AN2123

Input stage

3/15

3 Input stage

The TD351 is compatible with the use of both pulse transformers or optocouplers. The schematics shown

in Figure 4 can be considered as example of use with both solutions.

When using a pulse transformer, a 2.5 V reference point can be built from the 5 V VREF pin with a

resistor bridge. The capacitor between the Vref and the bridge middle point provides decoupling of the

2.5 V reference, and also insures a high level on IN input at power-up, in order to start the TD351 in the

OFF state.

When using an optocoupler, the IN pin can be pulled-up to Vref. The pull up resistor is to be chosen

between 5 k

Ω to 20 kΩ depending on the characteristics of the optocoupler. An optional filtering capacitor

can be added in case of a highly noisy environment, although the TD351 already includes filtering on

Figure 5). To turn TD351 output on, the input signal must be lower than 0.8 V for 220 ns minimum.

Conversely, the input signal must be higher that 4.2 V for 220 ns minimum in order to turn off TD351

output. A pulse width of about 500 ns at the threshold levels is recommended. In all cases, input signal at

the IN pin must be between 0 and 5 V.

Figure 4. Application schematic (pulse transformer at left; optocoupler at right)

Figure 5. Typical input signal waveforms with pulse transformer (left) or optocoupler (right)

IN

VREF

4K7

10nF

100pF

TD351

2

1

IN

VREF

TD351

10K

10K

10K

10nF

2

1

Pulse transformer

Optocoupler