LX1711 データシートの表示（PDF） - Microsemi Corporation

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LX1711

AUDIOMAX EVALUATION KIT USER'S GUDIE

Microsemi Corporation 

LXE1710 EVALUATION BOARD

USER GUIDE

MOSFET SELECTION

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.

=

POUT

PIN

=

I 2 RL

I 2[2(RNDS + RPDS + RIND) + RL] + PCROSS

Where

RL =

RNDS =

RPDS =

RIND =

PCROSS =

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.

  


MOSFET Power Loss = PDS = I 2[2(RNDS + RPDS)]

If

PO = 25W at 4Ω

Then I = P = 25 = 2.5A

R4

The LX1710 Evaluation Board is designed using

FDS4953 p-channel and FDS6612A n-channel

MOSFETS.

RNDS = 0.03Ω, RPDS = 0.095Ω

PDS = (2.5)2[2(0.03 + 0.095)] = 1.56W

MOSFET power loss is proportional to on-resistance.

      

MOSFET SwitchingLoss= PCROSS = CV 2 fSn

Where

C = Input Capacitance

V = Supply Voltage

fS = Switching Frequency

n = Number of MOSFETS

Assume C = 1000pF

V = 15VDC

fS = 500kHz

PCROSS = (1×10−9 )(152 )(500 ×103 )(4) = 0.45W

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 PDS and PCROSS.

Linfinity recommends that in selecting MOSFETs, RDS

0 1!1  2g <10nC. The table below provides

several MOSFET options.

FDS6612A FDS4953

Si4532ADY

IRF7105

Drain-Source On-Resistance

Drain-Source Voltage

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



VDSS (V)

n-channel

0.022

30

p-channel n-channel p-channel

0.053

0.053

0.08

-30

30

-30

n-channel

0.10

25

p-channel

0.25

-25

Drain Current (continuous)

ID(continuous) (A)

8.4

-5

4.9

-3.9

3.5

-2.3

Total Gate Charge

Qg (typical) (nC)

Manufacturer

  MOSFET Component Options

9

Fairchild

8

Fairchild

8

Vishay

Siliconix

10

Vishay

Siliconix

9.4

10

International International

Rectifier Rectifier

Copyright © 2000

Rev. 1.1, 2000-12-01

Microsemi

Linfinity Microelectronics Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 11

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