TS4994FC datasheet(30/35 Pages) STMICROELECTRONICS

30 / 35 page

Application information

TS4994FC

30/35

To calculate the maximum ambient temperature Tamb allowable, you need to know:

●

The value of the power supply voltage, VCC

●

The value of the load resistor, RL

●

The Rthja value for the package type

Example: VCC =5V, RL =8Ω, Rthja-flipchip = 100°C/W (100mm² copper heatsink)

Using the power dissipation formula given above in Equation 3 this gives a result of:

Pdissmax = 633mW

Tamb is calculated as follows:

Equation 5

Therefore, the maximum allowable value for Tamb is:

Tamb = 125-80x0.633=62°C

4.8

Decoupling of the circuit

Two capacitors are needed to correctly bypass the TS4994. 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 performance to that shown in the datasheet.

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 1µF, the 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).

4.9

Wake-up time: tWU

When the standby is released to put the device ON, the bypass capacitor Cb is not 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 the wake-up

time or tWU and is specified in Table 3 on page 5, with Cb=1µF. During the wake-up time, the

TS4994 gain is close to zero. After the wake-up time, the gain is released and set to its

nominal value.

If Cb has a value other than 1µF, refer to the graph in Figure 88 on page 22 to establish the

wake-up time.

amb

125

° CR

TJHA

dissmax

–

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