TS4972 datasheet(26/30 Pages) STMICROELECTRONICS

26 / 30 page

TS4972

Application Information

26/30

■ Note on how to use the PSRR curves

(page 7)

We have finished a design and we have chosen

the components values :

• Rin=Rfeed=22k

Ω

• 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

Ω) 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.

■ 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.

How we measure the PSRR ?

■ 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:

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.

Figure 85: PSRR changes with Cb

100

1000

10000

100000

-70

-60

-50

-40

-30

Cin=100nF

Cb=100

µF

Cin=100nF

Cb=1

µF

Vcc = 5, 3.3 & 2.6V

Rfeed = 22k, Rin = 22k

Rg = 100

Ω, RL = 8Ω

Tamb = 25

°C

Frequency (Hz)

Figure 86: PSRR measurement schematic

Vcc

Vripple

Vs-

Vs+

Rfeed

Rin

100 Ohms

Cin

Bias

AV = -1

Vin-

Vin+

Bypass

Standby

Vout 1

Vout 2

TS4972

PSRR dB

() = 20 x Log

Rms Vri ppl e

()

Rms Vs + - Vs-

()

---------------------------------------------

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