TS4871 ST Microelectronics, Inc., TS4871 Datasheet - Page 25

• Manufacturer Part Number: TS4871
• Description: 1W Mono Audio Amplifier With Standby Active High
• Manufacturer: ST Microelectronics, Inc.

We have finished a design and we have chosen
the components values :
• Rin=Rfeed=22k
• Cin=100nF
• Cb=1µF
Now, on fig. 13, we can see the PSRR (input
grounded) vs frequency curves. At 217Hz we have
a PSRR value of -36dB.
In reality we want a value about -70dB. So, we
need a gain of 34dB !
Now, on fig. 12 we can see the effect of Cb on the
PSRR (input grounded) vs. frequency. With
Cb=100µF, we can reach the -70dB value.
The process to obtain the final curve (Cb=100µF,
Cin=100nF, Rin=Rfeed=22k ) is a simple transfer
point by point on each frequency of the curve on
fig. 13 to the curve on fig. 12. The measurement
result is shown on the next figure.
Fig. 85 : PSRR changes with Cb
What is the PSRR ?
The PSRR is the Power Supply Rejection Ratio.
It's a kind of SVR in a determined frequency range.
The PSRR of a device, is the ratio between a
power supply disturbance and the result on the
output. We can say that the PSRR is the ability of
a device to minimize the impact of power supply
disturbances to the output.
How do we measure the PSRR ?
Note on how to use the PSRR curves
(page 7)
-30
-40
-50
-60
-70
10
Cin=100nF
Cb=1 F
100
Cin=100nF
Cb=100 F
Frequency (Hz)
Vcc = 5, 3.3 & 2.6V
Rfeed = 22k, Rin = 22k
Rg = 100 , RL = 8
Tamb = 25 C
1000
10000
100000
Fig. 86 : PSRR measurement schematic
• We fixed the DC voltage supply (Vcc), the AC
sinusoidal ripple voltage (Vripple) and no supply
capacitor Cs is used
The PSRR value for each frequency is:
Remark : The measure of the Rms voltage is not a
Rms selective measure but a full range (2 Hz to
125 kHz) Rms measure. It means that we
measure the effective Rms signal + the noise.
For their calculation, please check this "Frequency
Response Gain vs Cin, & Cfeed" graph:
PSRR d B = 20 x Log
High/low cut-off frequencies
Principle of operation
-10
-15
-20
-25
10
-5
5
0
10
Cin = 82nF
100
Cin = 22nF
Cin = 470nF
Frequency (Hz)
10
Cfeed = 330pF
-------------------------------------------- -
Rms Vs
Cfeed = 680pF
1000
R ms V
Rin = Rfeed = 22k
Tamb = 25 C
Cfeed = 2.2nF
r i p pl e
+
- Vs -
TS4871
10000
25/28