 MPS. All Rights Reserved.

Under-Voltage Lockout

chip from operating below the operational supply

voltage range. The UVLO-rising threshold is

about 4V while its trailing threshold is about 3.5V.

Soft-Start

Reference-type soft-start prevents the converter

output voltage from overshooting during startup.

When the chip starts, the internal circuitry

generates a constant current to charge external

SS capacitor. The soft-start (SS) voltage slowly

ramps up from 0V at a slow pace set by the soft-

when a short-circuit is removed, the SS ramps up

as if it is a fresh soft-start process. This prevents

output voltage overshoot.

Thermal Shutdown

Thermal

shutdown

prevents

the

chip

from

thermally running away. When the silicon die

temperature exceeds its upper threshold, the

thermal shutdown feature shuts down the whole

chip. When the temperature falls below its lower

threshold, the chip resumes function.

Floating Driver and Bootstrap Charging

The external bootstrap capacitor powers the

floating MOSFET driver. This floating driver has

its own UVLO protection, with a rising threshold

of about 2.4V with a hysteresis of about 300mV.

During this UVLO, the SS voltage of the

controller resets to zero. When the UVLO is

disabled, the controller follows the soft-start

process.

The

dedicated

internal

bootstrap

regulator

charges and regulates the bootstrap capacitor to

about 5V. When the voltage difference between

BST and SW falls below its working parameters,

BST turns on to charge the bootstrap capacitor.

SW. The external circuit must have enough

voltage headroom to accommodate charging.

bootstrap capacitor can charge. When the power

bootstrap capacitor cannot charge. The best

charging period occurs when the external free-

largest. When there is no current in the inductor,

Under higher duty cycle operating conditions, the

time period may not fully-charge the capacitor.

If the external circuit does not have sufficient

voltage

and

time

to

charge

the

bootstrap

capacitor, extra external circuitry can be used to

ensure the bootstrap voltage in normal operation

region.

Startup and Shutdown

appropriate thresholds, the chip starts operating.

The reference block starts first, generating stable

reference voltage and currents, and then enables

the internal regulator. The regulator provides

stable supply for the rest device.

While the internal supply rail is high, an internal

timer holds the power MOSFET off for about

50µsec to blank startup glitches. When the

internal soft-start block is enabled, it first holds its

SS output low and then slowly ramps up.

low,

and

junction

temperature

triggers

the

thermal shutdown threshold. For shutdown, the

signaling path is blocked first to avoid any fault

triggering. Internal supply rail are pulled down

then. The floating driver is not subject to this

shutdown command, but its charging path is

disabled.