lpcware

LPC15xx comparator large propagation delay

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Aug 12, 2016 by Zdravko Dimitrov
Content originally posted in LPCWare by p.h.v.heumen@gmail.com on Thu Mar 03 08:25:17 MST 2016
Hi,

While implementing a sensorless BLDC motor controller with the LPC15xx I encountered the problem that the analog comparators have a large propagation delay of several of hunderds microseconds. From the datasheet one would expect something in the order of nanoseconds instead.

The way I observe this large delay is in the following configuration:
- Inverting input is connected to the internal voltage divider.
- Non-inverting input is connected to a ADC pin and is driven by a function generator. The waveform applied is a sqaure wave from 0V to 2V with a rise-time of about 40 ns.
- The output is connected trough the switch matrix to an arbitrary pin.

For different settings for the voltage divider the delay of the output to the input square-wave varies from 50 us to 600 us.

I also tested to connect the Inverting input to a physical pin and apply a constant voltage to that pin. Here I do similar observations.

In the figure you see an example of a measurement. The yellow and blue trace is the non-inverting input and the purple trace is the comparator output.

Are there any commonly made mistakes that causes this large propagation delay?

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