STMicroelectronics Pop-free 120 mW stereo headphone amplifier DEMOTS488S DEMOTS488S データシート
製品コード
DEMOTS488S
Application information
TS488-TS489
26/32
Doc ID 11971 Rev 5
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
20 ms (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.
Typical wake-up time vs. bypass capacitance
Note:
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 cutoff frequency and PSRR values requested.
In order to reach low pop, C
in
must be charged to
V
CC
/2 in less than 20 ms. To follow this
rule, the equivalent input constant time (R
in
C
in
) should be less then 6.7 ms:
τ
in
= R
in
x C
in
< 0.0067 (s)
Example calculation:
In the typical application schematic R
in
is 20 k
Ω and C
in
is 330 nF. The lower cutoff
frequency (-3 db attenuation) is given by the following formula:
With the values above, the result is F
CL
= 25 Hz.
In this case,
τ
in
= R
in
x C
in
=6.6 ms.
This value is sufficient with regard to the previous formula, thus we can state that the pop is
imperceptible.
imperceptible.
t
WU
C
b
2.5
⋅
0.03125
---------------------
20 [ms;
μF ]
+
=
0
1
2
3
4
5
0
50
100
150
200
250
300
350
400
T
AMB
=25°C
Wa
ke-up
Time (ms)
Cb (
μ
F)
F
CL
1
2
π R
in
C
in
⋅
⋅
---------------------------------
=