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.

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.

 

If you eliminate the voltage doubler, the voltage will approximately drop in half and the current will double.

Increasing the output current is mainly using a coupled-inductor that's adequately large to generate the needed current.
The Coilcraft LPR4012-202LML looks okay for the task.

The IC current switch current limit of the IC should be more than adequate to generate the required output current and voltage.

 

If you eliminate the voltage doubler

Is it not a voltage tripler?

 

Is it not a voltage tripler?

With a unipolar source voltage, as is the case here, it acts as a voltage doubler.

 

With a unipolar source voltage, as is the case here, it acts as a voltage doubler.

I see what you mean.

 

Increasing the output current is mainly using a coupled-inductor that's adequately large to generate the needed current.
The Coilcraft LPR4012-202LML looks okay for the task.

If you don't mind me asking, how do you know that it is adequate? I'm not familiar with many terms in the datasheet, such as 'Isat' and 'Irms', and there is no 'Electrical Characteristics' table like in most datasheets. How can you tell that it can output that level of current?

 

how do you know that it is adequate? I'm not familiar with many terms in the datasheet, such as 'Isat' and 'Irms', and there is no 'Electrical Characteristics' table like in most datasheets.

Below is the "Electrical Characteristics" table.
Why do you think it is not?

Isat shows the primary winding saturation characteristics of the inductor and how the inductance drops with current.

Irms is the maximum RMS current that the primary winding can carry for the given temperature rise.

 

Below is the "Electrical Characteristics" table.
Why do you think it is not?

Isat shows the primary winding saturation characteristics of the inductor and how the inductance drops with current.

Irms is the maximum RMS current that the primary winding can carry for the given temperature rise.

View attachment 161894

Ah, I see. I was asking because I've seen other similar circuits that cannot handle that level of current, so wanted to understand myself why this one can. Thank you for the explanation.