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

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.

CROSS

L

IND

PDS

NDS

L

IN

OUT

P

R

R

R

R

I

R

I

P

P

+

+

+

+

=

=

]

)

(

2

[

2

2

Where

=

DC Resistance of Speaker

=

n-channel MOSFET on-resistance

=

p-channel MOSFET on-resistance

=

DC Resistance of Inductor

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

P

O

PDS

NDS

DS

5

.

2

4

25

Then

4

at

25

If

)]

(

2

[

Loss

Power

MOSFET

2

=

=

=

Ω

=

+

=

=

The LX1710 Evaluation Board is designed using

FDS4953

p-channel

and

FDS6612A

n-channel

MOSFETS.

W

P

R

R

DS

PDS

NDS

56

.

1

)]

095

.

0

03

.

0

(

2

[

)

5

.

2

(

095

.

0

,

03

.

0

2

=

+

=

Ω

=

Ω

=

MOSFET power loss is proportional to on-resistance.

n

f

CV

P

S

CROSS

2

Loss

Switching

MOSFET

=

=

Where

C

= Input Capacitance

V

= Supply Voltage

= Switching Frequency

n

= Number of MOSFETS

Assume

C

= 1000pF

V

= 15VDC

= 500kHz

W

45

.

0

)

4

)(

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

several MOSFET options

.

FDS6612A

FDS4953

Si4532ADY

IRF7105

n-channel p-channel n-channel p-channel n-channel p-channel

Drain-Source On-Resistance

RDS(ON)@VGS = +/-10V



0.022

0.053

0.053

0.08

0.10

0.25

Drain-Source Voltage

VDSS (V)

30

-30

30

-30

25

-25

Drain Current (continuous)

ID(continuous) (A)

8.4

-5

4.9

-3.9

3.5

-2.3

Total Gate Charge

9

8

8

10

9.4

10

Manufacturer

Fairchild

Fairchild

Vishay

Siliconix

Vishay

Siliconix

International

Rectifier

International

Rectifier

  MOSFET Component Options