Excessive Boost Converter Output

Question

I'm implementing a boost switching regulator based on a Texas LM5002 chip (datasheet), to get about 65V from a 12V supply. The design I've arrived at is shown below, inspired both from the data on the datasheet and the information here. I was not able to get all the parts with the necessary values, but I used the closest values I got my hands on. The design took into account an input current of about 50 mA to supply about 1mA on the output node.

^{simulate this circuit – Schematic created using CircuitLab}

Problem is, when I tie 12V to the input, I get, for a few seconds, around 900V (???), before the output shorting to ground. What is exactly could be causing this? Have I got something wrong on my circuit?

EDIT: It appears I made a mistake on my original question while drawing the schematic, the actual value of R4 is 1.6k and not 100 ohms.

Answer 1

EDIT - in case any other potential downvoters miss the point - the original question had R4 as 100 ohms and asked if there was something wrong on the schematic (it now says "circuit")!!

It looks like R4 is the wrong value i.e. far too low. It forms a potential divider with R3. With the values shown it has an attenuation of 0.00124. In normal operation you would expect to see 1.26V at the FB (feedback) pin. With your attenuator this implies a line output voltage of about 1,017 volts.

Here's an example from the data sheet for a 48 volt output: -

Note the ratio of resistor values surrounded by the red rectangle. They attenuate by a factor of 38.35 and with 48 volts on the output, the voltage on FB is 1.252 volts. Given that it should be 1.26 volts (according to the data sheet) I reckon this circuit will produce 48.3 volts.