MP28258 – 3A, 4.2V-20V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN QFN12 (2X3mm)

MP28258 Rev 1.14

www.MonolithicPower.com

12

9/25/2012

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with the SS voltage. Once SS voltage rises

the FB voltage. At this point, the soft start

finishes and it enters steady state operation. The

SS time is set about 1ms internally. When the EN

pin goes low, an internal current source

discharges the internal SS voltage. Once the SS

voltage

falls

below

the

voltage. The output voltage will decrease

smoothly with the SS voltage until the voltage

level zeros out.

Power-Good (PG)

The PG pin is the open drain of a MOSFET that

through a resistor (ex. 100k). The MOSFET turns

on with the application of an input voltage so that

the PG pin is pulled to GND before SS is ready.

is pulled high after a 0.5ms delay.

PG pin will be pulled low.

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

MP28258 has cycle-by cycle over-current limit

control. The inductor current is monitored during

the ON state. And it has two optional OCP/SCP

protection modes: latch-off mode and hiccup

mode.

For MP28258DD, the HS-FET turns off when the

inductor current exceeds the current limit and the

OCP timer—set at 50μs—starts. The OCP

triggers if the inductor current reaches or

exceeds the current limit every cycle in those

50μs. The MP28258DD short-circuit protection

(SCP) occurs when dead shorts occur—when the

inductor current exceeds the current limit and the

will trigger the OCP..

For MP28258DD-A, it enters hiccup mode, that

periodically restarts the part when the inductor

current peak value exceeds the current limit and

Typically, the UV threshold is 50% below the

REF voltage. In OCP/SCP, MP28258DD-A will

disable the output voltage power, discharge

internal soft-start cap, and then automatically try

to soft-start again. If the over-current circuit

condition still holds after soft-start ends, it

repeats this operation cycle until the over-current

circuit condition disappears, and output rises

back to regulation level.

Over/Under-voltage Protection (OVP/UVP)

MP28258 monitors the output voltage through a

resistor-divided feedback (FB) voltage to detect

over and under voltage on the output. When the

trigger the OVP. Once it triggers the OVP, the

LS-FET is always on while the HS-FET is off. It

needs to power cycle to turn on again.

Conversely, the UVP triggers when the FB

Usually UVP accompanies a drop in the current

limit and this results in SCP.

UVLO protection

MP28258 has under-voltage lock-out protection

(UVLO). When the input voltage is higher than

the UVLO rising threshold voltage, the MP28258

powers up. It shuts off when the input voltage is

lower than the UVLO falling threshold voltage.

This is non-latch protection.

Thermal Shutdown

The MP28258 employs thermal shutdown by

internally monitoring the junction temperature of

the IC. If the junction temperature exceeds the

threshold value (typically 150ºC), the converter

shuts off. This is non-latch protection. There is

about 25ºC hysteresis. Once the junction

temperature drops around 125ºC, it initiates a

soft start.