Datasheet SAN3021 Datasheet (SAMES)

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APPLICATION NOTE

SA2531/2 Single Chip Telephone:

Using dynamic microphone

as tone ringer

1 Scope

This Application Note describes how to modify the standard application AN1500A to use a dynamic microphone instead of the standard electret microphone. Additionally, an application is shown, which uses the dynamic microphone as tone ringer.

2 Key Features

•

Updating existing systems using dynamic handset microphone

•

No need for a piezo ringer when using the dynamic microphone as tone ringer

Table of Contents

1 SCOPE................................................................................................................................................................. 1

2 KEY FEATURES ................................................................................................................................................. 1

3 OTHER APPLICABLE DOCUMENTS AND PAPERS........................................................................................ 1

4 REVISION STATUS............................................................................................................................................. 1

5 GENERAL DESCRIPTION.................................................................................................................................. 2

6 HARDWARE CONFIGURATION......................................................................................................................... 2

6.1 U

6.1.1 Ringer part............................................................................................................................................. 2

6.1.2 Microphone amplifier part...................................................................................................................... 3

6.2 A

7 APPLICATION SCHEMATIC:............................................................................................................................. 4

8 LIABILITY AND COPYRIGHT STATEMENT...................................................................................................... 5

3 Other applicable documents and papers

Application Note AN1500A: SA2532K Single Chip Telephone Demo Board

4 Revision status

SING THE DYNAMIC MICROPHONE A S TONE RINGER (SCHEMATIC :A

MPLIFIER FOR GENERAL PURPOSE APPLICATIONS (SCHEMATIC :B

)................................................................. 2

)................................................................... 3

SAN3021 Application Note (this document) SAN3021 Schematic Rev. 1.1

SAN3021 PDS038-SA2531/2-004 Rev. B 21-03-00

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5 General description

Dynamic handset microphones have been widely replaced by electret microphones, mainly because of higher output level, low EMC sensitivity and price issues. Therefore, the standard Single Chip Telephone application, AN1500 is based on using an electret handset microphone.

However, there are situations where usage of a dynamic microphone is still desired, e.g.

•

when updating an existing design , using a dynamic handset microphone

•

when taking advantage of the higher speech quality of dynamic microphones

•

when using the dynamic microphone as tone ringer

6 Hardware configuration

Generally, because of the lower output level of a dynamic micr ophone (≈10dB less than an electret microphone) the microphone signal must be amplified by a simple 10dB amplifier before it can be fed into the microphone inputs, M1 and M2 of the SA2531/2.

In the following schematic, two basic considerations when using dynamic microphones are shown:

1. A single ended circuit, which uses the dynamic microphone for both microphone and tone ringer.

2. A differential circuit, which is fully balanced, to replace an electret microphone.

6.1 Using the Dynamic Microphone as Tone Ringer (Schematic :A)

6.1.1 Ringer part The piezo tone ringer used in AN1500 can be replaced by a dynamic m icrophone, when close attention is paid to

the following considerations : Since a maximum sound pressure is required in ringing mode, the maximum DC current and AC level

for the type of dynamic microphone used must not be exceeded. To limit the output of the ringer am plifier Q 4, the zener diode D4 should be reduced to 15V . R18 limits the output

current, it can also be used to set the ringer volume. Since the microphone (= the ringer) is connected single ended to V

, any noise on VDD would be amplified by the

microphone amplifier, causing higher noise for the transmitted signal. Therefore, D6 and C10 are added to reference the micr ophone amplifier input to V

(over C10) but at the same tim e keep the DC level above VDD in

ringing mode. The ringer cannot be referenced to V With the application s hown, the anode of D6 will be V

, because it would discharge VDD during ringing!

+0.7V, because during ringing VDD is supplied over the

ringing capacitor. (not shown).

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6.1.2 Microphone Amplifier Q5 is a simple “classic” amplifier, which amplifies the single ended signal by ≈10dB. Gain can be adjusted by

varying R21. R23 is the feedback resistor to set a stable DC bias point. R24 and R25 are setting the base voltage. The transistor type used (in this case BC549) should be a general purpose low noise transistor.

