LP8555 datasheet(49/64 Pages) TI1

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ISAT >

(VOUT ± VIN)

VOUT

VIN

Where D =

Where IRIPPLE =

(2 x L x f)

DQG'¶= (1 - D)

(VOUT ± VIN)

(VOUT)

+ IRIPPLE

IOUTMAX

'¶

LP8555

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SNVS857 – FEBRUARY 2014

9.2.1.4 Detailed Design Procedure

9.2.1.4.1

Inductor

There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor

current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current

rating specifications are followed by different manufacturers so attention must be given to details. Saturation

current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of

application should be requested from the manufacturer. Shielded inductors radiate less noise and should be

preferred.

The saturation current should be greater than the sum of the maximum load current and the worst case average

to peak inductor current.

Figure 44 shows the worst case conditions.

Figure 44. Calculating Inductor Maximum Current

•

IRIPPLE: peak inductor current

•

IOUTMAX: maximum load current

•

VIN: minimum input voltage in application

•

L : min inductor value including worst case tolerances

•

f : minimum switching frequency

•

VOUT: output voltage

•

D: Duty Cycle for CCM Operation

•

VOUT : Output Voltage

As a result the inductor should be selected according to the ISAT. A more conservative and recommended

approach is to choose an inductor that has a saturation current rating greater than the maximum current limit of

3.1 A. A 4.7-µH to 6.8-µH inductor with a saturation current rating of at least 3.1 A is recommended for most

applications. The inductor’s resistance should be less than 300 m

Ω for good efficiency.

9.2.1.4.2

Output Capacitor

A ceramic capacitor with 50-V voltage rating is recommended for the output capacitor. The DC-bias effect can

reduce the effective capacitance by up to 80% especially with small package size capacitors, which needs to be

considered in capacitance value and package selection. Typically one 10-µF or two 4.7-µF capacitors is

sufficient. Effectively the capacitance should be at least 2 µF at boost maximum output voltage.

9.2.1.4.3

LDO Capacitor

A ceramic capacitor with at least 10 V voltage rating is recommended for the output capacitor of the LDO. The

DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance

value selection. Typically 10 µF capacitor is sufficient.

9.2.1.4.4

VDD Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the VDD input capacitor. If input

voltage is higher, then the rating should be selected accordingly. The DC-bias effect can reduce the effective

capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically, a 1-

μF

capacitor is sufficient. X5R/X7R are recommended types.

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