U4082B Datasheet, PDF (25/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

U4082B
8.2.4
An acoustic loss of at least 23 dB is necessary to prevent instability and oscillations, commonly
referred to as “singing”. However, the following equations show that greater acoustic loss is nec-
essary to obtain proper level detection and switching.
Switching Thresholds
To switch comparator C1, currents I₁ and I₃ need to be determined. Referring to Figure 8-3 on
page 23, with a receive signal V_L applied to Tip/Ring, a current I₃ will flow through R₃ into RLI2
according to the following equation:
I₃
=
-V----^L-
R₃
×
G_HR ×
G_FO
×
G_R
×
G-----_S----A-
2
(4)
where the terms in the brackets are the V/V gain terms. The speaker amplifier gain is divided by
two since GSA is the differential gain of the amplifier, and V₃ is obtained from one side of that
output. The current I₁, coming from the microphone circuit, is defined by:
I₁
=
V-----_M-----×_{------G-----M----A-}
R₁
(5)
where V_M is the microphone voltage. Since the switching threshold occurs when I₁ = I₃, combin-
ing the above two equations yields:
V_M
=
V_L ×
R-----_1-
R₃
×
-^[--G-----^H---^R-----^×------G-----^F---^O-----^×------G----^R-----^×------G-----^S---^A----^]
G_MA × 2
(6)
This is the general equation defining the microphone voltage necessary to switch comparator C1
when a receive signal V_L is present. The highest V_M occurs when the receive attenuator is at
maximum gain (+6.0 dB). Using the typical numbers for equation 6 yields:
V_M = 0.52 × V_L
(7)
To switch comparator C2, currents I₂ and I₄ need to be determined. With sound applied to the
microphone, a voltage V_M is created by the microphone, resulting in a current I₂ into TLI1:
I₂
=
V-----_M--
R₂
×
G_MA
×
G_T ×
G-----_H----A-
2
(8)
Since G_HA is the differential gain of the hybrid amplifiers, it is divided by two to obtain the voltage
V₂ applied to R₂. Comparator C2 switches when I₄ = I₂. I₄ is defined by:
I₄
=
-V----^L-
R₄
[
G_HR
×
G_FO ]
(9)
Setting I₄ = I₂, and combining the above equations results in:
V_L
=
V_M ×
R-----_4-
R₂
×
-^[--G-----^M----^A----^×------G-----^T----^×------G-----^H---^A----^]
G_HR × G_FO × 2
(10)
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