MAX6966 a suitable sink driver for nixie tubes?

Question

I recieved some IN-14 nixie tubes for Christmas, and was hoping to implement them in a project i have been considering for some time. The details are unimportant, except that each nixie tube should have its own discrete pcb and driver. After looking at ICs for some time I found the MAXIM 6966, a LED sink driver with some nifty features (though i would have preferred an I2C interface instead of SPI). It seems perfect for my project (driving a single nixie tube) for the following reasons:

• Tiny 3x3mm QFN package
• Exactly 10 outputs
• Open-drain functionality (output pins either low or Hi-z)
• PWM functionality on outputs

My problem is this... the datasheet says the outputs have up to 7V protection, but i'm not sure if that applies to both Hi-Z and Low output configurations, or just the Low output configuration. With an output set to Hi-Z, the corresponding pin is likely to be biased to a high voltage such as 170V... well above the 7V protection. So that is to say, if the 7V max condition also applies to the Hi-Z pin configuration then I cannot use it.

I hope it will work fine, but I suspect it isn't suitable. Could someone confirm my suspicion before I dismiss this otherwise fantastic IC?

Here is a link to the product page: https://www.maximintegrated.com/en/products/interface/controllers-expanders/MAX6966.html

Thanks :)

Answer 1

No, that device is not suitable for driving Nixie tubes. When an output is switched off, it will see the full voltage at the output pin.

The datasheet clearly states that

I/O Port Outputs Are 7V-Rated Open Drain

That falls well short of the 170V you'll be using.

Development of drivers specifically designed for Nixie tubes pretty much reached a dead end with the 74141, but there are some modern parts around that will carry out serial to parallel conversion and drive high voltage open-drain outputs. Look at the Microchip HV5122 or HV5522 for example.

Answer 2

While the supply voltage vastly exceeds the maximum voltage for the driver IC, it is still possible to use them together if there is a transistor in series with the driver output in order to reduce the voltage below the upper limit.

This method is explored in TI's Application Report SLVA280, "Using TLC5940 With Higher LED Supply Voltages and Series LEDs". Note that while the title implies specific parts, the techniques described can be used with any open-drain driver and non-inductive load.