MP2249 – 6V, 3A, 1MHZ, SYNCHRONOUS STEP-DOWN CONVERTER

MP2249 Rev.1.0

www.MonolithicPower.com

8

11/2/2010

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Power Save Mode Operation

The MP2249 enters asynchronous mode as soon

as current of LS MOS goes below zero. When the

load is light enough and comp voltage is lower

than the set value, the part goes into power

skipping mode. This mode improves efficiency at

light load condition.

Dropout Operation

The MP2249 allows the main switch to remain on

for more than one switching cycle and increases

the duty cycle while the input voltage is dropping

close to the output voltage. When the duty cycle

reaches 100%, the main switch is held on

continuously to deliver current to the output up to

the switch’s current limit. The output voltage then is

the input voltage minus the voltage drop across the

main switch and the inductor.

Over-Current-Protection and Latchoff

The MP2249 provides cycle-by-cycle over current

limit when inductor current peak value exceeds the

current limit threshold. Meanwhile, output voltage

starts to drop until FB is below the Under-Voltage

(UV) threshold, typically 33% below the reference.

Once the UV is triggered, the MP2249 enters

latched off mode.

This protection mode is especially useful when the

output is dead-short to ground. The MP2249 exits

the latch off mode once the EN or input power is

recycled. This operation mode will be masked off

for the soft start stage.

Maximum Load current

The MP2249 can operate down to 2.5V input

voltage. However, the maximum load current

decreases at lower input due to large IR drop

across the main switch and synchronous rectifier.

The slope compensation signal reduces the peak

inductor current as a function of the duty cycle to

prevent sub-harmonic oscillations at duty cycles

greater than 50%. Conversely the current limit

increases as the duty cycle decreases.

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.

Output Discharge Function

When the device is disabled, the part goes into

output discharge mode automatically and its

internal discharge MOSFET provides a resistive

discharge path for the output capacitor. This

function is only suitable for discharge output

capacitor for the limited time. If the part is used in

a pre-bias application, the external power supply

providing the pre-bias should supply larger

current than the discharge MOSFET sinks.

PG Indicator

MP2249 provides an open-drain PG output that

goes high after output level reaches regulation

after startup. PG is pulled low immediately if the

output goes out of regulation level or when

device enters shutdown.