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SAN3021 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 2 3 4 5 6 SCOPE................................................................................................................................................................. 1 KEY FEATURES ................................................................................................................................................. 1 OTHER APPLICABLE DOCUMENTS AND PAPERS ........................................................................................ 1 REVISION STATUS............................................................................................................................................. 1 GENERAL DESCRIPTION .................................................................................................................................. 2 HARDWARE CONFIGURATION......................................................................................................................... 2
6.1 USING THE DYNAMIC MICROPHONE AS TONE RINGER (SCHEMATIC :A)................................................................. 2 6.1.1 Ringer part ............................................................................................................................................. 2 6.1.2 Microphone amplifier part ...................................................................................................................... 3 6.2 AMPLIFIER FOR GENERAL PURPOSE APPLICATIONS (SCHEMATIC :B) ................................................................... 3 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
SAN3021 Application Note (this document) SAN3021 Schematic Rev. 1.1
SAN3021
PDS038-SA2531/2-004
Rev. B
21-03-00
SAN3021
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 microphone (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 microphone, 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 amplifier Q4, 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 VDD, 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 microphone amplifier input to VSS (over C10) but at the same time keep the DC level above VDD in ringing mode. The ringer cannot be referenced to VSS, because it would discharge VDD during ringing! With the application shown, the anode of D6 will be VDD+0.7V, because during ringing VDD is supplied over the ringing capacitor. (not shown).
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SAN3021 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 220F. Since this supply is derived from the line output, there will be some small ripple from the transmitted signal at C14. In order not to form a positive feedback loop from LI over the microphone supply, Q5 and the internal microphone 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. Therefore 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 = 2k). Amplifier gain = RR21 / RR23B = 3:1 = 10dB. This application can directly replace an electret microphone. Because of the differential, balanced structure of the amplifier input , it is also suitable for longer distances between microphone and amplifier, e.g. corded handsets.
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SAN3021
7 Application schematic:
1 2 3 4 5 6 7 8
Applications Using Dynamic Handset Microphone
A
A. Using dynamic handset microphone as ringer in one-piece telephones.
21
+
C14 220u/25V
R21 6..10K
R24 360K
B. Amplifier for general applications.
A
R20 2K2 FCI LS
1 27
M1
23
C12 X M2 IC1
24
C11 100N
Q5 BC549
C16 1N
LI
C13 100N
C15 1N
RR20 2K2
LI
R23 2K R25 110K
27
+
CC14 100u/25V
RR21 3K
RR24 27K
off
28
CC11 100N
SA2531/2
CC13 100N
6
STB
RO2 AGND RR
3
5
25 7
CS CI
RR23A 1K
+
CC15 1N MIC
R7 220K
M2
12
24
1K1 RR26
B
8
5
RI
RO1
2
6 on 7 SW1_b
Q6 BC549
CC19 1N
M1
23
B
+
CC17 10U
CC18 10U
RR25 12K
R17 100K
10
1N4148
Q4 BC 547B D5
8 22
HS/DP MO MODE LLC
C1 C2 C3 C4 R1 R2 R3 R4 OSC
16 15 14 13 20 19 18 17
R26 1K1
RR23B 2K
+
R18 >=0
C
MIC
C8 10u/25V D4 15V 1N4148 R15 330K D6
9
C17 10n
11
C
4
VDD
26
+
C10 10u/16V
D11 5V6
+
C9 470u/16V
The information furnished here by SAMES is believed to be correct and accurate. However, SAMES shall not
D
VSS
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 Sh 01 of 01 Date : 3rd March 1997 Rev : 1.1
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SAN3021
8 Liability and Copyright Statement
Disclaimer: The information contained in this document is confidential and proprietary to South African 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. The information contained herein is current as of the date of publication; however, delivery of this document shall not under any circumstances create 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 document of any changes in the information contained herein, and SAMES expressly 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 information contained herein, will function without errors and as intended by the designer.
South African Micro-Electronic Systems (Pty) Ltd
P O Box 15888, Lynn East, 0039 Republic of South Africa, 33 Eland Street, Koedoespoort Industrial Area, Pretoria, Republic of South Africa
Tel: Fax:
012 333-6021 012 333-3158
Tel: Fax:
Int +27 12 333-6021 Int +27 12 333-3158
Web Site : http://www.sames.co.za
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