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LP3981 and LP2985 sourced from the same Li-Po battery

Dragana Maksimovic
Intellectual 310 points
Other Parts Discussed in Thread: LP3981, LP2985

Hello,

My device is souced from Li-Po rechargeable battery. Regulated 3.3V for all components (microcontroller and microSD card + passive) was initially acheived by LP3981.

Problem: write cycle to SD sard messes supply voltage, so ADC integrated in controller gives wrong sample values.

Then I separated card supply adding LP2985 regulator, so that both regulators are sourced from the same battery, but LP2985 sourcing only the card, and LP3981 for the rest of the circuit.

Problem: I still have the same ripple on battery voltage (~200mV drop), resulting in both 3.3V regulated supply ripple (~10mV), every 8ms, which is card write frequency.

Both regulator circuitries are as suggested in datasheets (2.2uF ceramic X7R low ESR capacitors - input and load, and 10nF bypass capacitor for LP2985 and 3.3nF for LP3981). I did try with larger capacitors and tantalum as well, but no luck.

Card power consuption in idle mode is <1mA, and ~50mA during write.

Question: How to stabilize supply voltages?

Thanks,

Dragana

  • 0
    TI__Mastermind 22610 points
    Ripple (ΔV) with a change in load current is typical battery behavior. The degree of change vs current depends on the specific battery chemistry, rated capacity, and state of charge.

    Without knowing exact voltages, my guess is that the battery voltage during load is falling just enough to impinge on the dropout range of the regulators, most likely the LP2985. Otherwise, a 200mV change in Vin would not normally cause a 10mV change in Vout due to line regulation.

    The LP2985, being a bipolar type device, has higher dropout voltage (typically 120mV at 50mA) and will be more susceptible to a low Vin disturbing the output voltage.

    My suggestion would be to use the CMOS LP3981 for both 3.3V legs as it has much lower dropout voltage (typically 22mV at 50mA).

    Another possible issue might be some resistance in the battery supply line, or in some common path, causing a voltage drop when current increases.