In my last blog, LDO basics: noise – part 1, I discussed how to lower output noise and control the slew rate by using a capacitor in parallel with the reference voltage (CNR/SS). In this post, I’d like to discuss another method to lower output noise: using a feed-forward capacitor (CFF).
What is a feed-forward capacitor?
A feed-forward capacitor is an optional capacitor placed in parallel with the top resistor of the resistor divider, as shown in Figure 1.
Figure 1: An NMOS low-dropout regulator (LDO) using a feed-forward capacitor
Much like the noise-reduction capacitor (CNR/SS), adding a feed-forward capacitor has multiple effects. Chief among these are improved noise, stability, load response and power-supply rejection ratio (PSRR). (The application report, “Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator,” covers these benefits extensively.) It’s also worth noting that a feed-forward capacitor is only viable when using an adjustable LDO because the resistor network is external.
As part of regulation, the error amplifier of the LDO uses the resistor network (R1 and R2) to increase the gain of the reference voltage, much like a noninverting amplifier, to drive the gate of the FET accordingly. The DC voltage of the reference will be gained up by a factor of . However, given the bandwidth of the error amplifier, you can also expect amplification in some portion of the AC elements of the reference voltage as well.
By adding a capacitor across the top resistor, you are introducing a shunt for a particular range of frequencies. In other words, you are keeping the AC elements in that frequency range within unity gain, where R1 simulates a short. (Keep in mind that the impedance properties of the capacitor you’re using determine this frequency range.)
You can see the reduction in noise of the TPS7A91 by using different CFF values in Figure 2.
Figure 2: TPS7A91 noise vs. frequency and CFF values
By adding a 100nF capacitor across the top resistor, you can reduce the noise from 9μVRMS to 4.9μVRMS.
Improved stability and transient response
Adding a CFF also introduces a zero (ZFF) and pole (PFF) into the LDO feedback loop, calculated with Equations 1 and 2:
ZFF = 1 / (2 x π x R1 x CFF) (1)
PFF = 1 / (2 x π x R1 // R2 x CFF) (2)
Placing the zero before the frequency where unity gain occurs improves the phase margin, as shown in Figure 3.
Figure 3: Gain/phase plot for a typical LDO using only feed-forward compensation
You can see that without ZFF, unity gain would occur earlier around 200kHz. By adding the zero, the unity-gain frequency pushes a little to the right (~300kHz) but the phase margin also improves. Since PFF is to the right of the unity-gain frequency, its effect on the phase margin will be minimal.
The added phase margin will be evident in the improved load transient response of the LDO. By adding phase margin, the LDO output will ring less and settle quicker.
Depending on the placement of the zero and pole, you can also strategically lessen the gain roll-off. Figure 3 shows the effect of the zero on gain roll-off starting at 100kHz. By increasing the gain in the frequency band, you will also improve the loop response for that band. This will lead to improvements in PSRR for that particular frequency range. See Figure 4.
Figure 4: TPS7A8300 PSRR vs. frequency and CFF values
As shown, increasing the CFF capacitance pushes the zero leftward. This will lead to better loop response and corresponding PSRR at a lower frequency range.
Of course, you must choose the value of CFF and the corresponding placement of ZFF and PFF so that you don’t introduce instability. You can prevent instability by following the CFF limits prescribed in the data sheet. A large CFF can also introduce other problems outlined in the aforementioned application report.
Table 1 lists some rules of thumb regarding how CNR and CFF affect noise.
Noise-reduction capacitor (CNR)
Feed-forward capacitor (CFF)
Table 1: Benefits of CNR and CFF versus frequency
As shown, adding a feed-forward capacitor can lead to improvements in noise, stability, load response and PSRR. Of course, you must carefully select the capacitor to maintain stability. When coupled with a noise-reduction capacitor, ac performance can be greatly improved. These are a just few tools to keep in mind for optimizing your power supply.
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