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LM5022: RT/SYNC Pin design

Eric Mo
Expert 1030 points

Part Number: LM5022
Other Parts Discussed in Thread: LM5002

Tool/software:

Hi Team,

My customer uses two LM5022 for boost design, with output voltages of 15V (2A) and 48V (200MA) respectively, with an input voltage of 12V. The schematic is shown below.The U5 be synchronized to the switching frequency of U4, And using the RT/SYNC Pin to synchronize LM5022 switchers,  which the U5 cannot output normally(after removing the R91 resistor, U5 output is normal). However, This circuit has been applied to the customer's old project and has been mass-produced, Could you please help to check and let us know the reason?

lm5002.sch.pdf

  • 0
    TI__Mastermind 19715 points

    Hello Eric,

    Thanks for reaching out to us via e2e.

    The voltage at the RT/SYNC pin is internally regulated to 2 V
    The capacitively coupled sync signal shall lift the voltage on the RT/SYNC pin to a peak voltage level greater than 3.8 V
    BUT - the Abs Max rating fort the RT/SYNC pin is 5.5 V !

    So, if you directly connect the capacitor to the OUT pin, this voltage will probably be too high in some cases (e.g. during start-up).
    This may damage the controller.

    R90 and R91 are already prepared to be used as voltage divider.bbbbbbbbbbbbbbbbbbbbb


    The RT resistor is always required, whether the oscillator is free-running or externally synchronized.
    The external clock must have a higher frequency than the free-running oscillator frequency set by the RT resistor.

    In your case, with R46 and R55, both set to 31K6, the frequency is the same, but not higher.


    Also, in chapter 7.4.1 of the datasheet, you will find the following:
    The sync pulse width must be set between 15 ns to 150 ns by the external components.

    I believe that the duty cycle of U4 does not match, so that pulse will be too long.

    Best regards
    Harry