LM2991QML: Clarification on LM2991 circuit

Hi Indu,
Received your word document below is the response to your questions:

We use the above circuit for generating -2.7V from -5V input which is used as the reference voltage for the ADC AD9012. When we have tested this with 274ohms load individually (10mA load current) performance was normal (pure DC). When it is connected to ADC ref pin (w/o load resistor), we are observing ouput oscillations.
Plot is attached. The actual load current for the ADC ref is in microamperes. Kindly provide a detailed explanation for the follwing. Your datasheet doesn’t provide a reference schematic for the above application.

(RK) Note as highlighted in the datasheet the part needs a min load of 5mA. Thus the reason why you are seeing issue when connected directly to ADC - possibly ADC does not provide adequate load to the LDO. You will need to ensure that you are meeting the min load specification.

We used this capacitor to avoid any external noise pickup to the adj pin based on earlier practise. Earlier we used LM137WG.·
2. Implication of using 0.1uF capacitor or any capacitor at the adjust pin. Whether it will lead to oscillations in the output voltage.

(RK) adding a capacitor from Adj pin to gnd adds a pole at parallel combination of R1//R3 with C2 that would be about 10kHz This can affect the stability of the part. However if you really want to add a cap then use a lower value i.e. 100pF or less or so thus the pole is at much higher frequency

• 3. C3 is a ceramic capacitor. If we use R2 of value 10 ohms or above will it lead to oscillations at the output. What is the max series resistance and capacitance that can be used at the output?

(RK) Adding a series resistor with output capacitor this adds a zero at about 1.6kHz in the loop. I would reduce the output resistor to 1 ohm or lower as 10 ohm seems large.

• 4. Below -3V, slope is different from above -3V. Why is it so?
(RK) which curve/ figure are you referring to.

• 5. Device says op voltage of –2V to -25V. Can we generate -2.7V?

(RK) Yes you should be able to generate -2.7V as your load is less. Note Vin > Vout + Vdo that is ensure that Vin is large enough.


• 6. Earlier, we have observed oscillations at -5V input when series resistance of R2 is not used. Why so? Oscillating pattern was saw tooth.

(RK) can you share the waveform are you saying you are seeing oscillation on the input of the part? Or on the output and what was the load on the device. What was oscillation frequency that was observed.


Also I am assuming that your application is for space application thus requires QML if not then there are other negative regulators for lower currents i.e. TPS7A3001-EP