TS4902 datasheet(16/19 Pages) STMICROELECTRONICS

16 / 19 page

TS4902

16/19

The efficiency is the ratio between the output

power and the power supply

The maximum theoretical value is reached when

Vpeak = Vcc, so

s Decoupling of the circuit

Two capacitors are needed to bypass properly the

TS4902, a power supply bypass capacitor Cs and

a bias voltage bypass capacitor Cb.

Cs has especially an influence on the THD+N in

high frequency (above 7kHz) and indirectly on the

power supply disturbances.

With 100µF, you can expect similar THD+N

performances like shown in the datasheet.

If Cs is lower than 100µF, in high frequency

increases, THD+N and disturbances on the power

supply rail are less filtered.

To the contrary, if Cs is higher than 100µF, those

disturbances on the power supply rail are more

filtered.

Cb has an influence on THD+N in lower frequency,

but its function is critical on the final result of PSRR

with input grounded in lower frequency.

If Cb is lower than 1µF, THD+N increase in lower

frequency (see THD+N vs frequency curves) and

the PSRR worsens up

If Cb is higher than 1µF, the benefit on THD+N in

lower frequency is small but the benefit on PSRR

is substantial (see PSRR vs. Cb curve : fig.12).

Note that Cin has a non-negligible effect on PSRR

in lower frequency. Lower is its value, higher is the

PSRR (see fig. 13).

s Pop and Click performance

Pop and Click performance is intimately linked

with the size of the input capacitor Cin and the bias

voltage bypass capacitor Cb.

Size of Cin is due to the lower cut-off frequency

and PSRR value requested. Size of Cb is due to

THD+N and PSRR requested always in lower

frequency.

Moreover, Cb determines the speed that the

amplifier turns ON. The slower the speed is, the

softer the turn ON noise is.

The charge time of Cb is directly proportional to

the internal generator resistance 50k

Ω.

Then, the charge time constant for Cb is

τb = 50kΩxCb (s)

As Cb is directly connected to the non-inverting

input (pin 2 & 3) and if we want to minimize, in

amplitude and duration, the output spike on Vout1

(pin 5), Cin must be charged faster than Cb. The

charge time constant of Cin is

τin = (Rin+Rfeed)xCin (s)

Thus we have the relation

τin << τb (s)

The respect of this relation permits to minimize the

pop and click noise.

Remark : Minimize Cin and Cb has a benefit on

pop and click phenomena but also on cost and

size of the application.

Example : your target for the -3dB cut off

frequency is 100 Hz. With Rin=Rfeed=22 k

Ω,

Cin=72nF (in fact 82nF or 100nF).

With Cb=1µF, if you choose the one of the latest

two values of Cin, the pop and click phenomena at

power supply ON or standby function ON/OFF will

be very small

50 k

Ωx1µF >> 44kΩx100nF (50ms >> 4.4ms).

Increasing Cin value increases the pop and click

phenomena to an unpleasant sound at power

supply ON and standby function ON/OFF.

Why Cs is not important in pop and click

consideration ?

Hypothesis :

• Cs = 100µF

• Supply voltage = 5V

• Supply voltage internal resistor = 0.1

Ω

• Supply current of the amplifier Icc = 6mA

At power ON of the supply, the supply capacitor is

charged

through

the

internal

power

supply

resistor. So, to reach 5V you need about five to ten

times the charging time constant of Cs (

τs =

0.1xCs (s)).

Then, this time equal 50µs to 100µs <<

τb in the

majority of application.

η =

POUT

Psup pl y

------------------------ =

πVPEAK

4VCC

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

----- = 78.5%

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