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LM2991 oscillates

takishin
Expert 2360 points
Other Parts Discussed in Thread: LM2991

Our customer has a question about LM2991.
I'm posting the question for the customer:

++++++++++++++++++++++++++++++++++++++++++++++

About Negative Low Dropout Adjustable Regulator:LM2991


In case of Vin=-17V,Vout=-16.3V,Iout=200mA;
The LM2991 oscillates.

In case of Vin=-17V,Vout=-14.8V,Iout=200mA;
The LM2991 which is a same board setting Vout=-14.8V works properly.
(I/O capacitors are aluminium electrolytic capacitors:470uF)

The device needs Vdrop=0.2V at 0.1A in datasheet.
Would you please let me know why the device oscllates as Vdrop=0.7V ?

++++++++++++++++++++++++++++++++++++++++++++++

Thanks in advance.

Best regards,
Takishin

  • 0
    Expert 2360 points

    Waveform when the LM2991 oscillates.

    ch1:output voltage  ch2:input voltage (negative voltage )

    Thanks,

    Takishin

  • 0 in reply to takishin
    TI__Mastermind 22610 points

    Vin=- 17V, Vout= -14.8V, Iout= 200mA, Cout= 470uF; the LM2991 works properly

    Vin= -17V, Vout= -16.3V, Iout= 200mA, Cout= 470uF; the LM2991 oscillates.

    I will presume that the 470uF Aluminum capacitor used for Cout is of reasonable quality and meets the min/max ESR requirement.

    In short, there is no obvious reason for the behavior, as described.

    Vout/Ch1 (yellow) shows a period of 1.5ms, which is about 649 Hz, generally too low to be considered a control loop stability issue. The 470uF will create a load pole around 4Hz, so Vout moving from -14.8V to -16.3V will not have a any perceptible effect on that.

    Looking at both wave forms: Vin/Ch2 (blue) is pulled down (less negative) as Vout goes more negative. From this I would presume that the output load current would be more than expected if Vin is being loaded.

    Vout ramps about 120mV more negative in about 300us, then ramps 120mV less negative in about 1.2ms. If Cout is 470uF, then the currents would be 188mA and 47mA respectively.

    Q1: Is it possible to confirm output load current (or input current) is 200mA with current probe?

    Q2: Is it possible to get the schematic, or values of the supporting components (i.e. R1, R2, Cff, etc, etc?)

    Q3: Have different Cout capacitors been tried?

  • 0 in reply to Donald Jones
    Expert 2360 points
    Hello,

    Thank you for your advice.

    I have confirmed the input current with current prove.

    During "Vin/Ch2 (blue) is pulled down (less negative)", the input current is near 1A.

    My suggestion is the following;

    The current is charging Cout and the other capacitors (actually total 2000uF).
    So the drop voltage of the LM2991 increases (Vdrop=0.8Vmax@Io=1A at Datasheet),
    then it cannot output the charging current, because the differential voltage
    between Vin(-17V) and Vout(-16.3V) is 0.7V.
    And, the capacitors are discharged so Vout decreases.

    How do you think?

    Best regards,
    Takishin