Charger IC cuases bad data to appear on shift registers outputs

Thread Starter

nir27

Joined Jun 18, 2021
2
Hello All,

This is a bit long so please bare with me...

A battery powered product I am developing contains 24 shift registers (MC74HC595A-D) which are daisy chained to one another. Each SR is coupled by a 0.1uF capacitor. Each output of the SR is controlling a FET with controls an LED. A microcontroller is used to send the serial data to all the 24 SRs and by that control 192 (24*8) LEDS on/off state. This is all working very well.

On the same PCB (4 layers), there is a charger IC from TI (BQ24192L) which is used to charge the battery. When charging, the product lights up 1 to 6 sequential leds to show the user the progress of charging (1 led on - low battery, 6 leds on - battery charger).

When charging with a low power settings, say 5W, everything is working, again, very well.

When high energy charging is taking place, say 10W or more, the display of the leds sometimes goes wrong. Usually what ill see is that one led which should be on is off and the one next to it, which should be off is now on. Sometimes 2 sequential leds will turn on, etc. Since I am updating the charge status once a second (the last led is always blinking at 1Hz), I see a lot of these 'mistakes'.

I have been trying to solve this problem for tens of hours but failed. I turn to this community for help!

A few more important notes:

1. When high charging is present, the charging IC gets very hot and goes into thermal regulation after a few seconds. The wrong leds problem is there before and after the thermal regulation. There is not a good way to cool it down on this PCB edit.

2. I have scoped all the control signals (data, serial clock, latch clock) going into the SRs and they all work great. No problem there.

3. I have tried to light up different leds in the array, even the 1st led of the first SR, but the problem is there also.

4. When scoping the charger IC (please see attached pictures and schematics) I see a 1.5Mhz noise from its internal buck converter appearing on the voltage rail of the system. The noise's peak to peak pulses are about 1V high on the VBUS inputs of the charging IC, 300mV peak to peak on the VBAT and VSYS pins of the charger IC. The same noise (300mV) is also present on each of the SR's power pin (16) and GND (8) pin.

5. Have tried adding larger +smaller caps at the VBUS input but did not help.

6. Have tried to filter the VBUS input with a LC filter (two caps and a coil) but that did not help.

I think the problem is the noise generated but I am not sure. Also, I do not know if it is a conducted noise or radiated one and, I do not know how to reduce it, if this is actually the cause of the problem I am facing.

Thank you for reading this and would love to hear your thoughts on this.

Nir.
 

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Sensacell

Joined Jun 19, 2012
2,850
Look at where the ground currents flow on the PCB, this would be my first suspect.
The second would be to look at suppressing that noise on the power bus.

Post the whole schematic and the PCB layout?
 

Papabravo

Joined Feb 24, 2006
16,463
The schematic of just the charger is not painting the complete picture. Where is the battery? Where are the shift registers? Who did the board layout? How are the grounds arranged? Where is the freewheeling diode for the buck regulator?
On the scope traces one of them is annotated 250ns. Is that per division or is that for the width of the trace?
 

Thread Starter

nir27

Joined Jun 18, 2021
2
Thank you guys.

The 250ns on the scope is per division. The PCB was done by a professional PCB editor - please find its BRD (inside the ZIP) file attached.

Also attached are a set of 8 schematics. 7 of them make up the main pcb of this product. The 8th ("Bottom PCB") includes just the USB type C connector from which the charging power is fed.

Please note the following about these schematics:

1. There are 3 more identical Leds pages (2,3,4) which are exactly the same as Leds1 so I have omitted them.

2. On the 'Power' schematics, R901 is installed and J8/U34 and all of their supporting components are not installed. On that same page, U35 (DC/DC 3 to 5V) is not installed, pin 2 and 5 on the PCB are short circuited and so the 5V signal is actually a 3V one (no need for it)

3. On the 'Charger' schematics, R903 is installed and U25 and all of its supporting components are not installed.

Nir.
 

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