I have constructed the SEPIC example shown on page 16 of the LT1307 data sheet (1307fa.pdf, Step-Up/Step-Down Converter). I have used 2 wuerth 74408943100 coils for the Inductors L1, which seem to be fairly similar to the Murata Erie LQH3C100 coils. I hooked the circuit up to a lab power supply together with an additional 100uF/16V electrolytic capacitor at the input, nothing else. The circuit does work, however, as soon as the input voltage reaches 3.75 volt, the circuit seems to become unstable, you see sort of random ripple on the output and irregular switching on pin 5. At 4 volt the circuit becomes regular again, switching in burst mode. Note this occurs at full load, output loaded with a 33 ohms resistor. I also tried wuerth 74438335100 coils, with te same results. Does anyone here have experience with the LT1307, especially used in SEPIC (Step-Up/Step-Down) configuration?
LT1307 switching normally at 3.73V input voltage. Yellow: Output. Red: Switching node (Pin5).
LT1307 at 3.75V input voltage, irregular cycles.
LT1307 at 4.0V, switching in burst mode at full load.
Thank you for your help!