Effective filtering

AN495

2/7

DocID1745 Rev 4

1

Effective filtering

In the application circuit (Figure 2) taken from the TDE1707 device datasheet, the use of

filter capacitors is recommended on both:

•

the power supply (+Vs pin 7).

•

the +5 V regulated voltage (Vreg pin 6).

–

To understand the function of the Vreg capacitor, it is necessary to consider that

the Vreg pin makes available a regulated voltage that can be used to supply

external circuits (typically proximity detectors); but the TDE1707 itself always uses

the Vreg to supply most of its internal circuits (see Figure 1: Block diagram).

Internally to the TDE1707, Vreg supplies all the functional blocks, except the

output power transistor (and the Driver-Led, when the Led is connected as in “load

to GND configuration”).

–

In a real application, the TDE1707 and associated circuitry can be significantly

farther away from the power supply and the long connection wires will act as a

serial inductor.

At power-on, or at turn-on of the power transistor, or in coincidence with load variations, this

inductance will react to the current variations with wide voltage variations. In proximity

sensor applications, for space reasons, the size of the filtering capacitors must be reduced

as much as possible.

The voltage oscillation can induce two kind of problems in the circuit:

1.

Overvoltage on the +Vs pin, connected to the +24 V bus, which can exceed the

TDE1707 rated limit.

2.

Disturbances inside the circuit (TDE1707), because the noise immunity level is

exceeded with disruption of the input/output function.

Filtering the overvoltage on the +Vs is possible only by an external capacitance between

+Vs (pin7) and Ground (pin4) to limit voltage spikes at the device’s rated values.

Figure 2. Application circuit

10nF

1mSec/nF

10nF

8

2

4

3

6

5

1

7

TDE1707

INPUT

VCC

GROUND

LOAD

10nf to 1uF

High side load

10nF

1mSec/nF

10nF

10nF to 1uF

1

2

4

3

6

5

7

8

TDE1707

INPUT

VCC

GROUND

LOAD

Low side load