NX2139A datasheet(13/21 Pages) MICROSEMI

13 / 21 page

NX2139A

Rev. 2.3

03/19/09

Based On Stability Requirement

ESR of the output capacitor can not be chosen

too low which will cause system unstable. The zero

caused by output capacitor's ESR must satisfy the re-

quirement as below:

ESR

OUT

ESR C4

=≤

×π ××

...(13)

Besides that, ESR has to be bigger enough so

that the output voltage ripple can provide enough volt-

age ramp to error amplifier through FB pin. If ESR is

too small, the error amplifier can not correctly dectect

the ramp, high side MOSFET will be only turned off for

minimum time 400nS. Double pulsing and bigger out-

put ripple will be observed. In summary, the ESR of

output capacitor has to be big enough to make the sys-

tem stable, but also has to be small enough to satify

the transient and DC ripple requirements.

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

ONS

1-D

××

=×

...(14)

When VIN = 22V, VOUT=1.8V, IOUT=7A, the result of

input RMS current is 1.9A.

For higher efficiency, low ESR capacitors are

recommended. One 10uF/X5R/25V and two 4.7uF/

X5R/25V ceramic capacitors are chosen as input

capacitors.

Power MOSFETs Selection

The NX2139A requires at least two N-Channel

power MOSFETs. The selection of MOSFETs is based

on maximum drain source voltage, gate source volt-

age, maximum current rating, MOSFET on resistance

and power dissipation. The main consideration is the

power loss contribution of MOSFETs to the overall con-

verter efficiency. In this application, one IRF7807 for

high side and one AO4714 with integrated schottky di-

ode for low side are used.

There are two factors causing the MOSFET

power loss:conduction loss, switching loss.

Conduction loss is simply defined as:

× ××

× −

××

HCON

OUT

DS(ON)

LCON

OUT

DS(ON)

TOTAL

HCON

LCON

D RK

(1 D) RK

=PP

...(15)

where the RDS(ON) will increases as MOSFET junc-

tion temperature increases, K is RDS(ON) temperature

dependency. As a result, RDS(ON) should be selected

for the worst case. Conduction loss should not exceed

package rating or overall system thermal budget.

Switching loss is mainly caused by crossover

conduction at the switching transition. The total

switching loss can be approximated.

OUT

SWS

= ×

×××

...(16)

where IOUT is output current, TSW is the sum of T

and T

F which can be found in mosfet datasheet, and

FS is switching frequency. Swithing loss PSW is fre-

quency dependent.

Also MOSFET gate driver loss should be consid-

ered when choosing the proper power MOSFET.

MOSFET gate driver loss is the loss generated by dis-

charg

ing the gate capacitor and is dissipated in driver

circuits.It is proportional to frequency and is defined

as:

gate

HGATE

HGS

LGATE

LGSS

××

...(17)

where QHGATE is the high side MOSFETs gate

charge,QLGATE is the low side MOSFETs gate

charge,VHGS is the high side gate source voltage, and

LGS is the low side gate source voltage.

This power dissipation should not exceed maxi-

mum power dissipation of the driver device.

Output Voltage Calculation

Output voltage is set by reference voltage and

external voltage divider. The reference voltage is fixed

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