NX2710
charge,VHGS is the high side gate source voltage, and
Power MOSFETs Selection
VLGS is the low side gate source voltage.
This power dissipation should not exceed maxi-
mum power dissipation of the driver device.
The NX2710 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
and power dissipation. The main consideration is the
power loss contribution of MOSFETs to the overall con-
verter efficiency. In 25A output application, five
IRFR3706 can be used, two for high side, three for low
side. They have the following parameters: VDS=30V, ID
=75A,RDSON =9mW,QGATE =23nC.
Over Current Limit Protection
Over current protection is achieved by sensing
current through the low side MOSFET. An internal cur-
rent source of 32uA flows through an external resistor
connected from OCP pin to SW node sets the over
current protection threshold. When synchronous FET
is on, the voltage at node SW is given as
VSW =-IL ´ RDSON
There are two factors causing the MOSFET
power loss:conduction loss, switching loss.
The voltage at pin OCP is given as
Conduction loss is simply defined as:
I
OCP ´ ROCP +VSW
P
HCON =IOUT2 ´ D´ RDS(ON) ´ K
LCON=IOUT2 ´ (1- D)´ RDS(ON) ´ K
PTOTAL =P + P
When the voltage is below zero, the over current
occurss as shown in figure 17.
P
...(20)
HCON
LCON
vbus
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 IRFR3706 datasheet. Con-
duction loss should not exceed package rating or overall
system thermal budget.
I
OCP
32uA
OCP
R
SW
OCP
OCP
comparator
Switching loss is mainly caused by crossover
conduction at the switching transition. The total
switching loss can be approximated.
Figure 17 - Over Current Protection
The over current limit can be set by the following
equation:
1
I
OCP ´ ROCP
PSW
=
´ V ´ IOUT ´ TSW ´ F
IN S
...(21)
ISET
=
2
K ´ RDSON
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.
If two MOSFETs RDSON=6.5mW, the worst case
thermal consideration K=1.5 and the current limit is
set at 40A, then
I
SET ´ K ´ RDSON 40A ´ 1.5´ 6.5mW
ROCP
=
=
= 6.1k W
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:
IOCP
32uA ´ 2
Choose ROCP=6kW.
For NX2710, if switching channel goes into hic-
cup current limit, the LDO will go to hiccup too.
LDO Selection Guide
NX2710 offers a LDO controller. The selection
of MOSFET to meet LDO is more straight forward.
The selection is that the Rdson of MOSFET should
Pgate = (QHGATE ´ VHGS + QLGATE ´ VLGS )´ FS
...(22)
where QHGATE is the high side MOSFETs gate
charge,QLGATE is the low side MOSFETs gate
Rev. 1.3
08/07/07
18
