HCF40106: suggest how to obtain a stable and smooth output

[Yo1]

Hello ST Community,

I am using HCF40106YM013TR in one of my products. One channel is configured with an RC timing network at the input, and its output is cascaded to another channel.

However, the output is chattering / oscillating near the positive trigger threshold voltage.

The RC values and operating conditions are as follows:

• R = 500 KΩ

• C = 4.7 µF

• VCC = 14 V 

• GND is connected through a series diode for reverse polarity protection

The Schematic and output waveform also attached herewith FYR,

Wave form 1 : VCC (DSO channel 1) vs Capacitor charging (DSO channel 2).

Wave form 2 : Capacitor charging (DSO channel 1) vs HCF 10406 output (DSO channel 2)

Kindly suggest how to obtain a stable and smooth output and whether any changes in RC values, grounding, or protection scheme are required.

Thank you for your support.

[Peter BENSCH · Accepted solution]

Welcome @Yo1, to the community!

Your problem is caused by several interacting factors:

• the diode you inserted for reverse polarity protection at GND
• resistor R1
• resistor R3
• high-impedance low-pass resistor R2

All together, these produce very unpredictable behaviour, which is also temperature-dependent.

To name just a few points:

• Although the HCF40106 is a Schmitt trigger, the data in the data sheet is only valid under the environmental conditions specified there. However, you are not operating it with a constant voltage, but with the voltage resulting at the divider R1+R3. This voltage, however, depends on the current consumption of the HCF40106, which in turn depends on its switching state, the switching process and the temperature. Since pin 14 is not regulated (e.g. with a zener diode), the VDD at pin 14 and thus also the threshold voltage of the Schmitt trigger shifts depending on the prevailing conditions. All of this together results in positive feedback via the supply voltage.
• During the switching process, the HCF40106 draws short current peaks from VDD, which results in voltage drops and thus also in a change in the threshold voltage of the Schmitt trigger. Since the time constant of R2 and C1 is extremely large and the charging curve remains at the switching threshold for a very long time, even the smallest disturbances of the VDD around this point have an immediate effect on the output.
• The VDD of the HCF40106 shifts statically depending on the temperature because the quiescent current is highly dependent on it. You can assume a few hundred millivolts, which in itself is not a major problem.
• However, the different switching currents result in noticeable jumps in VDD (pin 14). Since the signal at pin 11 passes through the trigger threshold extremely slowly in comparison, this is sufficient to trigger the observed oscillation.

Possible improvements:

1. Please do not use reverse polarity protection in the GND branch, but at the VDD side.
2. Reduce R2 by a factor of at least 5 to lower the sensitivity of the input at pin 11. If the time constant is to remain the same, C1 can be increased accordingly. Parallel to C1, you can place a ceramic capacitor of approx. 100 nF to intercept any coupling pulses.
3. Minimise the length of the supply lines to VSS, VDD and input E of the HCF40106 and use minimum inductance of these tracks. To do this, route the track from pin 8 as far away from pin 11 as possible, possibly with a GND guard plane in between.

Hope that helps?

Incidentally, it helps to get a response more quickly if community members don't have to go through the hassle of downloading and launching a PDF reader, but can see the relevant images directly in the thread when you insert them there using the camera button.

And one more thing: although taking a photo with your mobile phone is quick and easy, it will be blurry. However, your scope from GW Instek has a USB port below the screen that allows you to save screenshots directly to a USB stick. Screenshots like these dramatically increase people's willingness to help you!

Regards
/Peter