MP5505GL datasheet(11/15 Pages) MPS

11 / 15 page

MP5505 – ENERGY STORAGE AND MANAGEMENT UNIT

MP5505 Rev. 1.01

www.MonolithicPower.com

1/12/2015

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

APPLICATION INFORMATION

Setting the Storage Voltage

Set the storage voltage by choosing the external

feedback resistors R1 and R2 shown in Figure 5.

Cstorage

STRG

FBS

Figure 5: Storage Feedback Circuit

The storage voltage is determined by:

STORAGE

FBS

V(1

) V

Where VFBS is 0.79V typically. R1 and R2 are not

critical for normal operation. Select R1 and R2

higher than 10kΩ to account for the bleed current.

For example, if R2 is 14kΩ, R1 is then:

STORAGE

FBS

14k

)

Ω×

−

For a 12V storage voltage, R1 is 200kΩ.

Table 1 lists the recommended resistors for

different storage voltages.

Table 1: Resistor Pairs for VSTORAGE

VSTORAGE(V)

R1 (kΩ)

R2 (kΩ)

127

200

340

Select Release Voltage and Input Capacitors

Select the release voltage by choosing the

external feedback resistors R3 and R4 as shown

in Figure 6.

Similarly, the release voltage is:

RELEASE

FBB

V(1

) V

VFBB is also 0.79V typically. However, R3 and R4

not only determine the release voltage, but affect

stability. Since the release buck mode works in

COT mode, avoid

small resistor values to ensure a sufficient

voltage ramp. Generally, choose R3//R4≥20kΩ

for stable performance with CB=22μF. Table 2

lists the recommended resistor values for

different release voltages.

FBB

Figure 6: Release Feedback Circuit

Table 2: Resistor Pairs for VRELEASE

VRELEASE (V)R3 (kΩ)

R4 (kΩ)

4.2

105

24.3

2.9

107

40.2

Selecting the Storage Capacitor

The storage capacitor stores energy during

normal operation and releases this energy to VIN

when VIN loses input power. Use a general-

purpose electrolytic capacitor or low profile POS

capacitor for most applications.

Select a storage capacitor with a voltage rating

that exceeds the targeted storage voltage. The

stable voltage on the storage capacitor during

normal operation allows for full capacitor

utilization. Consider the capacitance reduce with

the DC voltage offset when choosing the

capacitors. Different capacitors have different

capacitance

derating

performance.

Choose

capacitor with enough voltage rating to guarantee

enough capacitance.

The required capacitance depends on the length

of the “dying gasp” for a typically application.

Assume the input release current is IRELEASE when

input voltage is regulated at VRELEASE for the DC-

DC converter, the storage is VSTORAGE, and the

required dying gasp time is τDASP. The required

storage capacitance is then:

RELEASE

STORAGE

DASP

RELEASE

−

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