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LMZ14203

jerry booth
Prodigy 200 points

The data sheet show a maximum input voltage of 19VDC for an output voltage of 1.2VDC.

But Web bench allows a design of 28 in and 1.2 VDC out.

Should I not allow an input greater than 19VDC for 1.2 V out or is Web bench correct ?.

What is the fault mode if the input is allowed to go above 19vdc when 1.2 VDC is the output voltage.

  • 0
    TI__Genius 15810 points

    Forwarding to our Webench applications engineer

  • 0 in reply to Akshay Mehta
    TI__Expert 6365 points

    Hello Jerry,

    The datasheet offers the suggested operating range for optimum efficiency and operate in the continuous mode.  If higher Vin was used beyond the suggested range, the device will still function. However, the efficiency will be worst and operating mode will be in the discontinues mode and the frequency may change too.   I hope I answered your question.

  • 0
    TI__Expert 4365 points

    Hello,

    The LMZ1420x and LMZ1200x series of regulator modules are based on Constant On-Time architecture. In this implementation, there is a RON resistor in the application that senses the magnitude of the input voltage and shortens the on-time as input voltage rises. In this manner the control system mimics constant frequency operation because switching frequency is essentially constant as input voltage changes. Back when these modules were developed it was noticed that the best efficiency occurred around 400kHz operation. The RON resistor values in the data sheet and application notes were selected for that switching frequency. Note that if you design for a different output voltage, the recommended RON value is adjusted accordingly to guide the user to the best power efficiency for the application. 

    However, for low output voltages such as 1.2V and an R-ON resistor set to give 400kHz operation, the control circuit will reach its specified minimum on-time limit at around 19V input. For input voltages above 19V the control loop maintains regulation by extending the Off-Time. With the On-time fixed at minimum, and an extended off-time the control loop maintains regulation by dropping switching frequency. It turns out the loop behaves properly all the way up to 42V input, without any impact on the load step performance. So to answer the original question. There is no problem at all operating above the data sheet recommended 19V maximum. The stated maximums shown were declared based on constant frequency operation, which in retrospect should not have been a requirement.

    Bench data to illustrates the frequency behavior vs input voltage. Excuse the tabular format

    Vin    Fsw

    12      387

    18      389

    24      368

    28      337

    32     306

    36     280

    40     258

    42     249

    Sorry this took so long to reply, I went back to the bench to test the application across these mode to make sure the application would behave properly.

    Alan Martin