XRP7714ILB-0X14-F【PDF 25/29 页】IC REG 5OUT BCK/LINEAR 40TQFN

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?2011 Exar Corporation

25/29

Rev. 1.1.6

EXTERNAL COMPONENT SELECTION

Inductor Selection

Select the Inductor for inductance L and saturation current Isat. Select an inductor with Isat higher

than the programmed over current limit. Calculate inductance from:

5 =

(5I5V5[ 5I5\5b5a) ?5I5\5b5a

55V5[

?/DIV>

1

55`

?/DIV>

1

55_5V5]

Where:

Vin is the converter input voltage

Vout is the converter output voltage

fs is the switching frequency

Irip is the inductor peak-to-peak current ripple (nominally set to 30% of Iout)

Keep in mind that a higher Irip results in a smaller inductance value which has the advantages of

smaller size, lower DC equivalent resistance (DCR), and allows the use of a lower output

capacitance to meet a given step load transient. A higher Irip, however, increases the output

voltage ripple, requires higher saturation current limit, and increases critical conduction. Notice

that this critical conduction current is half of Irip.

Capacitor Selection

" Output Capacitor Selection

Select the output capacitor for voltage rating, capacitance and Equivalent Series Resistance (ESR).

Nominally the voltage rating is selected to be at least twice as large as the output voltage. Select

the capacitance to satisfy the specification for output voltage overshoot/undershoot caused by the

current step load. A sudden decrease in the load current forces the energy surplus in the inductor

to be absorbed by Cout. This causes an overshoot in output voltage that is corrected by power

switch reduced duty cycle. Use the following equation to calculate Cout:

5 = 5? ?/DIV>

(5<

2

5<

1

)

2

5

55F

2

5I

55H5G

2

Where:

L is the output inductance

I2 is the step load high current

I1 is the step load low current

Vos is output voltage including the overshoot

Vout is the steady state output voltage

Or it can be expressed approximately by

5 = 5? ?/DIV>

(5<

2

5<

1

)

2

2 ?5I5\5b5a 5I

Here,

out

os

V

=

is the overshoot voltage deviation.

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