NX2113 datasheet(14/19 Pages) MICROSEMI

14 / 19 page

NX2113/2113A

Rev. 2.0

11/18/05

5. Calculate C

1 by setting compensator zero FZ

at 75% of the LC double pole.

C =

10k

0.75

6.5kHz

=10.7nF

×π ××

×π ×

Ω ××

Choose C

1=10nF.

6. Calculate C

2 by setting compensator pole

F at

half the swithing frequency.

10 k

300k Hz

=106pF

π ××

π ×

Ω×

Choose C

2=100pF.

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 and picture show the relationship

between

OUT

REF

and voltage divider..

REF

OUT

REF

-V

...(18)

whereR2 is part of the compensator, and the

value of R1 value can be set by voltage divider.

Choose R2=10k

Ω, to set the output voltage at

1.6V, the result of R1 is 10k

Ω.

Vout

Vref

Voltage divider

Figure 13 - Voltage divider load

In general, the minimum output load impedance

including the resistor divider should be less than 5k

Ω to

prevent overcharge the output voltage by leakage cur-

rent (e.g. Error Amplifier feedback pin bias current). A

minimum load for 5k

Ω less (<1/16w for most of applica-

tion) is recommended to put at the output. For example,

in this application,

Vout=1.6V

The power loss is 1/16W less

LOAD

1.6V 1.6V /(1/16W)

=Ω

Select minimum load is 1k

Ω should be good

enough.

Input Capacitor Selection

Input capacitors are usually a mix of high frequency

ceramic capacitors and bulk capacitors. Ceramic ca-

pacitors bypass the high frequency noise, and bulk ca-

pacitors supply current to the MOSFETs. Usually 1uF

ceramic capacitor is chosen to decouple the high fre-

quency noise. The bulk input capacitors are decided by

voltage rating and RMS current rating. The RMS current

in the input capacitors can be calculated as:

RMS

OUT

1-D

××

...(19)

VIN = 12V, VOUT=1.6V, IOUT=10A, using equation

(19), the result of input RMS current is 3.4A.

For higher efficiency, low ESR capacitors are

recommended. One Sanyo OSCON SP series

16SP270M 16V 270uF with 4.4A is chosen as input

bulk capacitor.

Power MOSFETs Selection

The NX2113 requires two N-Channel power

MOSFETs. The selection of MOSFETs is based on

maximum drain source voltage, gate source voltage,

maximum current rating, MOSFET on resistance and

power dissipation. The main consideration is the power

loss contribution of MOSFETs to the overall converter

efficiency. In this design example, two IRFR3706 are

used. They have the following parameters: V

DS=30V, ID

=75A,R

DSON =9mΩ,QGATE =23nC.

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