TS4900IST View Datasheet(PDF) - STMicroelectronics
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TS4900IST Datasheet PDF : 19 Pages
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TS4900
At power OFF of the supply, Cs is discharged by a ■ Remark on PSRR measurement conditions
constant current Icc. The discharge time from 5V
to 0V of Cs is
What is the PSRR ?
tDischCs = --5-I--c-C---c-s--- = 83 ms
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 the
Now, we must consider the discharge time of Cb. power supply disturbance and the result on the
At power OFF or standby ON, Cb is discharged by output. We can say that the PSRR is the ability of
a 100kΩ resistor. So the discharge time is about a device to minimize the impact of power supply
τbDisch ≈ 3xCbx100kΩ (s).
disturbances to the output.
In the majority of application, Cb=1µF, then
τbDisch≈300ms >> tdischCs.
t(s) ■ How to use the PSRR curves (page 7)
uc We have finished a design and we have chosen
d the components values :
Pro t(s) • Rin=Rfeed=22kΩ, Cin=100nF, Cb=1µF
te c Now, on fig. 13, we can see the PSRR (input
le du grounded) vs frequency curves. At 217Hz we have
so ro a PSRR value of -36dB.
In fact, we want a value of about -70dB. So, we
b P need a gain of +34dB !
- O te Now, on fig. 12 we can see the effect of Cb on the
) le PSRR (input grounded) vs. frequency. With
t(s o Cb=100µF, we can reach the -70dB value.
c bs The process to obtain the final curve (Cb=100µF,
du O Cin=100nF, Rin=Rfeed=22kΩ) is a simple transfer
ro ) - point by point on each frequency of the curve on
P t(s fig. 13 to the curve on fig. 12.
The measurement result is shown on figure A.
lete duc Fig. A : PSRR changes with Cb
Obso te Pro -30
oleCin=100nF
-40 Cb=1µF
Obs-50
Vcc = 5, 3.3 & 2.6V
Rfeed = 22k, Rin = 22k
Rg = 100Ω, RL = 8Ω
Tamb = 25°C
How do we measure the PSRR ?
Fig. B : PSRR measurement schematic
Vripple
Vcc
Rin
Cin
Rfeed
6
4 Vin- -
3 Vin+ +
Rg
100 Ohms
2 Bypass
1 Standby
Bias
Cb
Vout1 5
Vs-
RL
-
Vout2 8
Av=-1
Vs+
+
TS4900
■ Measurement process:
• Fix the DC voltage supply (Vcc)
• Fix the AC sinusoidal ripple voltage (Vripple)
• No bypass capacitor Cs is used
The PSRR value for each frequency is :
PSRR(d B) = 20 x Log 10
-----R-----m-----s----(--V-----r--i-p---p---l--e--)-----
Rms(Vs + - Vs-)
Remark : The measurement of the RMS voltage is
not a selective RMS measurement but a full range
(2 Hz to 125 kHz) RMS measurement. This means
Cin=100nF
-60
Cb=100µF
we have: the effective RMS signal + the noise.
-70
10
100
1000
10000
Frequency (Hz)
100000
17/19
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