LM27403 Unstable condition Problem

Let me see if I can find someone to help.

Please forward the converter schematic and PCB layout for review. What inductor are you using?

Regards,

Tim

LM27403-PWR Module.pdf

Note that if remote sensing on VOUT- is required, the connection from RS to GND should be a small resistance, e.g. 10 Ohms (this is inlcuded in the carrier board for the LM27403EVM-POL600). If remote sensing is not required, simply short RS to GND.

Also note that the compensation components for the LM27403EVM-POL600 design are selected assuming the module is mounted on a carrier board that has additional output capcitance. Perhaps you can include extra output capacitance to ensure a stable design. Alternatively, you can recalculate the compensation components based on the expected lower output capacitance (noting that the ceramic capacitance derates with applied voltage). Please consult the LM27403 quickstart tool for additional guidance.

Regards,

Tim

thank you very much for replying  Tim :)

I add Low ESR Tantalium Cap in Output and Input but the problem not solve. SW, LG and HG signals have Jitter. may this jitter cause unstable condition?! also when I touching board ,output voltage become 0V. output voltage show in figure:

Is RS shorted to GND? If the concern is that switching noise is propagating into the feedback circuit, you can slow down the high-side FET's turn on by placing a resistor (e.g. 5 Ohms) in series with the boot cap. This will reduce the voltage overshoot and ringing at the rising edge of the SW voltage.

Yes, 100mA is very high...according to the EVM user's guide, the input current at no load should be 40mA. Perhaps the core loss is very large at 600kHz with the particular inductor you're using - and this might also explain why it runs hot.

Maybe, but you would need to increase inductance to keep rpple current amplitude manageable. Are you using one of the inductor vendors listed in the EVM BOM, e.g. Coiltronics or Cyntec?

You can order the 600kHz EVM and check its operation. What is your ouptut voltage? Note that the effective capacitance of ceramic components reduces with increasing voltage. The cap vendor typically provides a software tool that plots capacitance vs. voltage.

Output voltage is 3V3. I put three numbers of 1uF tantalium Caps and four numbers of 100uF ceramic caps in output and ripple reduce to 9mV. Thanks you so much Tim for your helping.