MP2249 – 6V, 3A, 1MHZ, SYNCHRONOUS STEP-DOWN CONVERTER
MP2249 Rev.1.0 www.MonolithicPower.com 8
1/6/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
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.
