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LMR14020: Conventional BUCK Converter for Duty Cycle Problems in BUCK-BOOST Topologies

user3767427
Prodigy 40 points
Part Number: LMR14020
Other Parts Discussed in Thread: LMR16030, LMR16020, LMR16010

LMR14020 This chip I want to use as BUCK-BOOST topology, output negative voltage, input +18 to 30V output negative 24V, so need a relatively large duty cycle range, up to 0.6 or so, the specification says that when the switching frequency is 200kHz I can get the maximum duty cycle. I want to know the maximum duty cycle that this chip can provide at the highest switching frequency. Because I want to reduce the inductance, I want to use a chip with a higher switching frequency. Which more suitable models are recommended for me? Please give me some advice, thank you very much!

  • 0
    TI__Mastermind 27825 points

    You need a buck converter with operation Vin> at least 54V for 18V~30V to negative 24V output.

    What's load current of -24V?

    Pls check if LMR16020 and LMR16030 is OK for your application.

  • 0 in reply to Andy Chen
    Prodigy 40 points

    Thank you for your reply, Andy Chen, I realized the pressure problem after I posted it. If I change to LMR16020, there is still such a problem. If I set the switching frequency to the maximum, for example, 2.3MHz, this time. The PWM period is only 434ns, but there is always a minimum duty cycle adjustment width. How much duty cycle can I get at this time? Can you meet the needs of my application 60%?
    My application is to power a sensor. It requires a -24V supply, low ripple, and a maximum load current of 50mA. For cost reasons, I don't want to use the DCDC module to supply power. Is the LMR16020 suitable for my application?

  • 0 in reply to user3767427
    TI__Mastermind 27825 points

    Hi

    For Fsw=2.3MHz, The PWM period is only 434ns,

    the duty cycle is about 45% for 30Vin to -24Vout, Ton is 434*45%=195ns, this is bigger than Ton-min 90ns. 

    the duty cycle is about 60% for 18Vin to -24Vout, Toff is 434*40%=173ns, this is close to Toff-min 160ns. so we suggest to use 2MHz to increase Toff time to 200ns.

    LMR16010 is better choice as max load current only 50mA.

  • 0 in reply to Andy Chen
    Prodigy 40 points

    Hello, andy chen, I understand how to choose the switching frequency, thank you for your help!
    By the way, why is the posting time displayed after the successful posting of my post is not Beijing time? Are you replying to my question in China?