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I have a requirement for a 24V +- 15% rail to power a laser sensor off of a 24V lead acid battery pack. I implemented the reference design provided by renesas for a 12V buck-boost converter(ISL85403EVAL2Z userguide) with input range of 8V-40V. The only thing that i had to change was the feedback resistors in order to drive higher output voltage. This is the design below. I have also pasted the link for the gerber and kicad files if anyone is interested in the design.

isl85403

The issue i'm facing now is that the converter never enters boost mode. The condition for the converter to operate in boost mode according to the datasheet is that the EXT_BOOST voltage has to be between 200mV and 800mV. My choice of resistors R10 and R11 ensures that this voltage stays around 500mV. Also the output falls rapidly, almost instantaneously to ~1-2V as soon as i connect the load (laser sensor). I tested it by adding load resistors and with a load of 40 ohms the output falls down to 5V. The LGate pin has no activity at all when monitored on a scope. I use an old lecroy scope and wont be able to paste the output waveform here. I'm not experienced with power converter designing. My background is mostly robotics and mechatronics. If anyone could be kind enough to have a look at my design and point out the errors i'd be deeply grateful.

P.S. I have some extra pcb's for this design available. If anyone is interested in tinkering please let me know, i could have them shipped to you. https://github.com/Tal-seven/isl85403_referenceDesign

Regards

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The 1st problem is the high Ron resistance of Q1 = BS138 = 3.5 Ohms.
This ought to be ~10 mOhm ~100 mOhm . The IC BOM list BSZ100N06LS3G is 10mOhm is used in order to boost the voltage by sinking high current from the inductor then "flying back" that current into a boost voltage during off time.

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    \$\begingroup\$ Thank you for your help. I made the modifications and added a BUK7Y41-80E N-channel Mosfet which has an Rds of 40mOhm. The output now stays at the required level and the synchronous mos signal LGate is being driven. Thank you \$\endgroup\$ – Asusrog Oct 11 at 12:30

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