5.5

Decoupling of the circuit

Two capacitors are needed to bypass properly the TS4871, a power supply bypass capacitor Cs and a bias

voltage bypass capacitor Cb.

Cs has especially an influence on the THD+N in high frequency (above 7 kHz) and indirectly on the power supply

disturbances.

With 100 μF, you can expect similar THD+N performances like shown in the datasheet.

If Cs is lower than 100 μF, in high frequency increases, THD+N and disturbances on the power supply rail are

less filtered.

To the contrary, if Cs is higher than 100 μF, those disturbances on the power supply rail are more filtered.

Cb has an influence on THD+N in lower frequency, but its function is critical on the final result of PSRR with input

grounded in lower frequency.

If Cb is lower than 1 μF, THD+N increase in lower frequency (see THD+N vs frequency curves) and the PSRR

worsens up.

If Cb is higher than 1 μF, the benefit on THD+N in lower frequency is small but the benefit on PSRR is substantial

(see PSRR vs. Cb curve: fig.12).

Note that Cin has a non-negligible effect on PSRR in lower frequency. Lower is its value, higher is the PSRR (see

fig. 13).

5.6

Pop and Click performance

Pop and click performance is intimately linked to the size of the input capacitor Cin and the bias voltage bypass

capacitor Cb.

Size of Cin is due to the lower cut-off frequency and PSRR value requested. Size of Cb is due to THD+N and

PSRR requested always in lower frequency.

Moreover, Cb determines the speed that the amplifier turns ON. The slower the speed is, the softer the turn ON

noise is.

The charge time of Cb is directly proportional to the internal generator resistance 50 kW.

Then, the charge time constant for Cb is

τb = 50 kΩxCb (s)

As Cb is directly connected to the non-inverting input (pin 2 and 3) and if we want to minimize, in amplitude and

duration, the output spike on Vout1 (pin 5), Cin must be charged faster than Cb. The charge time constant of Cin

is

τin = (Rin+Rfeed)xCin (s)

Thus we have the relation

τin << τb (s)

The respect of this relation allows the pop and click noise to be minimized.

Remark : Minimize Cin and Cb has a benefit on pop and click phenomena but also on cost and size of the

application.

Example : your target for the -3 dB cut off frequency is 100 Hz. With Rin = Rfeed = 22 kΩ, Cin = 72 nF (in fact 82

nF or 100 nF).

With Cb = 1 μF, if you choose the one of the latest two values of Cin, the pop and click phenomena at power

supply ON or standby function ON/OFF will be very small

50 kΩx1 μF >> 44 kΩ x 100 nF (50 ms >> 4.4 ms).

Increasing Cin value increases the pop and click phenomena to an unpleasant sound at power supply ON and

standby function ON/OFF.

Why Cs is not important in pop and click consideration ?

Hypothesis :

Cs = 100 μF

Supply voltage = 5 V

Supply voltage internal resistor = 0.1 Ω

Supply current of the amplifier Icc = 6 mA

At power ON of the supply, the supply capacitor is charged through the internal power supply resistor. So, to

reach 5 V you need about five to ten times the charging time constant of Cs (τs = 0.1 x Cs (s)).

Then, this time equal 50 μs to 100 μs << τb in the majority of application.

TS4871

Application information

DS2547 - Rev 10

page 26/42