MP5455 – PEAK POWER ASSIST FOR LOW-POWER APPLICATIONS

MP5455 Rev. 1.01

www.MonolithicPower.com

11

2/2/2018

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floating ICH, the boost current limit is set to

around 500mA. After reaching the current limit,

the LS-FET turns off for the set minimum off-

time. At the end of this minimum off-time, if the

feedback voltage remains below the 0.79V

internal reference, the LS-FET turns on again.

Otherwise, the MP5455 waits until the voltage

drops below the threshold before turning on the

LS-FET.

Release

The MP5455 monitors the input and bus

voltages continuously. Once the bus voltage

drops below the selected release voltage (such

as when losing input power), the internal boost

converter stops charging and works in buck-

release mode. In buck mode, the MP5455

transfers energy from the high-voltage storage

capacitor to the low-voltage bus capacitor.

Determine the release voltage by selecting

resistor values for the bus resistor divider.

The released buck applies fixed-frequency

constant-on-time (COT) control and enables

fast transition between the charge and release

modes. The buck converter works at 100% duty

cycle until the storage capacitor voltage

approaches the bus voltage. Then the storage

and bus voltages drop until they reach the

DC/DC converter’s UVLO threshold (see Figure

3).

Figure 3: Release Times

Input Current Limit

The input current limiter controls the input

inrush current of the internal hot-swap MOSFET

carefully to prevent an inrush current from the

input to the bus. A capacitor connected to

DVDT sets the soft-start time. Despite the soft-

start process, ILIM can limit the steady-state

current.

Connect a resistor between ILIM and GND to

set the current limit.

Reverse-Current Protection

The hot-swapping circuit uses reverse-current

protection to prevent the storage energy from

transferring back to the input when energy is

released from the storage capacitors to bus.

The hot-swap MOSFET turns on when the input

voltage exceeds the VIN UVLO threshold

during start-up or when input voltage is about

0.2V higher than VB. The hot-swap MOSFET

turns off when input voltage falls below the bus

voltage during release.

Start-Up Sequencing

Connect a capacitor across DVDT to program

the soft-start time. During soft start, the energy

storage capacitors charge. Very short dV/dt

times can trigger the current-limit threshold.

Select the DVDT capacitor based on the

storage capacity.

VB

Release VB Regulation

Voltage