NX2142ACUTR datasheet(17/18 Pages) MICROSEMI

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NX2142/2142A

Rev. 1.1

10/28/07

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

××

...(25)

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 V

LGS is the

low side gate source voltage.

This power dissipation should not exceed maxi-

mum power dissipation of the driver device.

Over Current Limit Protection

Over current Limit for step down converter is

achieved by sensing current through the low side

MOSFET. For NX2142, the current limit is decided by

the R

DSON of the low side mosfet.

When synchronous

FET is on, and the voltage on SW pin is below 320mV,

the over current occurs. The over current limit can be

calculated by the following equation.

SET

DSON

320mV/R

The MOSFET R

DSON

is calculated in the worst

case situation, then the current limit for MOSFET

STM6912 is

SET

DSON

320mV

4.6A

1.2 57m

×Ω

Layout Considerations

The layout is very important when designing high

frequency switching converters. Layout will affect noise

pickup and can cause a good design to perform with

less than expected results.

There are two sets of components considered in

the layout which are power components and small sig-

nal components. Power components usually consist of

input capacitors, high-side MOSFET, low-side

MOSFET, inductor and output capacitors. A noisy en-

vironment is generated by the power components due

to the switching power. Small signal components are

connected to sensitive pins or nodes. A multilayer lay-

out which includes power plane, ground plane and sig-

nal plane is recommended .

Layout guidelines:

1. First put all the power components in the top

layer connected by wide, copper filled areas. The input

capacitor, inductor, output capacitor and the MOSFETs

should be close to each other as possible. This helps

25V 56uF 28m

Ω with 3.8A RMS rating are chosen

as input bulk capacitors.

Power MOSFETs Selection

The NX2142 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 STM6912 are

used. They have the following parameters: V

DS=30V, ID

=6A,R

DSON =57mΩ,QGATE =6.3nC.

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

...(23)

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, in which K approximately equals to

1.4 at 125oC according to datasheet. Conduction loss

should not exceed package rating or overall system

thermal budget.

Switching loss is mainly caused by crossover con-

duction at the switching transition. The total switching

loss can be approximated.

OUT

SWS

= ×

×××

...(24)

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