Datasheet

RIPPPLE

PEAK LOAD(MAX)

I I 1

æ ö

= ´ +

ç ÷

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bq24278

www.ti.com

SLUSB04 –JUNE 2012

APPLICATION INFORMATION

Output Inductor and Capacitor Selection Guidelines

When selecting an inductor, several attributes must be examined to find the right part for the application. First,

the inductance value should be selected. The bq24278 is designed to work with 1.5µH to 2.2µH inductors. The

chosen value will have an effect on efficiency and package size. Due to the smaller current ripple, some

efficiency gain is reached using the 2.2µH inductor, however, due to the physical size of the inductor, this may

not be a viable option. The 1.5µH inductor provides a good tradeoff between size and efficiency.

Once the inductance has been selected, the peak current must be calculated in order to choose the current

rating of the inductor. Use Equation 8 to calculate the peak current.

(8)

The inductor selected must have a saturation current rating less than or equal to the calculated I

PEAK

. Due to the

high currents possible with the bq24278, a thermal analysis must also be done for the inductor. Many inductors

have 40°C temperature rise rating. This is the DC current that will cause a 40°C temperature rise above the

ambient temperature in the inductor. For this analysis, the typical load current may be used adjusted for the duty

cycle of the load transients. For example, if the application requires a 1.5A DC load with peaks at 2.5A 20% of

the time, a Δ40°C temperature rise current must be greater than 1.7A:

TEMPRISE

= I

LOAD

+ D ×)I

PEAK

– I

LOAD

) = 1.5 A + 0.2 × (2.5 A – 1.5 A) = 1.7 A (9)

The bq24278 provides internal loop compensation. Using this scheme, the bq24278 is stable with 10µF to 200µF

of local capacitance. The capacitance on the SYS rail can be higher if distributed amongst the rail. To reduce the

output voltage ripple, a ceramic capacitor with the capacitance between 10µF and 47µF is recommended for

local bypass to SYS.

Product Folder Link(s) :bq24278