TS4871 datasheet(21/28 Pages) STMICROELECTRONICS

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TS4871

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s Power dissipation and efficiency

Hypothesis :

• Voltage and current in the load are sinusoidal

(Vout and Iout)

• Supply voltage is a pure DC source (Vcc)

Regarding the load we have:

and

Then, the average current delivered by the supply

voltage is:

The power delivered by the supply voltage is

Psupply = Vcc IccAVG (W)

Then, the power dissipated by the amplifier is

Pdiss = Psupply - Pout (W)

and the maximum value is obtained when:

and its value is:

Remark : This maximum value is only depending

on power supply voltage and load values.

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

TS4871, 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

VOUT = VPEAK sinωt (V)

IOUT =

VOUT

R L

----------------- (A)

POUT =

VPEAK

2RL

---------------------- (W)

ICC

AVG

= 2

VPEAK

πRL

-------------------- (A)

Pdi ss =

22 Vcc

π RL

---------------------- POUT

POUT (W)

–

∂Pdiss

∂POUT

---------------------- = 0

)

(

Vcc

max

Pdiss

η =

POUT

Psup pl y

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

πVPEAK

4VCC

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

----- = 78.5%

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