inrush current on ic

[milad.norouzi]

Hi. i'm designing some kind of touch key with lora antenna and ttp223 touch ics. the problem is that when i start to transmit or touching one of the touchpads , other ttp223 ics start to false detect. i read the lora ra01datasheet and i found out that the lora ra01 consume about 120mA of current when transmitting. is it my false detection because of this 120mA? and when i measure the voltage on lora ra01 with my multimeter, the ics start to act normal and doesnt detect false touch. maybe there is a huge capacitor in my multimeter?my design is attached, thanks for your time. Lora antenna is on bottom layer and touchpad and touch ic is on top layer. i tried to add 470uF and 680uF on vcc pin of lora ra01 but still same result.

 

[KlausST]

Hi,

no, I´m pretty sure it has nothing to do with supply current.
I rather think it´s HF interference.... combined with an unsuitable PCB layout.

I recommend to try a PCB layout with a really solid GND plane .. combined by following the dsign considerations for LORA and touch circuit.

Klaus

 

[milad.norouzi]

Thanks for your reply. i'm new on pcb design. you mean that i should place a Polygon on my pcb and connect it to my GND?

--- Updated Apr 13, 2022 ---

My Tx power on LoRa was 14 dBm, i reduced it to 4 dBm the pcb now is very much stable, but still the pad closer to LoRa ic detect touching sometimes. what can we conclude from this behavior?

 

[KlausST]

Hi,

this just would be a little better than before.
A "solid GND plane" has no cut, no traces ... just solid.

I don´t say that this is the one and only way, but for beginners it´s just a simple rule to get good performance.
Nowadays PCB layouts don´t work by just connecting some pads with some traces. There always is HF around - especially in your case: LORA.
So you have to care about "inductance" (that can´t be much improved by wider traces).

Since you work with touch sensors I recommend to read some "capacitive touch design guides". There are many documents in the internet.

I also recommend to watch a couple of videos about EMI/EMC PCB layout design.

Klaus

 

[milad.norouzi]

Thanks. but my PCB have 2 Layers and i cant make a "Solid" GND Plane i think. can i use a low pass filter on sense input of touch ics?

 

[FvM]

I'd start with small capacitors (a few pF) between touch pins and touch IC ground. Short connection is essential. Also unsufficient supply bypassing of the touch IC may cause problems. Series R between IC and touch pads (10 to 100k range) would be the next step.

 

[milad.norouzi]

I'd start with small capacitors (a few pF) between touch pins and touch IC ground. Short connection is essential. Also unsufficient supply bypassing of the touch IC may cause problems. Series R between IC and touch pads (10 to 100k range) would be the next step.

Thanks for your reply, i'm using 100nF parallel to ic and as close as possible. but there is no cap between touchpad and ground because i need the maximum sensitivity. i'll try the series resistor. i hope it solves the problem

 

[barry]

Thanks. but my PCB have 2 Layers and i cant make a "Solid" GND Plane i think. can i use a low pass filter on sense input of touch ics?

That’s kind of a garbage-in-garbage-out situation. As previously explained, you REALLY should have a ground plane. Now you’re stuck trying to fix a fundamental design flaw. Basically, try everything. put smaller caps across your 100nf ones. Try those series resistors. Without seeing a schematic it’s hard to make suggestions.