NX2139ACMTR MICROSEMI [Microsemi Corporation], NX2139ACMTR Datasheet - Page 13

• Manufacturer Part Number: NX2139ACMTR
• Description: SINGLE CHANNEL MOBILE PWM AND LDO CONTROLLER
• Manufacturer: MICROSEMI [Microsemi Corporation]

Based On Stability Requirement
too low which will cause system unstable. The zero
caused by output capacitor's ESR must satisfy the re-
quirement as below:
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
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:
input RMS current is 1.9A.
recommended. One 10uF/X5R/25V and two 4.7uF/
X5R/25V ceramic capacitors are chosen as input
capacitors.
Power MOSFETs Selection
power MOSFETs. The selection of MOSFETs is based
on maximum drain source voltage, gate source volt-
age, maximum current rating, MOSFET on resistance
Rev. 2.3
03/19/09
When V
ESR of the output capacitor can not be chosen
Besides that, ESR has to be bigger enough so
Input capacitors are usually a mix of high fre-
For higher efficiency, low ESR capacitors are
The NX2139A requires at least two N-Channel
F
I
D
RMS
ESR
T
ON
I
OUT
2
IN
F
= 22V, V
S
D
ESR C
1
1- D
OUT
=1.8V, I
OUT
...(14)
F
OUT
SW
4
=7A, the result of
...(13)
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.
power loss:conduction loss, switching loss.
tion temperature increases, K is R
dependency. As a result, R
for the worst case. Conduction loss should not exceed
package rating or overall system thermal budget.
conduction at the switching transition. The total
switching loss can be approximated.
and T
F
quency dependent.
ered when choosing the proper power MOSFET.
MOSFET gate driver loss is the loss generated by dis-
charg
circuits.It is proportional to frequency and is defined
as:
charge,Q
charge,V
V
mum power dissipation of the driver device.
Output Voltage Calculation
external voltage divider. The reference voltage is fixed
S
LGS
P
is switching frequency. Swithing loss P
where the R
where I
gate
is the low side gate source voltage.
P
P
P
Conduction loss is simply defined as:
Switching loss is mainly caused by crossover
Also MOSFET gate driver loss should be consid-
where Q
Output voltage is set by reference voltage and
i
P
F
HCON
LCON
TOTAL
This power dissipation should not exceed maxi-
ng the gate capacitor and is dissipated in driver
There are two factors causing the MOSFET
SW
which can be found in mosfet datasheet, and
HGS
(Q
=I
=I
LGATE
=P
OUT
OUT
OUT
1
2
HGATE
is the high side gate source voltage, and
HCON
HGATE
2
2
is output current, T
V
DS(ON)
is the low side MOSFETs gate
IN
(1 D) R
D R
V
P
is the high side MOSFETs gate
I
HGS
LCON
OUT
will increases as MOSFET junc-
DS(ON)
Q
T
DS(ON)
SW
LGATE
DS(ON)
K
F
S
K
SW
V
should be selected
DS(ON)
LGS
NX2139A
is the sum of T
) F
temperature
S
SW
.
...(15)
..(16)
...(17)
is fre-
13
R