The single ended signal is decoupled by C17. R26 is required to limit the input current to the amplifier during ringing.

C15 and C16 are EMC capacitors to limit the bandwidth of the amplifier. Frequencies >10kHz are being attenuated. The amplifier is supplied by (the initial electret microphone supply) R20 and C14. C14 should be increased to

220µF. Since this supply is derived from the line output, there will be som e small r ipple from the transm itted signal at C14.

In order not to form a positive f eedback

loop from LI over the m icrophone supply, Q5 and the internal m icrophone amplifier (M1/M2 to LI) , care must be taken to connect M1/M2 inputs in the correct manner as shown in the application.

With this connection, a negative feedback loop is formed, rejecting any ripple on C14.

Because of the unbalanced, single ended structure of this amplifier, its input is very sensitive. T herefore it is recommended to use this application only in one-piece telephones with shortest possible wires between microphone and amplifier !

6.2 Amplifier for General Purpose Applications (Schematic :B)

This amplifier is similar to the amplifier described in pt. 6.1, however the microphone is balanced (RR23B and RR26 = 2kΩ).

Amplifier gain = RR21 / RR23B = 3:1 = 10dB.

This application can directly replace an electret microphone. Becaus e of the differential, balanced str ucture of the amplifier input , it is also suitable for longer distances between microphone and amplifier, e.g. corded handsets.

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7 Application schematic:

1 2 3 4 5 6 7 8

Applications Using Dynamic Handset Microphone

A. Using dynamic handset

microphone as ringer

R20

2K2

C14 220u/25V

R21

6..10K

R24 360K

B. Amplifier for general

applications.

in one-piece telephones.

IC1

AGND

SA2531/2

C9 470u/16V

RO1

RO2

OSC

C12

C11

100N

C13

BC549

100N

VSS

264

R23 2K

C17

10n

C16

C15

R26 1K1

R25 110K

2K2

RR20

CC14 100u/25V

CC11

CC13

100N

BC549

RR21 3K

RR23A 1K

RR23B 2K

CC19

CC15

MIC

CC17 10U

1K1

RR26

RR24 27K

RR25 12K

CC18 10U

FCI

off

R17 100K

SW1_b

C8 10u/25V

C10 10u/16V

756

220K

R15 330K

15V

BC 547B

MIC

R18

>=0

1N4148

D11

5V6

STB

CI RR

HS/DP

MODE

LLC

VDD

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The information furnished here by SAMES is believed to be correct and accurate. However, SAMES shall not

be held liable to any party for any damages including but not limited to personal injury, property damage, loss of profits, loss of use, interuption of business or indirect, special, incidental or consequential damages, of any kind, in connection with or arising out of the furnishing, performance or use of the technical data. No obligation or liability to any third party shall arise or flow out of SAMES rendering technical or other services.

SAMES Telecom

Sch.

SAN3021 Pn# Single Chip Telephone Application Circuit

01 01

3rd March 1997Date :of

Rev :

1.1

1 2 3 4 5 6 7 8

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8 Liability and Copyright Statement

Disclaimer:

Micro-Electronic Systems (Pty) Ltd ("SAMES”) and may not be copied or disclosed to a third party, in whole or in part, without the express written consent of SAMES. T he information c ontained herein is current as of the date of publication; however, delivery of this document shall not under any circum stances cr eate any implication that the information contained herein is correct as of any time subsequent to such date. SAMES does not undertake to inform any recipient of this docum ent of any changes in the information contained herein, and SAMES express ly reserves the right to make changes in such information, without notification,even if such changes would render information contained herein inaccurate or incomplete. SAMES makes no representation or warranty that any circuit designed by reference to the inform ation contained her ein, will func tion without error s and as intended by the designer

The information contained in this document is confidential and proprietary to South African

South African Micro-Electronic Systems (Pty) Ltd

P O Box 15888, 33 Eland Street, Lynn East, Koedoespoort Industrial Area, 0039 Pretoria, Republic of South Africa, Republic of South Africa

Tel: 012 333-6021 Tel: Int +27 12 333-6021 Fax: 012 333-3158 Fax: Int +27 12 333-3158

W eb Site : http://www.sames.c o.za

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