AN2123 STMicroelectronics, AN2123 Datasheet - Page 3

• Manufacturer Part Number: AN2123
• Description: TD351 Advanced IGBT Driver Principles of operation and application
• Manufacturer: STMicroelectronics

AN2123
3 Input stage
The TD351 is compatible with the use of both pulse transformers or optocouplers. The schematics shown
in
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 kΩ 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
input signals and rejects signals smaller than 135 ns (t
Waveforms from the pulse transformer must comply with the t
Figure
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)
Figure 4
Pulse transformer
5). To turn TD351 output on, the input signal must be lower than 0.8 V for 220 ns minimum.
can be considered as example of use with both solutions.
10K
10K
10K
10nF
1
2
IN
VREF
TD351
Optocoupler
onmin
specification).
onmin
100pF
4K7
and V
10nF
ton
/V
toff
1
2
specifications (see
IN
VREF
TD351
Input stage
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