# LT1308A - High Voltage Modification

I am trying to understand how to modify the circuit below to output ~180V at 4mA minimum for a multiplexed nixie design.

LT1308A Datasheet - Circuit on Page 17​

Voltage:
The 1:12 transformer is being replaced with a 1:10 coupled inductor, which should drop the voltage right off the bat. Modifying the resistors connected to the FB pin for a specific voltage should allow the IC to regulate the output voltage by changing its duty cycle, though only within a certain range. It appears the upper right portion of the circuit is a voltage multiplier, so could that be modified to drop the voltage?

Current:
I don't know how to go about modifying the current capabilities. My guess would be that it bottlenecks somewhere, since other 'Typical Application' circuits in the datasheet can handle much higher current loads. Is this output possible with the circuit?

David Forbes' nixie watch uses one of these ICs in his schematic, though it is the B variant. There are several forums in which he says one of these can output up to 4mA, but I don't know how to achieve this output.

[EDITED to replace circuits and restate the current part.]

First, please note that the output is raised using a voltage tripler.

So the first thing can be to replace it with a voltage doubler.

Then you have to change the ratio of 10M and 34.8k resistors so that for 180V output you get 1.2V at FB pin. For example instead of 34k8 you put 68k.

The current seems to be limited by 50% efficiency of circuit. The converter alone peaks 90% efficiency at 200mA switch current. If this is the case, and efficiency is not improved, you can aim at 2.5mA max output.

Source of circuits

• Are you sure the output current of the original application is limited to 1.2mA because of ripple requirements only? Just wondering how you determined that. Also, I may be mistaken but I think the application circuit the op posted is in fact a doubler... Commented Oct 18, 2018 at 23:07
• You're right, I think the 1.2mA is limited by in(efficiency). The converter alone has an efficiency of 90% for 200mA switching current. 1.2mA corresponds more or less with an efficiency of 50% of the whole circuit, which is not too bad. In this case one can aim at 2.5mA maximum. Regarding the tripler. Yes, it triples the voltage accross the two capacitors. Commented Oct 19, 2018 at 1:32