TS4994 ST Microelectronics, Inc., TS4994 Datasheet - Page 21

• Manufacturer Part Number: TS4994
• Description: 1.2W Differential Input Audio Power Amplifier With Selectable Standby
• Manufacturer: ST Microelectronics, Inc.

Application Information
4.4 Low and high frequency response
In the low frequency region, C
effect. C
-3dB cut-off frequency. F
In the high-frequency region, you can limit the
bandwidth by adding a capacitor (C
with R
cut-off frequency. F
While these bandwidth limitations are in theory
attractive,
performance in terms of capacitor precision (and
by consequence in terms of mismatching), they
deteriorate the values of PSRR and CMRR.
We will discuss the influence of mismatching on
PSRR and CMRR performance in more detail in
the following paragraphs.
Example: A typical application with input coupling
and feedback capacitor with F
F
4.5 Calculating the influence of mismatching
On PSRR performance:
For this calculation, we consider that C
have no influence.
We use the same kind of resistor (same
tolerance) and R is the tolerance value in %.
The following equation is valid for frequencies
ranging from DC to about 1kHz. Above this
frequency, parasitic effects start to be significant
and a literal equation is not possible to write.
The PSRR equation is ( R in %):
This equation doesn’t include the additional
performance
filtering. If a bypass capacitor is added, it acts,
together with the internal high output impedance
bias, as a low-pass filter, and the result is a quite
CH
=8kHz. We assume that the mismatching
PSRR
feed
in
F
. It forms a low-pass filter with a -3dB
forms, with R
CH
F
CL
in
20
provided
2
2
practice,
Log
CH
R
is in Hz.
(
feed
10000
R
in
1
1
CL
, a high-pass filter with a
in
by
R
is in Hz.
C
C
because
in
100
feed
in
bypass
starts to have an
R
(
Hz
2
feed
CL
)
(
Hz
)
=50Hz and
in
) in parallel
)
(
and C
dB
capacitor
of
)
feed
low
between R
we sweep the frequency from DC to 20kHz we
observe the following with respect to the PSRR
value:
important PSRR improvement with a relatively
small bypass capacitor.
The complete PSRR equation ( R in %, C
microFarad and F in Hz) is:
Example: With R=0.1% and C
PSRR would be -60dB. With a 100nF bypass
capacitor, at 100Hz the new PSRR would be
-93dB.
This example is a worst case scenario, where
each
illustrates the fact that with only a small bypass
capacitor, the TS4994 produce high PSRR
performance.
20
PSRR
l
l
l
From
decreases from infinite to a finite value and
the C
neglected. Due to the tolerance of C
must introduce a mismatch (R
R
performance.
From 200Hz to 5kHz, C
enough to be neglected compare to R
C
neglected. In this range, we can reach the
PSRR performance of the TS4994 itself.
From 5kHz to 20kHz, C
be neglected compare to R
impedance decreases to a finite value. Due
to tolerance of C
mismatching (R
that will decrease the PSRR performance.
Log
in2
feed
resistor
xC
(
feed
10000
impedance is high enough to be also
in1,2
in2
DC
) that will decrease the PSRR
impedance is high enough to be
and C
has
to
R
2
feed1
)
feed1,2
200Hz,
R
extreme
feed1,2
xC
1
100
in
feed1
F
can be neglected. If
in
2
impedance is low to
, we introduce a
impedance is low
Cb
b
C
=0, the minimum
tolerance
in
2
R
in
feed2
22
impedance
in1
and C
2 .
TS4994
in1,2
xC
xC
in
(
in1
feed2
21/32
dB
, we
b
and
and
feed
)
in
)