TS4890 datasheet(26/32 Pages) STMICROELECTRONICS

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TS4890

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s Power amplifier design examples

Given :

• Load impedance : 8

Ω

• Output power @ 1% THD+N : 0.5W

• Input impedance : 10k

Ω min.

• Input voltage peak to peak : 1Vpp

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

• THD+N in 20Hz to 20kHz < 0.5% @Pout=0.45W

• Ambient temperature max = 50°C

• SO8 package

First of all, we must calculate the minimum power

supply voltage to obtain 0.5W into 8

Ω. See curves

in fig. 15, we can read 3.5V. Thus, the power

supply voltage value min. will be 3.5V.

Following

the

maximum

power

dissipation

equation :

with 3.5V we have Pdissmax=0.31W.

Refer to power derating curves (fig. 24), with

0.31W the maximum ambient temperature will be

100°C. This last value could be higher if you follow

the example layout shows on the demoboard

(better dissipation).

The gain of the amplifier in flat region will be :

We have Rin > 10k

Ω. Let's take Rin = 10kΩ, then

Rfeed = 28.25k

Ω. We could use for Rfeed = 30kΩ

in normalized value and the gain will be Gv = 6.

In lower frequency we want 20 Hz (-3dB cut off

frequency). Then

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

gives 16Hz.

In Higher frequency we want 20kHz (-3dB cut off

frequency). The Gain Bandwidth Product of the

TS4890 is 2MHz typical and doesn't change when

the amplifier delivers power into the load.

The first amplifier has a gain of

and the theoretical value of the -3dB cut of higher

frequency is 2MHz/3 = 660kHz.

We can keep this value or limiting the bandwidth

by adding a capacitor Cfeed, in parallel on Rfeed.

Then

So, we could use for Cfeed a 220pF capacitor

value that gives 24kHz.

Now, we can choose the value of Cb with the

constraint THD+N in 20Hz to 20kHz < 0.5% @

Pout=0.45W. If you refer to the closest THD+N vs

frequency measurement : fig. 71 (Vcc=3.3V,

Gv=10), with Cb = 1µF, the THD+N vs frequency

is always below 0.4%. As the behaviour is the

same with Vcc = 5V (fig. 67), Vcc = 2.6V (fig. 67).

As the gain for these measurements is higher

(worst case), we can consider with Cb = 1µF, Vcc

= 3.5V and Gv = 6, that the THD+N in 20Hz to

20kHz range with Pout = 0.45W will be lower than

0.4%.

In the following tables, you could find three

another examples with values required for the

demoboard.

Remark : components with (*) marking are

optional.

Application n°1 : 20Hz to 20kHz bandwidth and

6dB gain BTL power amplifier.

Components :

)

(

Vcc

max

Pdiss

INPP

OUT

INPP

OUTPP

795

Rin

Designator

Part Type

22k / 0.125W

Short Cicuit

R7*

(Vcc-Vf_led)/If_led

10k / 0.125W

470nF

100µF

Rin

Rfeed =

265

FEED

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