TS4871ID 查看數據表（PDF） - STMicroelectronics

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TS4871ID

OUTPUT RAIL TO RAIL 1W AUDIO POWER AMPLIFIER WITH STANDBY MODE

STMicroelectronics 

TS4871

s 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.

Fig. 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

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 ?

Fig. 86 : PSRR measurement schematic

Vripple

Vcc

Rin

Cin

Rfeed

6

4 Vin- -

3 Vin+ +

Rg

100 Ohms

2 Bypass

1 Standby

Bias

Cb

7

Vout1 5

-

Av=-1

+

Vout2 8

Vs-

RL

Vs+

TS4871

s Principle of operation

• 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:

PSRR(d B) = 20 x Log 10

-----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.

sHigh/low cut-off frequencies

For their calculation, please check this "Frequency

Response Gain vs Cin, & Cfeed" graph:

10

5

0

-5

-10

-15

-20

-25

10

Cfeed = 330pF

Cfeed = 680pF

Cin = 470nF Cfeed = 2.2nF

Cin = 22nF

Cin = 82nF

Rin = Rfeed = 22kΩ

Tamb = 25°C

100

1000

Frequency (Hz)

10000

25/28

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