TS4961T datasheet(36/49 Pages) STMICROELECTRONICS

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Application component information

TS4961T

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4.1

Common mode feedback loop limitations

The common mode feedback loop allows the output DC bias voltage to be averaged at

VCC/2 for any DC common mode bias input voltage.

However, because of the Vicm limitation in the input stage (see Table 2: Operating conditions

for audio amplifier section on page 3), the common mode feedback loop can only fulfill its

role within a defined range. This range depends upon the values of VCC and Rin (Av). To

obtain a good estimation of the Vicm value, the following formula can be used (no tolerance

on Rin):

with

and the result of the calculation must be in the range:

Due to the +/-9% tolerance on the 150 k

Ω resistor, it is also important to check Vicm in these

conditions:

If the result of the Vicm calculation is not in the previous range, input coupling capacitors

must be used (with VCC from 2.4 V to 2.5 V, input coupling capacitors are mandatory).

For example:

With VCC =3V, Rin =150 kΩ and VIC = 2.5 V, we typically find Vicm = 2 V and this is lower

than 3 V - 0.8 V = 2.2 V. With 136.5 k

Ω we find 1.97 V, and with 163.5 kΩ we have 2.02 V.

Therefore, no input coupling capacitors are required.

icm

150k

()

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

(V)

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

(V)

0.5V

icm

0.8V

–

≤≤

136.5k

()

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

icm

163.5k

()

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

≤≤

Obsolete

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- Obsolete

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