At power OFF of the supply, Cs is discharged by a constant current Icc. The discharge time from 5 V to 0 V of Cs

is:

tDiscℎCs= 5CSIcc=83ms

Now, we must consider the discharge time of Cb.

At power OFF or standby ON, Cb is discharged by a 100 kΩ resistor. So the discharge time is about

τbDisch ≈ 3 x Cb x 100 kΩ (s).

In the majority of application, Cb = 1 μF, then

τbDisch ≈ 300 ms >> τdischCs.

5.7

Power amplifier design examples

Given :

Load impedance : 8 Ω

Output power @ 1% THD+N : 0.5 W

Input impedance : 10 kΩ min.

Input voltage peak to peak : 1 Vpp

Bandwidth frequency : 20 Hz to 20 kHz (0, -3 dB)

Ambient temperature max = 50 °C

MiniSO8 package

First of all, we must calculate the minimum power supply voltage to obtain 0.5 W into 8 Ω. With curves in fig. 16,

we can read 3.5 V. Thus, the power supply voltage value min. is 3.5 V.

Following the maximum power dissipation equation

Pdissmax = 2VCC2π2RL= W

with 3.5 V we have Pdissmax = 0.31 W

Refer to power derating curves (fig. 20), with 0.31 W the maximum ambient temperature is 87 °C. This last value

could be higher if you follow the example layout shown on the demoboard (better dissipation).

The gain of the amplifier in flat region is:GV=VOUTPPVINPP=22RLPOUT

VINPP =5.65

We have Rin > 10 kΩ. Let's take Rin = 10 kΩ, then Rfeed = 28.25 kΩ. We could use for Rfeed = 30 kΩ in

normalized value and the gain is Gv = 6.

In lower frequency we want 20 Hz (- 3dB cut-off frequency). Then:

So, we could use for Cin a 1 μF capacitor value

CIN= 12πRinFCL=795nF

which gives 16 Hz.

In higher frequency we want 20 kHz (- 3dB cut off frequency). The gain bandwidth product of the TS4871 is 2

MHz typical and does not change when the amplifier delivers power into the load.

The first amplifier has a gain of:

RfeedRin=3

and the theoretical value of the - 3 dB cut-off higher frequency is 2 MHz/3 = 660 kHz.

We can keep this value or limit the bandwidth by adding a capacitor Cfeed, in parallel on Rfeed.

Then:

TS4871

Application information

DS2547 - Rev 10

page 27/42