TS4984 Datasheet, PDF (22/29 Pages) STMicroelectronics – 2 x 1W Stereo audio power amplifier with active low standby mode

TS4984
Application Information
4.5 Decoupling the circuit
Two capacitors are needed to correctly bypass the TS4984. A power supply bypass capacitor C_S and a
bias voltage bypass capacitor C_B.
C_S has particular influence on the THD+N in the high frequency region (above 7 kHz) and an indirect
influence on power supply disturbances. With a value for C_S of 1 µF, you can expect similar THD+N
performances to those shown in the datasheet. For example:
l In the high frequency region, if C_S is lower than 1 µF, it increases THD+N and disturbances on the
power supply rail are less filtered.
l On the other hand, if C_S is higher than 1 µF, those disturbances on the power supply rail are more
filtered.
C_b has an influence on THD+N at lower frequencies, but its function is critical to the final result of PSRR
(with input grounded and in the lower frequency region), in the following manner:
l If C_b is lower than 1µF, THD+N increases at lower frequencies and PSRR worsens.
l If C_b is higher than 1µF, the benefit on THD+N at lower frequencies is small, but the benefit to PSRR
is substantial.
Note that C_in has a non-negligible effect on PSRR at lower frequencies. The lower the value of C_in, the
higher the PSRR.
4.6 Wake-up time, T_WU
When the standby is released to put the device ON, the bypass capacitor C_b will not be charged
immediately. As C_b is directly linked to the bias of the amplifier, the bias will not work properly until the C_b
voltage is correct. The time to reach this voltage is called wake-up time or T_WU and specified in electrical
characteristics table with C_b = 1 µF.
If C_b has a value other than 1 µF, please refer to the graph in Figure 66 to establish the wake-up time
value.
Due to process tolerances, the maximum value of wake-up time could be establish by the graph in
Figure 67.
Figure 66: Typical wake-up time vs. C_b
Figure 67: Maximum wake-up time vs. C_b
600
Tamb=25°C
500
400
Vcc=2.6V
300
Vcc=3.3V
600 ^Tamb=25°C
500
400
Vcc=2.6V
Vcc=3.3V
300
200
100
0
0.1
Vcc=5V
1
2
3
4
4.7
Bypass Capacitor Cb ( F)
200
100
0
0.1
Vcc=5V
1
2
3
4
4.7
Bypass Capacitor Cb ( F)
Note: Bypass capacitor C_b as also a tolerance of typically +/-20%. To calculate the wake-up time with this tolerance,
refer to the previous graph (considering for example for C_b = 1 µF in the range of 0.8 µF ≤ 1 µF ≤ 1.2 µF).
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