TS4900(2002) 查看數據表（PDF） - STMicroelectronics

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TS4900
(Rev.:2002)

300mW at 3.3V SUPPLY AUDIO POWER AMPLIFIER WITH STANDBY MODE ACTIVE HIGH

STMicroelectronics 

TS4900

At power OFF of the supply, Cs is discharged by a

constant current Icc. The discharge time from 5V

to 0V of Cs is

tDischCs = --5-I--c-C---c-s--- = 83 ms

Now, we must consider the discharge time of Cb.

At power OFF or standby ON, Cb is discharged by

a 100kΩ resistor. So the discharge time is about

τbDisch ≈ 3xCbx100kΩ (s).

In the majority of application, Cb=1µF, then

τbDisch≈300ms >> tdischCs.

s 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 fact, we want a value of 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 figure A.

Fig. A : 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

s Remark on PSRR measurement conditions

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 the

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. 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

s 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

we have: the effective RMS signal + the noise.

17/19

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