If you're forced to position the regulator a distance away from the devices being powered and you can't dedicate a power plane (or the board is done and things look bad), try using bus bars to reduce impedances and clean up the power. They can be used for both power and ground connections. Don't forget to study the effects of airflow when using bus bars. If employed properly, they can be used to direct airflow where most needed.
You may also consider running a short sense trace back from the device you're powering. This can help set the proper voltage at the load, though it may result in additional noise issues if the sense line isn't carefully routed and compensated. Keep in mind that the sense line drives an input to the regulator's error amplifier, which controls the overall gain. Long sense traces can cause instabilities, so you need to properly study the regulator's control loop.
CAPACITORS AREN'T ALL THE SAME
Capacitor technology has seen steady improvements over the last few years to help provide lower ESR and larger capacitance in smaller surface-mount packages. Aluminum electrolytics are still the lowest-cost bulk capacitors, but they're physically large and typically exhibit high ESR. Tantalum capacitors have been the workhorse for bulk capacitance, yet they too are large, and their ESRs may be too high for the capacitance needed with high-transient load situations. Some manufacturers are starting to offer new types of electrolytic capacitors with specialty polymers for the cathode material. These components, however, are still quite pricey, have a low ESR, and come in small sizes that reflect the lower voltage ranges of new process technologies.
MEASURE THE NOISE
Noise on a power bus often results in high EMI, which may bring up regulatory issues during system testing. Load transients and ripple voltage across a power bus cause radiation at the frequencies involved and their harmonics. Reducing noise via the methods discussed may significantly reduce system EMI.
Too often, intermittent system problems relate back to power problems that could have been rectified during the beginning stages in a design cycle. A solid study of the power planes early in the debug stage might uncover fixable specification violations, thereby eliminating potential residual problems before they surface. Take the following worst-case measurements.
When measuring noise on a power bus, understand what you see. Large amounts of energy radiate from power buses, so a less-than-ideal probe without proper ground connections can cause improper measurements. Use one or two high-frequency probes with a small ground stub, as shown in Figure 5. A ground wire of as short as two inches will result in bad measurements.
The oscilloscope must clearly display and trigger up to the noise frequency to which your circuit might react. At minimum, use a 200-MHz oscilloscope and probe. For best results, use one probe for the power connection and another for the ground connection. Then set the scope to a math function that adds the two inputs, with one probe set to invert. This differential method reduces the risk of ground currents that may occur when using a single probe. When making differential measurements, it's often best if no ground wires are used on the probes.
Trigger the oscilloscope properly using normal mode, with the threshold adjusted for extreme peaks. Make two measurements, one with the scope triggering on the positive edge for the positive peak voltage measurement, and the other with the scope set for negative slope trigger for the negative peak measurement. The difference between the two numbers is the peak-to-peak noise voltage. Measure the frequency of the noise by simply inverting the periodic time of the specific noise being viewed.
To measure high-frequency transients, set the scope for a faster sweep. Look at components above a few megahertz and use the technique described previously to measure the peak-to-peak voltage and frequency. If you see more than one noise component, pay attention to the one with the highest amplitude. In some cases, noise components may be at frequencies higher than the device can respond to, and thus not cause problems. To view noise components in the kilohertz range, set the scope to a low sweep frequency. Low-frequency noise often results from system operation, possibly a function of a processor executing high-performance subroutines along with low-power idle modes. Changing the software operation of the devices being powered can help identify the noise source and potentially lower peak current.
Use a spectrum analyzer to quickly identify noise components that may cause problems. It will also help identify the noise source. The ripple voltage created by a switcher may be riding on another lower-frequency component, and will almost always have higher-frequency noise on it created by high-speed digital processing.
Taking the time to properly observe the power supplied to high-speed devices is worth the effort. Some bench time early in the prototype debug stage can save a lot of debug time later on.
Very often I see system power problems surface as unrelated system anomalies. Power problems can be quickly resolved or avoided altogether by investing sufficient time in your power design from the earliest stages.