NX2124 datasheet(12/17 Pages) MICROSEMI

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NX2124/2124A

Rev.1.8

02/28/08

4.Calculate R

3 value by the following equation.

OSC

OUT

ESR

REF

1.5V

30kHz 1.5uH1

6.5m

2.0mA/V

1.8V

0.8V

=14.6k

×π ××

××

×π ××

××

Ω

Choose R

3 =14.7kΩ.

5. Calculate C

1 by setting compensator zero FZ

at 75% of the LC double pole.

C =

14.7k

0.75 2.3kHz

=6.3nF

×π ××

×π ×

Ω ××

Choose C

1=6.8nF.

6. Calculate C

2 by setting compensator pole

at half the swithing frequency.

C =

14 .7 k

300kHz

=72pF

π ××

Ω×

Choose C

1=68pF.

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)

where R2 is part of the compensator, and the

value of R1 value can be set by voltage divider.

See compensator design for R

1 and R2 selection.

Vout

Vref

Voltage divider

Figure 8 - Voltage divider

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 switching current to the MOSFETs. Usu-

ally 1uF ceramic capacitor is chosen to decouple the

high frequency noise.The bulk input capacitors are de-

cided by voltage rating and RMS current rating. The RMS

current in the input capacitors can be calculated as:

RMS

OUT

1-D

××

...(19)

VIN = 5V, VOUT=1.8V, IOUT=9A, using equation (19),

the result of input RMS current is 4.3A.

For higher efficiency, low ESR capacitors are rec-

ommended. One Sanyo OS-CON 16SP270M 16V 270uF

18m

Ω with 4.4A RMS rating are chosen as input bulk

capacitors.

Power MOSFETs Selection

The power stage 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.

There are two factors causing the MOSFET power

loss:conduction loss, switching loss.

Conduction loss is simply defined as:

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