NX2139A datasheet(15/21 Pages) MICROSEMI

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NX2139A

Rev. 2.3

03/19/09

Power Good Output

Power good output is open drain output, a pull

up resistor is needed. Typically when softstart is

finised and FB pin voltage is over 90% of V

REF, the

PGOOD pin is pulled to high after a 1.6ms delay.

Smart Over Output Voltage Protection

Active loads in some applications can leak cur-

rent from a higher voltage than V

OUT, cause output volt-

age to rise. When the FB pin voltage is sensed over

112% of V

REF, the high side MOSFET will be turned off

and low side MOSFET will be turned on to discharge

the V

OUT. NX2139A resumes its switching operation af-

ter FB pin voltage drops to V

REF.

If FB pin voltage keeps rising and is sensed over

125% of V

REF, the low side MOSFET will be latched to

be on to discharge the output voltage and over voltage

protection is triggered. To resume the switching opera-

tion, resetting voltage on pin VCC or pin EN is neces-

sary.

Under Output Voltage Protection

Typically when the FB pin voltage is under 70%

of V

REF, the high side and low side MOSFET will be

turned off. To resume the switching operation, VCC or

ENSW has to be reset.

LDO Selection Guide

NX2139A offers a LDO controller. The selection

of MOSFET to meet LDO is more straight forward.

The MOSFET has to be logic level MOSFET and its

Rdson at 4.5V should meet the dropout requirement.

For example.

LDOIN =1.8V

LDOOUT =1.5V

Load =2A

The maximum Rdson of MOSFET should be

RDSON

LDOIN

LDOOUT

LOAD

(1.8V 1.5V)/ 2A

0.15

−×

−

=Ω

Most of MOSFETs can meet the requirement.

More important is that MOSFET has to be selected

right package to handle the thermal capability. For LDO,

maximum power dissipation is given as

LOSS

LDOIN

LDOOUT

LOAD

(1.8V 1.5V) 2A

0.6W

−×

−

×=

Select MOSFET SI4800 with 33m

Ω R

DSON

sufficient.

LDO Compensation

The diagram of LDO controller including VCC

regulator is shown in the following figure.

Rf1

Rf2

LDO input

Rload

ESR

Vref

LDOFB

LDODRV

Figure 13 - NX2139A LDO controller.

Rb and Cb have fixed value which is used to com-

pensate the comparater of the LDO controller. Set

Rb=50ohm, Cb=33nF.

For most low frequency capacitor such as elec-

trolytic, POSCAP, OSCON, etc, the compensation pa-

rameter can be calculated as follows.

f1m

ESR

C =

1+g

ESR

×π ×

××

where F

O is the desired crossover frequency.

Typically, when the POSCAP and electrical ca-

pacitor is chosen as output capacitor, crossover fre-

quency F

O has to be 2 to 3 times higher than zero

caused by ESR. In this example, we select Fo=150kHz.

m is the forward trans-conductance of MOSFET.

For SI4800, g

m=19.

Select Rf1=7.5kohm.

Output capacitor is Sanyo POSCAP 4TPE150MI

with 150uF, ESR=18mohm.

19S 18m

C =

=36pF

150kHz 7.5k

1+19S 18m

×Ω

×π ×

Ω

×Ω

Typically C

C is chosen to be 1 to 1.5 times smaller

than calculated value to compensate parasitic effect.

NX2139A Datasheet(PDF) 15 Page - Microsemi Corporation

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