TS4984 datasheet(22/29 Pages) STMICROELECTRONICS

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TS4984

Application Information

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4.5 Decoupling the circuit

Two capacitors are needed to correctly bypass the TS4984. 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

performances to those shown in the datasheet. For example:

l In the high frequency region, if CS 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 CS is higher than 1 µF, those 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), in the following manner:

l If Cb is lower than 1µF, THD+N increases at lower frequencies and PSRR worsens.

l If Cb is higher than 1µF, the benefit on THD+N at lower frequencies is small, but the benefit to PSRR

is substantial.

Note that Cin has a non-negligible effect on PSRR at lower frequencies. The lower the value of Cin, the

higher the PSRR.

4.6 Wake-up time, TWU

When the standby is released to put the device ON, the bypass capacitor Cb will not be 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 wake-up time or TWU and specified in electrical

characteristics table with Cb =1µF.

If Cb 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.

Note:

Bypass capacitor Cb 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 Cb = 1 µF in the range of 0.8 µF ≤ 1µF ≤ 1.2 µF).

Figure 66: Typical wake-up time vs. Cb

Figure 67: Maximum wake-up time vs. Cb

100

200

300

400

500

600

4.7

0.1

Tamb=25

°C

Vcc=2.6V

Vcc=3.3V

Vcc=5V

Bypass Capacitor Cb ( F)

100

200

300

400

500

600

Tamb=25

°C

4.7

0.1

Vcc=5V

Vcc=3.3V

Vcc=2.6V

Bypass Capacitor Cb ( F)

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