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Tool/software:
Hello.
I have designed an isolated DCDC converter using this IC and am testing it right now.
I'm asking this question because I ran into a problem during testing that I can no longer resolve.
In the startup sequence of the IC, I get the first pulse, but the next pulse is not output.
I have confirmed that Vcs has reached 0.28V and that the RUN signal is High.
Please let me know if there is anything else to check.
thank you.
C1: PWML
C2: RUN
C5: Vcs
C6: Current sensing shunt
Hi Hirotsugu-San,
Thank you for reaching out on UCC28780.
If only one PWML pulse with a width of 2 µs is observed, three main possibilities may exist.
1. CS pin related fault - shorted or too low voltage . If this is the case, the issue can be resolved by removing the short/ proper current sense resistor sizing. Or CS pin might not reach 280mv within 2us. In this case Lm value might need to be checked.
2. Input voltage is too low. Increase the input voltage to 90% of brown-in voltage .
Please follow this document on debugging tips for starting the converter.
Regards,
harish
Hi Harish.
Thank you for your responce.
As you can see in the oscilloscope image, Vcs reaches 0.28V and the PWM pulse width is less than 1us.
The pulse width seems to get longer as the supply voltage is lowered,
so the operation seems to be working properly until it turns off when Vcs reaches 0.28V.
I also checked if 3 PWM pulses are produced with resistor Ropp open,
but only one PWM pulse of less than 1us was produced.
Best regards.
Suzuki.
Hi Hirotsugu-San,
Can you please forward the schematic and let us know their input output voltage conditions and load?
Thank you
Regards,
Harish
Hi Harish-san
I'm sorry, but company policy does not allow us to openly disclose schematics to the public.
Can we continue our discussion in a more closed area?
Best regards.
Suzuki
Hi Hirotsugu-San,
Please reach out through internal mail where we can share schematics.
Thank you
Regards,
Harish