U4082B Datasheet, PDF (21/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

U4082B
Figure 7-11. V_B Power Supply Rejection versus Frequency Characteristics and V_B Capacitor
80
C_VB = 1000 µF
500 µF
60
200 µF
100 µF
50 µF
40
20
0.3
1
f (kHz)
2
3
8. Design Guidelines
8.1 Switching Time
The switching time of the U4082B circuit is determined primarily by C_T (pin 14, refer to Figure
3-3 on page 7), and secondarily by the capacitors at the level detector outputs (RLO1, RLO2,
TLO1, TLO2). For more information, please refer to Figure 1-2 on page 2.
The time to switch from idle to receive or transmit mode is determined by the capacitor at C_T,
together with the internal current sources. The switching time is:
∆MinimalT
=
-^∆----M-----i-n---i--m-----a---l-V------^×------C----^T-
I
=
-2---4---0----M-----i-n---i--m-----a---l---^×-----5--
60
=
20.0 ms
where
∆V = 240 mV
C_T = 5 µF
I = 60 µA
If the circuit switches directly from receive to transmit mode (or vice versa), the total switching
time would be 40 ms.
The switching time depends upon the mode selection. If the circuit is going to “fast idle”, the time
constant is determined by the C_T capacitor, and the internal 2 kΩ resistor. With C_T = 5 µF, the
time constant is approximately 10 ms, giving a switching time to idle of approximately 30 ms (for
95% change). Fast idle is an infrequent mode, however, occurring when both speakers are talk-
ing and competing for control of the circuit. The switching time from idle back to either transmit or
receive mode is described above.
By switching to “slow idle” the time constant is determined by the C_T capacitor and R_T, the exter-
nal resistor (see Figure 3-4 on page 9). With C_T = 5.0 µF and R_T = 120 kΩ, the time constant is
approximately 600 ms, giving a switching time of approximately 1.8 seconds (for 95% change).
The switching period to slow idle begins when both speakers have stopped talking. The switch-
ing time back to the original mode will depend on how soon that speaker begins speaking again.
The sooner the speaking starts during the 1.8s period, the quicker the switching time since a
smaller voltage excursion is required. The switching time is determined by the internal current
source as described above.
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4743D–CORD–03/06