EM5845VT datasheet(12/13 Pages) EXCELLIANCE

12 / 13 page

2016/02/24

A.2

EM5845

Output Capacitor Selection

An output capacitor is required to filter the

inductor current. Output ripple and transient

response are two critical factors when choosing the

output capacitance. The COT mode allows for the

usage of low ESR ceramic capacitors and thus

smaller board layout, electrolytic capacitors may

also be used. The following equations allow

calculation of the required capacitance to meet a

desired output ripple voltage.

For the ceramic capacitors (low ESR):

OUTRIPPLE







When using electrolytic capacitors (High ESR):

ESR

OUTRIPPLE







Regarding transient response considerable, a good

starting point is to determine the allowable

overshoot in Vo if the load is suddenly released. In

this case, energy stored in the inductor will be

transferred to Co causing its voltage to rising, so

choosing the needs capacitance should be cover for

both ripple and transient requirement. The most of

applications will require a minimum of 22uF output

capacitance. Like the input capacitor multilayer

ceramic capacitors are X7R or X5R types. The

maximum RMS current rating of the capacitors is:

RMS

MAX

CORMS

)

(

)

(

)

(









Layout Considerations



The VIN, VCC decoupling capacitors should be

as closed to the IC VIN and VCC pin.



The FB voltage divider network should be as

closed to the IC FB pin and away from

switching node.



Use widely trance for the High current path,

From VIN, SW, Vo to GND path.



Widely ground plan for good heat sinking and

better noise immunity.

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