NX2139A
Based On Stability Requirement
and power dissipation. The main consideration is the
ESR of the output capacitor can not be chosen power loss contribution of MOSFETs to the overall con-
too low which will cause system unstable. The zero verter efficiency. In this application, one IRF7807 for
caused by output capacitor's ESR must satisfy the re- high side and one AO4714 with integrated schottky di-
quirement as below:
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:
F
1
SW
F
=
£
ESR
...(13)
2´ p ´ ESR´ COUT
4
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.
P
HCON =IOUT2 ´ D´ RDS(ON) ´ K
LCON=IOUT2 ´ (1- D)´ RDS(ON) ´ K
PTOTAL =P + P
P
...(15)
HCON
LCON
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.
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:
1
PSW
=
´ V ´ IOUT ´ TSW ´ F
IN S
...(16)
2
where IOUT is output current, TSW is the sum of TR
and TF 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-
charging the gate capacitor and is dissipated in driver
circuits.It is proportional to frequency and is defined
as:
IRMS = IOUT
´ D ´ 1-D
...(14)
D = TON ´ FS
When VIN = 22V, VOUT=1.8V, IOUT=7A, the result of
input RMS current is 1.9A.
P
= (QHGATE ´ VHGS + QLGATE ´ VLGS )´ FS
...(17)
gate
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.
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
VLGS is the low side gate source voltage.
This power dissipation should not exceed maxi-
mum power dissipation of the driver device.
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
Output Voltage Calculation
Output voltage is set by reference voltage and
external voltage divider. The reference voltage is fixed
Rev. 2.3
03/19/09
13
