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Original question:

LM25037-Q1 Push-Pull topology

LM25037-Q1: LM25037-Q1 Push-Pull D=0.5

user4382362
Prodigy 80 points
Part Number: LM25037-Q1
Other Parts Discussed in Thread: LM25037

1. How to ensure D=0.5 all the time?

2. Whether there is a crisis if  LM25037 works in open loop mode?

  • 0
    TI__Expert 3095 points

    Hi user4382362,
    Follow the datasheet to configure the controller to operate in voltage mode control. This is described in Section 7.4.1 in the datasheet.
    Disable the internal error amplifier by shorting the FB pin to ground. This is described in Section 7.3.4 of the datasheet.
    A simple pull down resistor on the COMP pin will set the voltage on this pin, this voltage should be adjusted to provide the desired duty cycle. The relationship between COMP voltage and duty cycle will vary according to the ramp amplitude that you have selected.

    This method will allow you to program any desired duty cycle. Two things to watch out for:
    1) COMP has an internal pull-up resistor to VREF. This internal resistor will have a wide tolerance. If you require the duty cycle to be accurate across parts and temperature I suggest driving this pin from a relatively low impedance fixed voltage so that you are not affected by the tolerance of the internal pull-up resistor.
    2) If you generate your ramp using VIN then the duty cycle will drop as input voltage rises. This may suit you since it will tend to regulate the output for changes in input voltage. If it does not suit, then I suggest that you generate the ramp using a constant voltage level instead of VIN to ensure fixed duty cycle. If the ramp amplitude is fixed and the COMP pin voltage are fixed the duty cycle will be fixed, since the PWM output is produced by comparing these two levels.
    Thanks
    Joe Leisten

    5125.E2EPicture.docx

  • 0 in reply to Joe Leisten
    Prodigy 80 points

    Is there any calculation to set RAMP and COMP voltage in order to achieve D=0.5?

    My design is connect RAMP to CS, so I think D varies as the Vin changes, COMP has a pull-down resistor, so this pin can be given a fixed voltage by REF.

    I want to calculate exact RAMP and COMP voltage at D=0.5, thanks!

  • 0 in reply to user4382362
    TI__Expert 3095 points
    Hi user4382362,
    Imagine that you connect the COMP pin to VREF and the RAMP pin to CS.
    In this case the Gate Drive output will be high for the time taken for the RAMP pin to voltage reach VREF-1V.
    During this time the RAMP pin charges at a rate determined only by the external capacitor and resistor that you connect.
    To set duty cycle you should select the RAMP pin components so that the output high period is half the oscillator period.
    The oscillator frequency can be set using Figure 6 of the datasheet.
    I hope this is clear.
    Thanks
    Joe Leisten