DocID1745 Rev 4

3/7

AN495

Effective filtering

7

Immunity problems in the circuit are essentially related to the +5 V Vreg filtering, because

the noise on this voltage can induce errors in the low-voltage circuits.

The best solution to avoid immunity problems is to put the filter capacitor between Vreg and

Ground even if the Vreg does not supply any external circuits. Compared to the +Vs

capacitor filtering effect, which also improves the immunity, the Vreg capacitor offers two

advantages:

1.

The capacitive value can be significantly lower, for the same filtering effect, because

the Vreg filter does not have to sustain the load current variations.

2.

The nominal voltage of the capacitor is lower (6 V instead of 25-50 V), so that the

physical size of the capacitor is also smaller.

The higher effectiveness of filtering Vreg instead of +Vs is always evident. On average, in a

typical application, a capacitor put on Vreg can, with the same filtering effectiveness of a

capacitor on +Vs, be 20 times smaller in value.

Nonetheless, a capacitor on +Vs, although not of high value, is recommended. Its value

should be adapted in accordance with the equivalent inductance of the supply connections

and of the value of the load.

For instance, the values used in the test circuit (Figure 3):

•

325

μH are too high to be found in practice.

•

4.7 nF on +Vs is barely sufficient with 325

μH (with 325 μH, 10 nF would be preferable).

•

4.7 nF on Vreg is sufficient in most practical cases.

•

Using the test circuit shown in Figure 3, with the input trigger of the TDE1707 in pulsed

mode, the effect of the capacitor on Vreg (pin 6) can be noted in Figure 4 and Figure 5:

–

Figure 4 shows the voltage on +Vs (Ch1) and Vreg (Ch2) at turn-on and turn-off

edges, without the filter capacitor on pin 6.

–

Figure 5 shows the same signals but with 4.7 nF capacitor connected between

Vreg (pin 6) and Ground.

The improvement of the immunity level and the low values of the capacitors makes the

suggested filtering solution the best for proximity detector applications.