In both the voltage regulator and the voltage reference, the range of dynamic impedance from one device type to another device type can vary greatly and change dramatically as a function of operating or bias current.
Unless the device is specifically designed for use with low equivalent series resistance (ESR) capacitors (such as ceramic), the addition of such a capacitor can drastically degrade the control loop stability.
In the case of voltage regulators, the datasheet often includes a chart that defines the region of stable performance with a range of capacitance ESR vs load current. While this is somewhat helpful, the definition of stability is not included, which seems to mean that the circuit will not oscillate. Also, this data is often provided for only one capacitance value. Within the allowable range, then, the stability can still be quite poor. An example of such a stability plot is shown in Figure 1.
Figure 1: Example of a stable range of ESR values. (Texas Instruments refers to this as "The tunnel of death" in their application note SLVA072 -- Technical Review of Low Dropout Voltage Regulator Operation and Performance -- Application Report.)
Many voltage reference datasheets include frequency-dependent output impedance plots, such as the example in Figure 1. While the peaking in the impedance due to the addition of a capacitor is evident, the phase margin is not defined, nor is the ESR of the capacitor.
Figure 2: Example of voltage reference dynamic output impedance plot from the LM4050 datasheet. The dashed lines represent the impedance of the load capacitor and the voltage reference without the output capacitor. The peaking indicates the poor stability with the output capacitor included.
Poor control loop stability can lead to high noise and large voltage perturbations even in the case of a very precise voltage reference or regulator. The measurement in Figure 3 shows the ringing that results in a LT1009 voltage reference with the addition of a 0.1uF ceramic output capacitor.
This measurement shows many interesting effects. First, note that the frequency of the ring is significantly dependent on the load current. Each current level reflects a different frequency. Even more notable is the harmonic content of the ringing voltage. The harmonic content shows that the nonlinearity of a voltage reference can act as a frequency mixer. The voltage spectrum includes the ringing frequency associated with each edge as well as all sum and different frequencies of the ringing frequency and the repetition rate of the step load. In this example, the step rate is 3kHz.
Figure 3: Step load response on LT1009 voltage reference with a 0.1uF capacitor. Note the drastically different responses related to the higher and lower current levels. Also note the complex harmonic structure that includes each of the two ringing frequencies and all of the mixing products between these frequencies and the repetition rate of the step load, which is 3kHz in this example.
These harmonics can then propagate through the printed circuit board through the input and/or output of the voltage reference, resulting in either specific frequency noise or broadband noise.
An additional concern with this ringing is that, while Figure 3 shows the natural damped response, it is also possible for the repetition frequency to occur at the ringing (resonant) frequency.
When the step load occurs at the ringing frequency, the resulting output voltage response is much larger than in the natural damped response. These levels can further exacerbate the noise, and in some cases the ringing can become large enough to cause circuit stress or circuit function issues.
Figure 4: Step load response of a LM317 voltage regulator also shows the two ringing frequencies resulting from each of the two load currents in the step. This figure also shows the impact of setting the step rate equal to the resonant frequency.
The poor stability and respecting ringing can and should be improved by selection of appropriate capacitor ESR or the addition of a resistor either in series with the capacitor or between the regulator and the capacitor.
About the Author
Steve Sandler is the founder of AEi Systems, a leading US company that
specializes in the worst case analysis and troubleshooting of satellite systems. He is also the founder of Picotest, a high performance measurement company, supporting high technology companies in more than 25 countries. Steve is the author of several books related to SPICE modeling and simulation and is currently writing his fifth book for McGraw-Hill on high performance measurement, due for completion in 2014.