MP5455 – PEAK POWER ASSIST FOR LOW-POWER APPLICATIONS

MP5455 Rev. 1.01

www.MonolithicPower.com

13

2/2/2018

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Selecting the Storage Capacitor

The storage capacitor stores energy during

normal operation and releases this energy

general-purpose electrolytic capacitor or low-

profile POS capacitor for most applications.

One 4.7µF ceramic capacitor is recommended

if the electrolytic capacitor ESR is high.

Select a storage capacitor with a voltage rating

that exceeds the targeted storage voltage.

Consider the capacitance reduction with the DC

voltage offset when choosing the capacitors.

Different capacitors have different capacitance

de-rating performances. Choose a capacitor

with enough voltage rating to guarantee enough

capacitance.

The required capacitance depends on the

length of the dying gasp for a typical application.

capacitance is then calculated with Equation (4):

2

2

2

RELEASE

STORAGE

DASP

RELEASE

RELEASE

S

V

V

I

V

C













(4)

Consider the power loss during release. The

buck converter can run up to 85% efficiency in

most applications. Select storage capacitance

capacitance is 280μF.

For typical applications using a 5V input supply,

set the storage voltage above 10V to utilize the

high-voltage energy fully and minimize storage

capacitance requirements. Use a 16V POS

capacitor or 25V electrolytic capacitors.

Selecting the External Diode

An external diode parallel with the high-side

power MOSFET (HS-FET) is optional for

normal charge mode operation. This diode

improves the boost efficiency if the boost peak

current is high. The voltage rating should be

higher than the storage voltage, and the current

rating should be higher than the current

programmed by ICH.

Setting the Input Hot-Swap Current Limit

Connect a resistor from ILIM to GND to set the

current limit value. For example, a 1.2kΩ

resistor sets the current limit to about 4.1A.

Table 3 lists the recommended resistors for

different current limit values.

4.6

1.07

4.1

1.2

3.7

1.4

1.6

3.2

Selecting the Inductor

The inductor is necessary to supply constant

current to the load. Since boost mode and buck

mode share the same inductor and the buck

mode current is generally higher, an inductor

that at least supports the buck mode releasing

current is recommended.

Select the inductor based on the buck release

fixed at 500kHz. The inductance value can be

calculated with Equation (5):

RR

LSW

S

VV

L(1

)

IF

V

 



(5)

current, which can be set in the range of 30% to

40% of the full releasing current.

The inductor should not saturate under the

maximum inductor peak current.

Setting the Bus Voltage Rise Time

Connect a capacitor to DVDT to set the bus

voltage start-up slew rate and soft-start time.

Leave DVDT floating for the default soft-start

time (around 0.9ms from 0V to 5V). Table 4 lists

the recommended capacitors for different soft-

start times at a 5V input condition.

Table 4: Soft Start vs. Capacitor Value

10

10

100

100