“Adding a feedforward capacitor can improve noise, stability, load response, and PSRR. Of course, you have to choose capacitors carefully for stability. When used in conjunction with noise reduction capacitors, AC performance can be greatly improved. These are just some of the tools to keep in mind when optimizing your power supply.
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In LDO Basics: Noise – How Noise Reduction Pins Can Improve System Performance, we discussed how to use capacitors in parallel with the reference voltage (CNR/SS) to reduce output noise and control slew rate. In this article, we’ll discuss another way to reduce output noise: using a feed-forward capacitor (CFF).
What is a feedforward capacitor?
A feed-forward capacitor is an optional capacitor in parallel with the resistor on top of the resistor divider, as shown in Figure 1.
Figure 1: Low Dropout Regulator (LDO) Using Feedforward Capacitors
Similar to noise reduction capacitors (CNR/SS), adding a feedforward capacitor has several effects. These effects include improved noise, stability, load response, and power supply rejection ratio (PSRR). These advantages are detailed in the application report “Pros and Cons of Using Feedforward Capacitors and Low Dropout Regulators”. Also, it’s worth noting that feedforward capacitors are only feasible when using an adjustable LDO, since the resistor network is external.
improve noise
As part of the voltage regulation control loop, the error amplifier of the LDO uses a resistor network (R1 and R2) to increase the gain of the reference voltage, similar to the noninverting amplifier circuit driving the gate of the field effect transistor so that (VOUT = VREF × ( 1 + R1/R2). This increase means that the DC voltage of the reference will increase by a factor of 1 + R1/R2. Within the bandwidth of the error amplifier, the AC components of the reference voltage, such as noise, will also be amplified.
By adding a capacitor across the top resistor (CFF), an AC shunt is introduced for a specific frequency range. In other words, the AC components in this frequency range remain within unity gain. Remember that the impedance characteristics of the capacitors you use will determine this frequency range.
Figure 2 demonstrates the TPS7A91 noise reduction (by using different CFF values).
Figure 2: TPS7A91 Noise vs. Frequency and CFF Value
By adding a 100nF capacitor to the top resistor, you can reduce the noise from 9μVRMS to 4.9μVRMS.
Improved stability and transient response
Adding CFF also introduces a zero (ZFF) and a pole (PFF) in the LDO feedback loop, calculated using Equations 1 and 2:
ZFF = 1 / (2 × π × R1 × CFF) (1)
PFF = 1 / (2 × π × R1 // R2 × CFF) (2)
Placing the zero before the frequency at which unity gain occurs improves phase margin, as shown in Figure 3.
Figure 3: Gain/phase diagram of a typical LDO using only feedforward compensation
You can see that without ZFF unity gain occurs earlier, around 200kHz. By adding a zero, the unity-gain frequency is pushed slightly to the right at about 300kHz, but the phase margin is also improved. Since the PFF is to the right of the unity gain frequency, its effect on phase margin will be minimal.
The additional phase margin will be noticeable after improving the load transient response of the LDO. By increasing the phase margin, the LDO output will have less ringing and settle faster.
Improve PSRR
Depending on where the zeros and poles are, you can also strategically reduce the gain roll-off. Figure 3 shows the effect of the zero on the gain roll-off starting at 100kHz. By increasing the gain in a band, you will also improve the loop response for that band, resulting in improved PSRR for a specific frequency range. See Figure 4.
Figure 4: TPS7A8300 PSRR vs. Frequency and CFF Values
As you can see, increasing the CFF capacitor pushes the zero to the left, improving the loop response and corresponding PSRR in the lower frequency range.
Of course, you have to choose the value of CFF and the corresponding positions of ZFF and PFF to avoid causing instability. You can avoid instability by following the CFF limits stated in the datasheet, but we generally recommend choosing a value between 10nF and 100nF. Larger CFFs may present additional challenges outlined in the aforementioned pros and cons application report.
Table 1 lists some rules of thumb on how CNR and CFF affect noise.
Table 1: Advantages of CNR and CFF vs. Frequency
Epilogue
Adding a feedforward capacitor can improve noise, stability, load response, and PSRR. Of course, you have to choose capacitors carefully for stability. When used in conjunction with noise reduction capacitors, AC performance can be greatly improved. These are just some of the tools to keep in mind when optimizing your power supply.
The Links: PS11034 G220SW01-V0