TS4974 datasheet(19/25 Pages) STMICROELECTRONICS

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TS4974

Application information

19/25

4.4

Decoupling of the circuit

Two capacitors are needed to correctly bypass the TS4974. A power supply bypass

capacitor CS and a bias voltage bypass capacitor Cb.

CS 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 CS of 1 µF, you can

expect similar THD+N performance to that shown in the datasheet.

In the high frequency region, if CS 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 CS is higher than 1 µF, the disturbances on the power supply rail are

more filtered.

Cb 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 (tWU)

When the standby is released to put the device ON, the bypass capacitor Cb is not charged

immediately. As Cb is directly linked to the bias of the amplifier, the bias will not work

properly until the Cb voltage is correct. The time to reach this voltage is called the wake-up

time or tWU and is specified in the tables in Section 3: Electrical characteristics on page 5,

with Cb=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 Cb has a value other than 1 µF, refer to Figure 40 to establish the wake-up time.

Figure 40.

Startup time vs. bypass capacitor

4.6

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.

Note:

In shutdown mode, the Bypass pin and Vin+, Vin- pins are short-circuited to ground by

internal switches. This allows a quick discharge of Cb and Cin capacitors.

0,4

0,8

1,2

1,6

2,0

Tamb=25

°C

Vcc=2.6V

Vcc=3.3V

Vcc=5V

Bypass Capacitor Cb (

μF)

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