1.2 Totem Pole High Voltage Output

The high side, or “top-side,” switch in a totem pole configuration requires complicated drive and level translation schemes as mentioned earlier in this application note. A simple way of switching an output between positive and negative voltage rails directly from 5.0V logic is to use 2 N-channel MOSFETs with a single HT0440. Again, the floating and isolated outputs of the HT0440 are used to directly drive the top and bottom side N-channel MOSFETs. The logic control inputs can be exercised to produce break-before-make switching that eliminates potential short circuiting of the positive and negative rails. This easy technique enables the output to be switched between +350 and −350V by simply applying a +5.0V logic control to both logic inputs of the HT0440. No other interfacing is required. The circuit is ideal for lower switching speed applications. An external clock can be used at a lower frequency than the on-chip clock of 1 MHz to reduce current input demand into the logic control pins. In this case, Pin 2 is simply connected to the external clock instead of ground.

Compared to previously used schemes for driving MOSFETs, e.g., pulse transformers and optical isolators, this technique utilizes smaller and less costly components, thereby saving board space and cost.