XRP7714ILB-0X14-F【PDF 26/29 页】IC REG 5OUT BCK/LINEAR 40TQFN

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?2011 Exar Corporation

26/29

Rev. 1.1.6

Select ESR such that output voltage ripple (Vrip) specification is met. There are two components in

Vrip. First component arises from the charge transferred to and from Cout during each cycle. The

second component of Vrip is due to the inductor ripple current flowing through the output

capacitors ESR. It can be calculated for Vrip:

55_5V5] = 5<5_5V5] ?/DIV>

?/DIV>

55F5E

2

+ ?/DIV>

1

8 ?565\5b5a ?5S5`

?/DIV>

2

Where:

Irip is the inductor ripple current

fs is the switching frequency

Cout is the output capacitance

Note that a smaller inductor results in a higher Irip, therefore requiring a larger Cout and/or lower

ESR in order to meet Vrip. With the current generation of ultra-low ESR ceramic capacitors it is

common to operate with Irip e 30% of Iout.

When trying to optimize control loop bandwidth, particularly at switching frequencies below

600kHz, an effective ESR in the range of 7 to 20mohm can help significantly. The Digital Power

Studio design tool is used to verify what will work best in your application.

" Input Capacitor Selection

Select the input capacitor for Voltage, Capacitance, ripple current, ESR and ESL. Voltage rating is

nominally selected to be at least twice the input voltage. The RMS value of input capacitor current,

assuming a low inductor ripple current, can be approximated as:

55V5[ = 5<5\5b5a ??7 ?/DIV>

(

1 57

)

Where:

Iin is the RMS input current

Iout is the DC output current

D is the duty cycle

In general, the total input voltage ripple should be kept below 1.5% of VIN. The input voltage

ripple also has two major components: the voltage drop on the main capacitor

Cin

V and the

voltage drop due to ESR -

ESR

V

. The contribution to Input voltage ripple by each term can be

calculated from:

2

)

(

out

in

out

Cin

V

C

f

V

I

V

=

)

5

.

0

(

rip

out

ESR

I

ESR

V

+

?/DIV>

=

Total input voltage ripple is the sum of the above:

ESR

Cin

Tot

V

+

=

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