Hi,
For a project, I have designed a fly-buck using a LM76005-Q1 (schematic below).
While the circuit provides a correct voltage on the primary (V5P5_PRI, yellow curve) and secondary (V5P0_NO_REG, orange curve) outputs, I have noticed an instability with the duty cycle (SW, green curve). As can be seen on the measurements, the duty cycle oscillates from ~35% to ~5% every cycle. The chip is set in forced PWM with a constant switching frequency, and I was expecting an (almost) constant duty cycle when used with a constant load.
I have tried multiple modifications to solve the issue but without any acceptable results:
- Increasing the load on the primary / secondary / both up to 1.6 A → still unstable
- Disconnecting the secondary (remove D300 and C300) → becomes stable
- Removing or Increasing C321 (referred as CFF in the datasheet) up to 200 pF → still unstable
- Dividing R304/R308 (referred as RFBT / RFB in the datasheet) by a factor of 10 and adapting C321 → still unstable.
- Doubling the primary output capacitance (C319, C315, C320, C316) from 40 µF to 80 µF and adapting C321 → still unstable.
- Shorting the two grounds (GND_PRI and GND_SEC) → still unstable.
- While keeping the two grounds shorted, removing C321 (CFF) and adding a 9 pF capacitance in parallel to R308 → becomes stable (but the instability comes back if the 2 grounds are disconnected)
Do you have any pieces of advice on how to solve this issue ?
Kind regards.
