NX2142ACUTR datasheet(16/18 Pages) MICROSEMI

16 / 18 page

NX2142/2142A

Rev. 1.1

10/28/07

Output Voltage Calculation

Output voltage is set by reference voltage and

external voltage divider. The reference voltage is fixed

at 0.8V. The divider consists of two ratioed resistors

so that the output voltage applied at the Fb pin is 0.8V

when the output voltage is at the desired value. The

following equation applies to figure 15, which shows

the relationship between

OUT

REF

and voltage di-

vider.

Vout

Vref

Figure 15 - Voltage divider

REF

OUT

REF

-V

...(21)

where R

2 is part of the compensator, and the value

of R1 value can be set by voltage divider.

Input Capacitor Selection

Input capacitors are usually a mix of high fre-

quency ceramic capacitors and bulk capacitors. Ce-

ramic capacitors bypass the high frequency noise, and

bulk capacitors supply switching current to the

MOSFETs. Usually 1uF ceramic capacitor is chosen

to decouple the high frequency noise.The bulk input

capacitors are decided by voltage rating and RMS cur-

rent rating. The RMS current in the input capacitors

can be calculated

as:

RMS

OUT

INMIN

1-D

××

...(22)

VINMIN = 8V, VOUT=1.05V, IOUT=10A, the result of

input RMS current is 3.4A.

For higher efficiency, low ESR capacitors are

recommended. One Sanyo OSCON CAP 25SVP56M

×π ××

×π ×

Ω×

ESR

OUT

ESRC

20.5m

1360uF

5.7kHz

2.Set R

2 equal to10kΩ. Using equation 18, the

final selection of R

1 is 4.7kΩ.

3. Set crossover frequency at 1/10~ 1/5 of the

swithing frequency, here FO=60kHz.

4.Calculate R

3 value by the following equation.

OSC

OUT

ESR

REF

60kHz 2.2uH1

20.5m

2.5mA/V

2.5V

0.8V

=5k

×π ××

××

×π ××

××

Ω

Choose R

3 =5kΩ.

5. Calculate C

1 by setting compensator zero FZ

at 75% of the LC double pole.

C =

0.75

2.9kHz

=14nF

×π ××

×π ×

Ω ××

Choose C

1=15nF.

6. Calculate C

2 by setting compensator pole

at half the swithing frequency.

5 k

600k Hz

=54pF

π ××

π ×

Ω×

Choose C

2=52pF.

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