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LM2991 Driving large ceramic capacitance

Dave Hart
Prodigy 20 points
Other Parts Discussed in Thread: LM2991

I need to drive a distributed ceramic capacitive load of ~80uF, 144 capacitors across a 1 sqft multi-layer PCB. Can I have a tantalum close to the LM2991 for stability, then add a series resistor before I drive the output power plane and the 80uF. The resistor's purpose is to isolate the low ESR ceramic capacitance to keep the LDO stable. I'll be using VIN = -15V, VOUT = -14.5V, I = 800mA. I can tolerate the IR drop of the resistor.

Quote LM2991 datasheet :

Ceramic Bypass Capacitors
Many designers place distributed ceramic capacitors whose value is in the range of 1000 pF to 0.1 μF at the
power input pins of the IC's across a circuit board. These can cause reduced phase margin or oscillations in LDO
regulators.
The advent of multi-layer boards with dedicated power and ground planes has removed the trace inductance that
(previously) provided the necessary "de-coupling" to shield the output of the LDO from the effects of bypass
capacitors.
These capacitors should be avoided if possible, and kept as far away from the LDO output as is practical.

  • 0
    TI__Mastermind 22610 points

      " ... a tantalum close to the LM2991 for stability ..."

    This would be required. In this case I would suggest at least ~10x larger value than the datasheet recommendation.

     

    "... then add a series resistor before I drive the output power plane ..."

     

    Use the largest value series R that you can tolerate.

  • 0 in reply to Donald Jones
    TI__Prodigy 20 points

    OK, thanks, I'll use Co > 10uF tantalum and probably a 5 ohm 5 watt series resistor before the ceramic capacitance.

    Is there any trick to control the in-rush current into the LM2991? I will be powering up with it OFF, then turn ON later. My power supply is poor and sensitive to large load currents, I'd like to minimize the input current surge when I turn it ON if possible.