TS4900 datasheet(17/19 Pages) STMICROELECTRONICS

17 / 19 page

TS4900

17/19

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

constant current Icc. The discharge time from 5V

to 0V of Cs is

Now, we must consider the discharge time of Cb.

At power OFF or standby ON, Cb is discharged by

a 100k

Ω resistor. So the discharge time is about

τb

Disch ≈ 3xCbx100kΩ (s).

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

τb

Disch≈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=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 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=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 figure A.

Fig. A : PSRR changes with Cb

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

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 :

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.

tDischCs =

5Cs

Icc

-------------- = 83 ms

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)

Vripple

Vcc

Rin

Cin

100 Ohms

Rfeed

Vin-

Vin+

Bypass

Standby

Bias

Vout1

Vout2

Av=-1

TS4900

Vs-

Vs+

PSRR d B

() = 20 x Log

Rm s Vrippl e

()

Rm s Vs + - Vs-

()

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

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