Voltage Inverter

Voltage Inverter

A negative supply voltage may be obtained with the use of a special inverter IC. but such ICs are not always readily available. Fortunately. if the current drawn is not excessive, a standard HC-MOS chip may be used. The present circuit is based on Type 74HC14, which contains six Schmitt triggers, whose combined gates can deliver a fairly high output current. Moreover, such devices can be made to oscillate readily. Parallel switching is, however, a problem with Schmitt triggers, even if they are contained in the same chip. This problem is particularly acute in the case of slow input signals. In the present circuit, this problem is resolved by driving the parallel-switched input via a gate that is not part of the parallel circuits. Since this is also a Schmitt trigger, the output signal has steep edges. The parallel-switched gates together with ICI. form a rectangular-wave generator, whose output frequency is about 125 kHz. The output signal is converted by a charge pump into a negative supply voltage. The diodes used for that purpose are Schottky types that, owing to their low threshold voltage, do not lower the load voltage by as much as silicon diodes. Under no-load conditions, the output voltage is about 6 V and the IC draws a quiescent current of around 100 pA. When the load current is about 1 mA, the output voltage drops to 4 V. If this voltage can drop even further (down to half the supply voltage), a load current of up to 10 mA is possible. A higher load current or a smaller drop in the output voltage cannot be obtained by raising the value of the capacitors in the charge pump since the IC can-not cope with this. Note also that the circuit is not short-circuited proof: the IC will not give up the ghost immediately, but it does not take all that long.

 

Voltage Inverter

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