TS4972 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'TS4972' PDF : 30 Pages View PDF
TS4972
Application Information
■ Note on how to use the PSRR curves
(page 7)
We have finished a design and we have chosen
the components values :
• Rin=Rfeed=22kΩ
• 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=22kΩ) 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.
Figure 85: PSRR changes with Cb
-30
Cin=100nF
-40 Cb=1µF
Vcc = 5, 3.3 & 2.6V
Rfeed = 22k, Rin = 22k
Rg = 100Ω, RL = 8Ω
Tamb = 25°C
-50
-60
-70
10
Cin=100nF
Cb=100µF
100
1000
10000
Frequency (Hz)
100000
How we measure the PSRR ?
Figure 86: PSRR measurement schematic
Vripple
Vcc
Rin
Cin
Rfeed
6
1 Vin- -
7 Vin+ +
Rg
100 Ohms
3 Bypass
5 Standby Bias
Cb
2
-
AV = -1
+
Vout 1 8
Vout 2 4
Vs-
RL
Vs+
TS4972
■ Principle of operation
• We fixed the DC voltage supply (Vcc)
• We fixed the AC sinusoidal ripple voltage
(Vripple)
• No bypass capacitor Cs is used
The PSRR value for each frequency is:
PSRR(dB) = 20 x Log10
-----R-----m-----s----(--V-----r--i-p---p---l--e---)----
Rms(Vs + - Vs-)
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.
■ Note on PSRR measurement
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.
26/30
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