LPC15xx comparator large propagation delay

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Oct 4, 2018 by Daniel Hajek
Content originally posted in LPCWare by p.h.v.heumen@gmail.com on Thu Mar 03 08:25:17 MST 2016

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?