MP5455 – PEAK POWER ASSIST FOR LOW-POWER APPLICATIONS

MP5455 Rev. 1.01

www.MonolithicPower.com

12

2/2/2018

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APPLICATION INFORMATION

Selecting Input Capacitor and TVS

Capacitors at VIN are recommended to absorb

possible voltage spikes during input power turn-

on, input switch hard-off (during power-off), or

other

special

conditions.

The

application

determines the capacitor. For example, if the

input power trace is too long (with higher

parasitic inductance) during the input switch

hard-off period, more energy pumps into the

input. This means more input capacitors are

needed to ensure that the input voltage spike

remains in a safe range. Use a capacitor 0.1µF

or larger based on the spike condition.

Consider inrush current requirements when

selecting an input capacitor. Typically, more

input capacitors result in a higher input inrush

current during hot-plugging. A smaller input

capacitor is needed for a smaller inrush current.

The MP5455 works normally with a very small

input capacitor. However, this leads to a

possible high voltage spike. An efficient solution

is to add a TVS diode at the input to absorb the

possible input voltage spike. At the same time,

keep the inrush current small during hot-

plugging. A typical TVS diode, like SMA6J5.0A,

is recommended.

Setting the Storage Voltage

Set the storage voltage by choosing the

external feedback resistors (R9 and R10) (see

Figure 4).

Figure 4: Storage Feedback Circuit

The storage voltage is determined with

Equation (1):

STORAGE

FBS

R9

V(1

) V

R10





(1)

not critical for normal operation.

Select a higher R9 and R10 value to account

for the bleed current. For example, if R10 is

14kΩ, calculate R9 with Equation (2):

STORAGE

FBS

FBS

14k

(V

V

)

R9

V







(2)

For a 12V storage voltage, R9 is 200kΩ.

Table 1 lists the recommended resistors for

different storage voltages.

R9 (kΩ)

R10 (kΩ)

8

127

14

12

200

14

20

340

14

Selecting the Release Voltage and VB

Capacitors

Select the release voltage by choosing the

external feedback resistors R1 and R2 (see

Figure 5).

Figure 5. Release Feedback Circuit

Similarly, the release voltage is calculated with

Equation (3):

RELEASE

FBB

R1

V(1

) V

R2





(3)

R1 to be about 10kΩ and CB to be 22µF to

47µF. Table 2 lists the recommended resistor

values for different release voltages.

R1 (kΩ)

R2 (kΩ)

4.2

10.5

2.43

2.9

10.7

4.02

R9

R10

Cstorage

STRG

FBS

R1

R2

VB

FBB