Where have all my electrons gone?

I think I found the reason the +30V boost regulator is getting turned on, and it isn't pretty.

Both the LTC3494 boost regulator and the SN6505 transformer driver have active-high enable inputs. The control inputs in the circuits I designed to drive the vacuum fluorescent display's grids and anodes are NPN transistors in common emitter configurations. The printer and keyclick sounder circuit inputs are N-channel FETs in common source configurations. All of these want to see inputs of no more than 0.3 to 0.5 volts to remain off.

When a Spartan 6 FPGA resets it has to load its configuration from external storage. During this period it normally configures its I/O pins with soft pull-ups. In this state, each pin could source up to 500 µA. Because this would turn on all my external circuits, I disabled this behavior by strapping the HSWAPEN configuration pin high.

Apparently I didn't notice this paragraph in application note XAPP974, Indirect Programming of SPI Serial Flash PROMs with Spartan-3A FPGAs:

Once the FPGA has been configured with the JTAG-to-SPI core, the unused I/Os are set to PULLUP, activating the internal pull-up on the I/O. The user can still pull down the I/O using a 1.1 kΩ resistor if dictated by system requirements.

Gee, thanks Xilinx! Wouldn't it have been better if the JTAG-to-SPI core honored the state of the HSWAPEN input?

With all 25 of the VFD drivers enabled, the +30V boost regulator is trying to drive 25 pull-down resistors presenting a load of about 752 ohms, and that's without the VFD itself attached. This circuit isn't intended to source 40 mA or more, and needs to be shut down to avoid damage to the circuit due to overloading.

If I disable the +30V boost regulator it doesn't matter if the VFD drivers are enabled, so I won't bother with them. Thus there are five circuits that may need to have their pull-down resistors reduced from 100K, listed here in decreasing importance:

1. The VFD +30V boost regulator
2. The printer hammer driver
3. The printer motor driver
4.  The VFD filament transformer drive
5. The keyclick sounder drive

The choice of resistor value is a combination of the need to keep the voltage at the input pins below their threshold values with 500 µA sourced from the FPGA pin, and the limited selection of 0603-sized resistors I have on hand. I've chosen to use 750 ohm resistors.

I probably won't change the keyclick sounder driver pull-down from 100K. Doing so would reduce the draw on the +7.5V supply by 7.5 mA during Flash ROM programming, at the cost of an additional 4.4 mA from the +3V3 supply. But that extra draw on the +3V3 supply would occur any time the keyclick sound is being generated. So that's probably a bad thing, and nothing terrible will happen to the piezoelectric sounder if it's energized for a minute or so during Flash ROM programming.