inrush current on ic

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milad.norouzi

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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.
 

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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
 

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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 ---

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?
 
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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
 

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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?
 

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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.
 

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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
 

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milad.norouzi said:
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?
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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.
 

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