nx2139a Microsemi Corporation, nx2139a Datasheet - Page 15

• Manufacturer Part Number: nx2139a
• Description: Single Channel Mobile Pwm And Ldo Controller
• Manufacturer: Microsemi Corporation

Power Good Output
up resistor is needed. Typically when softstart is
finised and FB pin voltage is over 90% of V
PGOOD pin is pulled to high after a 1.6ms delay.
Smart Over Output Voltage Protection
rent from a higher voltage than V
age to rise. When the FB pin voltage is sensed over
112% of V
and low side MOSFET will be turned on to discharge
the V
ter FB pin voltage drops to V
125% of V
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
of V
turned off. To resume the switching operation, VCC or
ENSW has to be reset.
LDO Selection Guide
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.
More important is that MOSFET has to be selected
right package to handle the thermal capability. For LDO,
maximum power dissipation is given as
Rev. 2.3
03/19/09
REF
OUT
Power good output is open drain output, a pull
If FB pin voltage keeps rising and is sensed over
NX2139A offers a LDO controller. The selection
V
V
I
The maximum Rdson of MOSFET should be
Most of MOSFETs can meet the requirement.
R
Active loads in some applications can leak cur-
Load
Typically when the FB pin voltage is under 70%
, the high side and low side MOSFET will be
LDOIN
LDOOUT
RDSON
. NX2139A resumes its switching operation af-
=2A
REF
REF
=1.8V
=1.5V
(1.8V 1.5V) / 2A
, the high side MOSFET will be turned off
, the low side MOSFET will be latched to
(V
LDOIN
V
LDOOUT
REF
) I
.
0.15
OUT
LOAD
, cause output volt-
REF
, the
sufficient.
LDO Compensation
regulator is shown in the following figure.
pensate the comparater of the LDO controller. Set
Rb=50ohm, Cb=33nF.
trolytic, POSCAP, OSCON, etc, the compensation pa-
rameter can be calculated as follows.
pacitor is chosen as output capacitor, crossover fre-
quency F
caused by ESR. In this example, we select Fo=150kHz.
For SI4800, g
with 150uF, ESR=18mohm.
than calculated value to compensate parasitic effect.
C =
Select MOSFET SI4800 with 33m
C
Typically C
P
Rb and Cb have fixed value which is used to com-
For most low frequency capacitor such as elec-
where F
g
Select Rf1=7.5kohm.
Output capacitor is Sanyo POSCAP 4TPE150MI
LOSS
C =
Typically, when the POSCAP and electrical ca-
The diagram of LDO controller including VCC
m
2
C
Rf1
Rf2
is the forward trans-conductance of MOSFET.
Figure 13 - NX2139A LDO controller.
O
2
(V
(1.8V 1.5V) 2A
has to be 2 to 3 times higher than zero
150kHz 7.5k
LDOIN
O
m
=19.
C
is the desired crossover frequency.
LDOFB
1
is chosen to be 1 to 1.5 times smaller
F
1
O
Vref
V
R
Rc
LDOOUT
f1
1+g
) I
g
m
Cc
LDODRV
m
LOAD
0.6W
1+19S 18m
ESR
19S 18m
ESR
NX2139A
Rb
Cb
R
LDO input
ESR
Co
DSON
Rload
=36pF
is
15