TS488 Datasheet, PDF (26/32 Pages) STMicroelectronics – Pop-free 120mW stereo headphone amplifier

Application information
TS488-TS489
4.8
Wake-up time
When the standby is released to put the device ON, the bypass capacitor C_b is 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 plus a time delay of
20ms (pop precaution) is called the wake-up time or t_WU; it is specified in the electrical
characteristics table with C_b = 1µF.
If C_b has a value other than 1µF, t_WU can be calculated by applying the following formulas or
can be read directly from Figure 83.
t_WU
=
-^C----^b----^â‹------²---^.--⁵-- + 20
0.03125
[ms;μF ]
Note:
Figure 83. Typical wake-up time vs. bypass capacitance
400
T =25°C
AMB
350
300
250
200
150
100
50
0
0
1
2
3
4
5
Cb (μF)
It is assumed that the C_b voltage is equal to 0V. If the C_b voltage is not equal to 0V, the
wake-up time is shorter.
4.9
POP performance
Pop performance is closely related to the size of the input capacitor C_in. The size of C_in is
dependent on the lower cut-off frequency and PSRR values requested.
In order to reach low pop, C_in must be charged to V_CC/2 in less than 20ms. To follow this
rule, the equivalent input constant time (R_inC_in) should be less then 6.7ms:
τ _in = R_in x C_in < 0.0067 (s)
Example calculation:
In the typical application schematic R_in is 20kΩ and C_in is 330nF. The lower cut-off frequency
(-3db attenuation) is given by the following formula:
F_CL
=
------------------¹-------------------
2π ⋅ R_in ⋅ C_in
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