TS4974_07 Datasheet, PDF (19/25 Pages) STMicroelectronics – 1W differential audio power amplifier with up/down digital volume control pins

TS4974
Application information
4.4
Decoupling of the circuit
Two capacitors are needed to correctly bypass the TS4974. 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 performance to that shown in the datasheet.
In the high frequency region, if C_S is lower than 1 µF, it increases the THD+N, and
disturbances on the power supply rail are less filtered.
On the other hand, if C_S is higher than 1 µF, the 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).
4.5
Wake-up time (t_WU)
When the standby is released to put the device ON, the bypass capacitor C_b is not 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 the wake-up
time or t_WU and is specified in the tables in Section 3: Electrical characteristics on page 5,
with C_b=1 µF. During the wake-up phase, the TS4974 gain is close to zero. After the wake-
up time, the gain is released and set to its nominal value.
If C_b has a value other than 1 µF, refer to Figure 40 to establish the wake-up time.
Figure 40. Startup time vs. bypass capacitor
15
Tamb=25°C
Vcc=5V
10
Vcc=3.3V
Vcc=2.6V
5
0
0,4
0,8
1,2
1,6
2,0
Bypass Capacitor Cb (μF)
4.6
Note:
Shutdown time
When the standby command is set, the time required to put the two output stages in high
impedance and the internal circuitry in shutdown mode is a few microseconds.
In shutdown mode, the Bypass pin and Vin+, Vin- pins are short-circuited to ground by
internal switches. This allows a quick discharge of C_b and C_in capacitors.
19/25