Arduino to HV5122 interface for driving high voltage nixie tubes.

[Zog]

Hey all,
I have been looking at the HV5122 https://ww1.microchip.com/downloads/en/DeviceDoc/20005418B.pdf and interfacing it directly to the outputs of an arduino board.
Looking around I see a lot of other peoples circuits that use this for direct driving nixies and it looks like a good choice.

Something I have noticed is that some circuits have level shifted the clock and other digital inputs to +12V from +5V. Looking at the spec sheet I have no idea why you would do that ?

Wouldn't the HV5122 work just fine with +5V on the inputs.

I am missing something but for the life of me I can't see it.

Here is a link to a circuit that is doing the level shifting. https://create.arduino.cc/projecthub/gra_and_afch/arduino-shield-ncs314-nixie-tubes-clock-in-14-20932b Last one down is a good example.

Why ?

[Ian.M]

The HV5122 datasheet (table 3-2) says it needs  12V Vdd (+/- 10%) to run, and its logic '1' threshold is Vdd-2V,  so you need level shifting to drive it.  A 5V logic signal wont even reach 50% of Vdd at its min. rated Vdd so is vanishingly unlikely to work at all.

[Zog]

Thanks Ian.
This is doing my head in.

Supertex data sheet is showing

Recommended Operating Conditions
VDD Logic voltage supply 10.8 12 13.2 V
HVOUT High voltage output -0.3 - 225 V
VIH High-level input voltage VDD -2.0 - VDD V
VIL Low-level input voltage 0 - 2.0 V
f
CLK Clock frequency - - 8.0 MHz
TA Operating free-air temperature
Plastic -40 - +85 OC
Ceramic -55 - +125


But the Microchip one is showing this.

ABSOLUTE MAXIMUM RATINGS†
Supply voltage, VDD ...-0.5V to +15V
Supply voltage, VPP ...-0.5V to +250V
Logic input levels ... -0.5V to VDD+0.5V
Ground current1 ... 1.5A
Continuous total power dissipation2... 1200mW
Operating temperature range... -40°C to +85°C
Storage temperature range ... -65°C to +150°C

Not at all sure which one to believe here.

I have seen other designs that use +5VDD so.... erm ?

Edit: looking at the wrong section. Datasheet agree.

But how can these guys be using 5V.... can't be right. Right ?

https://doayee.co.uk/files/NixieSchematic.pdf

[Ian.M]

Its the difference between hobby hackers and professional design . . .

They are running the HV5122 at 5V Vdd , so the logic levels are 5V compatible.  However 5V probably isn't enough to turn the output MOSFETs fully on so the chip runs hot as its dropping too much voltage sinking current,  and odds are the chip's much much slower than the datasheet specs so malfunctions if clocked too fast.   Maybe you get lucky and get chips from a batch that sorta works at 5V, then when you want to build another one the chips from another batch don't work at all till you give them higher Vdd.

[westfw]

The "Absolute max ratings" are not the same as the recommended operating conditions.

The Microchip datasheet has the "Recommend" values off in section 3 (for some reason separate from the "electrical specifications.")

[tttonyyy]

Found this thread right after looking at the Doayee schematic and then the part datasheet. I was surprised to see that the recommended operating voltage was 12V, but in the schematic it is driven 5V.

Shame because if it could take 3V3 it would have been perfect for my project.