MP4569 datasheet(11/17 Pages) MPS

11 / 17 page

MP4569―75V, 0.3A, SYNCHRONOUS STEP-DOWN CONVERTER

MP4569 Rev. 1.0

www.MonolithicPower.com

9/12/2014

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

Under-Voltage Lockout

VIN under voltage lockout (UVLO) protects the

chip from operating below the operational supply

voltage range. The UVLO-rising threshold is

about 4.2V while its trailing threshold is about

3.75V.

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-

start time. When VSS is less than the VREF, VSS

overrides VREF so the FB comparator uses VSS

instead of VREF as the reference. When VSS is

higher than VREF, VREF resumes control.

VSS is also associated with VFB. Though VSS can

be much smaller than VFB, it can only barely

exceed VFB. If somehow VFB drops, VSS tracks

VFB. This function prevents output voltage

overshoot in short-circuit recovery -- when the

short circuit is removed, the SS ramps up as if it

is a fresh soft-start process.

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 HS_FET 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 resets to zero.

When the UVLO is disabled, the regulator 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,

a PMOS pass transistor connected from VIN to

BST turns on to charge the bootstrap capacitor.

The current path is from VIN to BST and then to

SW. The external circuit must have enough

voltage headroom to accommodate charging.

As long as VIN is sufficiently higher than SW, the

bootstrap capacitor can charge. When the

HS_FET is ON, VIN is about equal to SW so the

bootstrap capacitor cannot charge. The best

charging period occurs when the LS_FET is on

so that VIN - VSW is at its largest. When there is

no current in the inductor, VSW equals VOUT so the

difference between VIN and VOUT can charge the

bootstrap capacitor.

If the internal 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

If both VIN and VEN are higher than their

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 soft-

start block is enabled, it first holds its SS output

low and then slowly ramps up.

Three events shut down the chip: VEN low, VIN

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.

Power OK (POK)

POK is an open drain power good output. “HIGH”

output indicates VOUT is higher than 90% of its

nominal value. POK is pulled down in shutdown

mode.

Reference Voltage Output (VREF)

VREF pin output 1V reference voltage. It has up

to 500uA source current capability.

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