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LM25184-Q1: Switching Frequency, Enable and Soft start query

Asha C
Intellectual 375 points
Part Number: LM25184-Q1
Other Parts Discussed in Thread: LM25184, LM5180EVM-S05

Tool/software:

Hello,

We are using LM25184 for stepping up 15V to 20V with isolation btwn input and output. Please find the below schematic. Note: C63 is DNP in design.

1. Switching frequency:

With input 15V and Load of 0.120A , observed switching frequency is 375kHz. Please find the waveform below.

But datasheet says max switching frequency is 350kHz. Could you please provide your feedback.

2. Enable Voltage and soft start

We have set UVLO rising as 12V and UVLO Falling as 11.5V but 

a. Unloaded condition on output(20V_ISO) , UVLO Rising observed is 11.9V

b. In loaded condition(0.12A on output ie 20V_ISO), output is rising after input voltage reaches 15V. Please find the below waveform. Could you please provide your feedback wrt UVLO Rising/Enable and Soft start in this case.

...

  • 0
    TI__Genius 15476 points

    Hi Asha,

    -I asked the design team to check fsw 375kHZ. That value seems high to me so I would confirm if that is possible or how we can address this.

    -Regarding the UVLO rising 11.9V compared with 12V when unloaded, this is normal as we have to take into account the variation of the resistances values and also the parasitic inductance of the layout which provide some extra unwanted impedance.

    -When output is loaded 15V is very high compared to 12V. Could you please share the waveform (was not attached above). Also, please share your layout to be reviewed.

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    Intellectual 375 points

    Hello Manuel,

    Sorry! Yes, i have missed the waveform prior. Please find the below.

    And regarding the layout - Will be sharing soon!

  • 0 in reply to Asha C
    Intellectual 375 points

    Could you please provide your feedback on this.

    We are waiting for the reply. This is stopping us from going forward to the next phase in design.

  • 0 in reply to Asha C
    TI__Genius 15476 points

    Hi Asha,

    I just followed up with design team regarding the switching frequency. Regarding the UVLO. It is interesting that Vout starts ramping up when VIN=15V. There must be some delayed happening at Cout so that it is reflecting later when the device starts switching. Could you please take (SW-GND) in addition to (VIN-GND) and Vout? We need to confirm that device starts switching after reaching 1.5V at UVLO pin (12V at VIN).

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    TI__Genius 15476 points

    Hi Asha,

    The max. switching frequency value can reach 375kHz in some cases according to the experts. I hope this helps.

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    Intellectual 375 points

    Is it because of internal tolerance?

    What would be the maximum switching frequency we can expect from this converter, is it 375kHz?

  • 0 in reply to Manuel Alva Hernandez
    Intellectual 375 points

    Hello Manuel,

    Please find the waveform below.

  • 0 in reply to Asha C
    TI__Genius 15476 points

    Hi Asha,

    I took waveform in the LM5180EVM-S05 (link below where you can check Schematic) and checked the Soft Start to confirm this behavior. As you can see below, the device starts switching when VIN=~9.5V (UVLO_on), but Vout (5V in this case) has not increased much yet. This is due to the high load condition (Iout=500mA in my case). During Soft Start, the controller delivers power to the secondary side with a constant fsw of 9khZ<fsw<15kHz while increasing Vin and this charges Cout slowly. After the soft start time (6ms in this case), the control loop feedback takes control and the controller switches at higher frequencies to rump up Cout sharply. This is different at no load conditions, where Cout get charged quicker as there is no need for a lot of power to be delivered to the output side.

    I hope this helps. Regarding the questions about the switching frequency tolerances, I assigned this question to the previous team expert.

    Thank you