TS4851

Application Information

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5.3 Low frequency response

In low frequency region, the effect of Cin starts. Cin with Zin forms a high pass filter with a -3dB cut off

frequency.

Zin is the input impedance of the corresponding input:

•20k

Ω for Phone In IHF input

•50k

Ω for the 3 other inputs

Note: For all inputs, the impedance value remains constant for all gain settings. This means that the lower cut-off

frequency doesn’t change with gain setting. Note also that 20k

Ω and 50kΩ are typical values and there are

tolerances around these values (see Electrical Characteristics on page 7).

5.4 Decoupling of the circuit

Two capacitors are needed to bypass properly the TS4851, 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 7kHz) and indirectly on the power

supply disturbances.

With 1µF, you could expect similar THD+N performances like shown in the datasheet.

If Cs is lower than 1µF, THD+N increases in high frequency and disturbances on the power supply rail are

less filtered.

To the contrary, if Cs is higher than 1µF, those disturbances on the power supply rail are more filtered.

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

input grounded in lower frequency:

•

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

•

If Cb is higher than 1µF, the benefit on THD+N and PSRR in the lower frequency range is small.

5.5 Startup time

When the TS4851 is controlled to switch from the full standby mode (output mode 0) to another output

mode, a delay is necessary to stabilize the DC bias. This delay depends on the Cb value and can be

calculated by the following formulas.

Typical startup time = 0.0175 x Cb (s)

Max. startup time = 0.025 x Cb (s)

(Cb is in µF in these formulas)

These formulas assume that the Cb voltage is equal to 0V. If the Cb voltage is not equal to 0V, the startup

time will be always lower.

The startup time is the delay between the negative edge of Enable input (see Description of SPI operation

on page 3) and the power ON of the output amplifiers.

Note: When the TS4851 is set in full standby mode, Cb is discharged through an internal switch.. The time to reach 0V

of Cb voltage is about ms.

)

Hz

(

Cin

Zin

2

1

F

CL

π

=