# Excessive Boost Converter Output

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.

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.

• I made a mistake whe drawing the above schematic. R4 is actually 1.6k. Feb 25, 2015 at 16:31
• You asked if there was something wrong with your schematic and this I identified. Feb 25, 2015 at 16:41
• The 10mH inductor used for energy storage seems might high - if this inductor has a self resonant frequency close to 1MHz (your switching speed) bizarre voltages could also be generated. I would definitely check this out and/or increase RT to something more manageable like sub-100kHz operation. Feb 25, 2015 at 19:02
• I'm not sure what the SRF of this particular inductor is, but looking at similar parts on several catalogs makes me believe it is around 250-300kHz (I bought it as bulk, no datasheet was provided). I will try to increase Rt anyway. Feb 26, 2015 at 13:51
• I suspect it is also so I would definitely look at lowering the inductance. Feb 26, 2015 at 13:57