TS4985 datasheet(21/29 Pages) STMICROELECTRONICS

21 / 29 page

TS4985

Application Information

21/29

The TS4985 has two independent power amplifiers, and each amplifier produces heat due to its

power dissipation. Therefore, the maximum die temperature is the sum of the each amplifier’s

maximum power dissipation. It is calculated as follows:

Pdiss L = Power dissipation due to the left channel power amplifier

Pdiss R = Power dissipation due to the right channel power amplifier

Total Pdiss =Pdiss L +Pdiss R (W)

In most cases,

Pdiss L = Pdiss R, giving:

or, stated differently:

4.5

Decoupling the circuit

Two capacitors are needed to correctly bypass the TS4985. A power supply bypass capacitor

CS and a bias voltage bypass capacitor CB.

CS has particular influence on the THD+N in the high frequency region (above 7kHz) and an

indirect influence on power supply disturbances. With a value for

CS of 1µF, you can expect

similar THD+N performances to those shown in the datasheet. For example:

●

In the high frequency region, if

CS is lower than 1µF, it increases THD+N and disturbances

on the power supply rail are less filtered.

●

On the other hand, if

CS is higher than µF, those disturbances on the power supply rail are

more filtered.

Cb has an influence on THD+N at lower frequencies, but its function is critical to the final result

of PSRR (with input grounded and in the lower frequency region), in the following manner:

●

If Cb is lower than 1µF, THD+N increases at lower frequencies and PSRR worsens.

●

If Cb is higher than 1µF, the benefit on THD+N at lower frequencies is small, but the benefit

to PSRR is substantial.

Note that

Cin has a non-negligible effect on PSRR at lower frequencies. The lower the value of

Cin, the higher the PSRR.

4.6

Wake-up time, TWU

When the standby is released to put the device ON, the bypass capacitor

Cb will not be charged

immediately. As

Cb is directly linked to the bias of the amplifier, the bias will not work properly

until the

Cb voltage is correct. The time to reach this voltage is called wake-up time or TWU and

specified in electrical characteristics table with

Cb = 1µF.

Cb has a value other than 1µF, please refer to the graph in Figure 63 to establish the wake-up

time value.

Total P

diss

dis sL

(W)

Total P

diss

42V

π R

-----------------------P

out

–

()

TS4985 Datasheet(PDF) 21 Page - STMicroelectronics

TS4985 Datasheet(HTML) 21 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Datasheet Download

Link URL