
LXE1710 E
VALUATION
B
OARD
MOSFET S
ELECTION
As seen in previous sections, the user can design the
output filter of the amplifier to meet performance or
costs targets. In addition, the amplifier’s power stage
(selection of MOSFETs) can be selected depending on
these tradeoffs. The efficiency of the amplifier circuit
can be approximated by the following equation.
P
+
(
Where
R
L
=
DC Resistance of Speaker
R
NDS
=
n-channel MOSFET on-resistance
U
SER
G
UIDE
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 11
Copyright
2000
Rev. 1.1, 2000-12-01
CROSS
P
L
IND
PDS
NDS
L
IN
OUT
P
R
R
R
R
I
R
I
+
+
+
=
=
]
)
2
2
R
PDS
=
p-channel MOSFET on-resistance
R
IND
=
DC Resistance of Inductor
P
CROSS
=
MOSFET Switching Loss
The overall efficiency is a function of primarily the
MOSFETs and output filter inductors. The “Inductor”
section’s contribution will be considered later. The
MOSFET Power loss is a function of the on-resistance
and gate charge.
A
R
P
I
W
P
R
R
I
at
P
25
=
O
PDS
NDS
DS
5
4
25
Then
4
If
)]
(
Loss
Power
MOSFET
2
=
=
=
+
=
=
The LX1710 Evaluation Board is designed using
FDS4953
p-channel
and
MOSFETS.
FDS6612A
n-channel
W
P
R
R
DS
PDS
NDS
56
.
=
)]
095
.
03
.
)
095
.
,
03
.
2
+
=
=
=
MOSFET power loss is proportional to on-resistance.
n
S
f
CV
P
CROSS
2
Loss
Switching
MOSFET
=
=
Where
Assume
C
= Input Capacitance
V
= Supply Voltage
f
S
= Switching Frequency
n
= Number of MOSFETS
C
= 1000pF
V
= 15VDC
f
S
= 500kHz
W
P
CROSS
45
.
)
)(
10
500
)(
15
)(
10
1
3
2
9
=
×
×
=
MOSFET switching loss is proportional to total gate
charge, supply voltage, and switching frequency.
There are a few other important parameters to
consider when selecting the output power components
besides the on-resistance and gate charge of the
MOSFETs. The drain-source voltage must provide
ample margin for circuit noise and high speed
switching transients. Since the amplifier configuration
requires output bridge operation at the supply voltage,
the MOSFETs should have a drain-source voltage of
at least 50% greater than the supply voltage. The
power dissipation of the MOSFETs should also be
able to dissipate the heat generated by the internal
losses and be greater than the sum of P
DS
and P
CROSS
.
Linfinity recommends that in selecting MOSFETs, R
DS
0 1!1
2
several MOSFET options
.
g
<10nC. The table below provides
FDS6612A FDS4953
n-channel p-channel n-channel p-channel n-channel p-channel
Si4532ADY
IRF7105
Drain-Source On-Resistance
RDS(ON)@VGS = +/-10V
0.022
0.053
0.053
0.08
0.10
0.25
Drain-Source Voltage
Drain Current (continuous)
Total Gate Charge
VDSS (V)
30
-30
30
-30
25
-25
ID(continuous) (A)
8.4
-5
4.9
-3.9
3.5
-2.3
Q
g
(typical) (nC)
9
8
8
10
9.4
10
Manufacturer
Fairchild
Fairchild
Vishay
Siliconix
Vishay
Siliconix
International
Rectifier
International
Rectifier
MOSFET Component Options