MP5455 – PEAK POWER ASSIST FOR LOW-POWER APPLICATIONS

MP5455 Rev. 1.01

www.MonolithicPower.com

10

2/2/2018

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

© 2018 MPS. All Rights Reserved.

OPERATION

The MP5455 is an energy storage and

management unit in a QFN-20 (3mmx4mm)

package. The MP5455 provides a very compact

and efficient energy management solution for

applications requiring power back-up or hold-up

supplies. MPS’s patented lossless energy

storage and release management circuits use a

bidirectional buck/boost converter to achieve

optimal energy transfer and provide the most

cost-effective energy storage solution.

The integrated boost converter raises the

energy-storage voltage level. The storage

feedback resistor divider sets the storage

voltage. If the input shuts down suddenly, the

internal buck converter transfers the energy

from the storage capacitor to the bus and holds

the bus voltage when the system consumes

energy from the storage capacitor. The buck

converter can work in 100% duty cycle

operation

to

deplete

the

stored

energy

completely.

Start-Up

When VIN starts up, the bus voltage (VB) is

charged from 0 to VIN, approximately. The VB

rising slew rate is controlled by the dV/dt

capacitance. This function prevents input inrush

current and provides protection to the entire

system.

EHCH is used to enable the storage charge and

release circuitry. If ENCH is already high before

VB finishes the dV/dt process, the storage

charge circuitry works automatically when VIN

is higher than the under-voltage lockout (UVLO)

threshold (typically 2.5V). The storage charge

circuitry operates in two modes: pre-charge

mode (where STRG is charged to VB using a

current source) and boost mode (where STRG

is charged to set the voltage). The pre-charge

mode charges STRG up to VB using a near-

constant current source (around 130mA). When

STRG is close to VB and VB is higher than a

certain threshold (where the corresponding FBB

is higher than 0.813V), boost mode is initiated.

Boost mode charges STRG to the target

voltage. Figure 1 shows the charging build-up

process when ENCH is high before VB starts

up.

It is strongly recommended to enable ENCH

after VB has settled (see Figure 2). Since

release mode is triggered when FBB is lower

than 0.79V (although there is a 23mV

hysteresis between boost mode and release

mode), VB may be pulled back low and enter

release mode accidently. To prevent this,

enable ENCH after VB settles. In some high-

current

charges,

boost

mode

can

be

programmed by ICH. Figure 2 shows the

charging build-up process when ENCH is

enabled after VB settles.

Figure 1: Charging Process

Figure 2: Charging Process when EN and ENCH

Are Separated

Storage Voltage

After the start-up period, the internal boost

converter

regulates

the

storage

voltage

automatically to a set value. The MP5455 uses

burst mode to minimize the converter’s power

loss. When the storage voltage drops below the

set voltage, burst mode initiates and charges

the storage capacitor. During the burst period,

the current limit and the low-side MOSFET (LS-

FET) control the switch. When the LS-FET

turns on, the inductor current increases until it

reaches its current limit. The boost-current limit

can be programmed by an ICH resistor. By

EN&ENCH

Power-On Delay Time

STRG Pre-Charge Mode

VB

VB DVDT

Charge-Up

STRG Boost Mode

FBB=0.813V

EN

Power-On Delay Time

STRG Pre-Charge Mode

VB

VB DVDT

Charge-Up

ENCH

STRG Boost Mode