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Quick VRM design with perfectly flat output impedance

17 Aug 2015  | Steve Sandler

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This is one of those places where a measurement is beneficial, as these low value current sense resistors aren't always what they appear to be [1]. Using a 4-wire ohmmeter, the in-circuit resistance of R11 is measured to be approximately 12mΩ, resulting in a reduced power stage transconductance of 8.3. The actual transconductance is verified by measuring the error amplifier output voltage as a function of load current. The resulting measurements, the graph, and the extracted trendline are shown in figure 3. Note the current limit is a bit shy of 10A due to this increased resistance, but the resistor is retained for convenience as it's difficult to change.

Figure 3: Measure error amplifier output voltage vs output current as measured on the evaluation board. The curve fit trendline indicates a transconductance of 8.6, close to the value obtained using the 4-wire ohmmeter.

Error amplifier and compensation

With the power stage and overall transconductance values determined, the error amplifier gain is determined to be:

The error amplifier associated portion of the evaluation schematic is shown in figure 4. The complete evaluation design information can be found in [2].

Figure 4: A portion of the evaluation board schematic shows the voltage divider (R3 and R4) which must be adjusted to change the output voltage from 5V to 3.3V. Several other changes are also made.

Several component changes are required to obtain the flat impedance characteristic. Specifically, resistors R3 must be changed in order to adjust the 5V output to approximately 3.37V at 0A to result in 3.3V at the mean current of 5A. Capacitor C6 is shorted using a 0Ω resistor, since we do not want a low frequency recovery, but a flat impedance. C5 may also require modification to cancel the zero created by the output capacitor ESR.

First, R3 is changed from 3.74KΩ to 2.2kΩ resulting in the desired 3.37V output. The error amplifier gain is then set by the ratio of R10 and R3.

Since the installed value is very close at 18kΩ it is retained as is. C5 is selected in order to offset the zero from the output capacitor and ESR with a pole from R3 and C5.

Since the original 100pF capacitor had to be removed, I measured the blank pads where C5 mounts to be 20pF, requiring 236p. A 220pF capacitor is installed.

Output inductor
Though the output inductor is not a significant contributor to the output impedance, it is important to use an appropriate value. The relationship between the inductance the ripple current is:

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