LDO Basics: Noise - Part 2

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LDO Basics: Noise - Part 2 [Copy link]

 By Aaron Paxton, Texas Instruments In my last post, LDO Basics: Noise – Part 1, I discussed how to reduce output noise and control slew rate by adding a capacitor in parallel with the reference voltage (CNR/SS). In this post, I will discuss another method for reducing output noise: using a feedforward capacitor (CFF).

What is a feedforward capacitor?

The feedforward capacitor is an optional top capacitor that is connected in parallel with the top resistor of the resistor divider, as shown in Figure 1. Figure 1: NMOS low dropout regulator (LDO) using feedforward capacitors 85)]Similar to the noise reduction capacitor (CNR/SS), adding a feedforward capacitor has several effects. The most important is noise reduction, but also includes improved stability, load response, and power supply rejection ratio (PSRR). (The application report, “Advantages and Disadvantages of Using Feedforward Capacitors in Low-Dropout Regulators,” discusses these benefits in detail.) It is important to note that the feedforward capacitor can only be used when using an adjustable LDO, because the resistor network is external.

Noise Reduction

The error amplifier used in LDO regulation uses a resistor network (R1 and R2) to increase the gain of the reference voltage to drive the gate of the FET, much like a non-inverting amplifier. The DC voltage of the reference is increased by a factor of ????. However, given the bandwidth of the error amplifier, you can also expect some amplification of the AC component of the reference voltage.

By adding a capacitor in parallel with the top half of the resistor divider, you have introduced a shunt for a specific frequency range. In other words, you have made the AC component contribute to unity gain over that frequency range, with R1 simulating a short circuit. (Keep in mind the impedance properties of the capacitors used to determine this frequency range.) In Figure 2, you can see the noise reduction effect of the TPS7A91 when using different values of CFF.

Figure 2: TPS7A91 Noise vs. Frequency and CFF Values By placing a 100nF capacitor in parallel with the top half of the resistor divider, the noise can be reduced from 9μVRMS to 4.9μVRMS. Improving Stability and Transient Response Adding a CFF also introduces a zero (ZFF) and a pole (PFF) into the LDO feedback loop, which are calculated in 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) Forming a zero before the frequency at which unity gain occurs improves the phase margin, as shown in Figure 3.Figure 3: Gain/Phase Plot of a Typical LDO Using Only Feedforward Compensation You can see that without the ZFF, unity gain occurs about 200kHz earlier. By adding the zero, the unity gain frequency is moved a little to the right (~300kHz), but the phase margin is also increased. Since the PFF is to the right of the unity gain frequency, it has the least effect on the phase margin.

After improving the LDO's load transient response, you will see a significant increase in phase margin. With the increased phase margin, the LDO output will have less ringing and settle more quickly.

Improving PSRR

Depending on the placement of the zeros and poles, you can also subtly reduce gain drift. Figure 3 The effect of the zero on the gain reduction starting at 100kHz is shown. By increasing the gain within the band, you will also improve the loop response in that band. This will improve the PSRR for that particular frequency range. See Figure 4. Figure 4: TPS7A8300 PSRR vs. Frequency and CFF Value As shown in the figure, increasing the CFF capacitor value pushes the zero point to the left, resulting in better loop response and corresponding PSRR in the lower frequency range. Of course, you must choose the value of CFF and add the zeros ZFF and poles PFF appropriately so that instability does not occur. Adhering to the CFF limits given in the data sheet above will prevent instability. Large values of CFF will cause other problems as described in the previous application report. Table 1 lists rules of thumb for how CNR and CFF affect noise.

Table 1: CNR and CFF vs frequency

[color=rg b(85, 85, 85)]Conclusion

As this article has shown, adding a feedforward capacitor reduces noise and improves stability, load response, and PSRR. Of course, you must choose the capacitor carefully to maintain stability. If a noise-reducing capacitor is used, the AC performance will be greatly improved. These are a few things you need to keep in mind to optimize your power supply.

85)]After improving the load transient response of the LDO, you will see a significant increase in phase margin. With the increased phase margin, the LDO output will have less ringing and settle more quickly.

Improving PSRR

Depending on the placement of the zero and pole, you can also subtly reduce gain drift. Figure 3 shows the effect of the zero on the gain rolloff starting at 100kHz. By increasing the gain within the band, you will also improve the loop response in that band. This will improve the PSRR for that particular frequency range. See Figure 4.

85)][/size ]

Figure 4: TPS7A8300 PSRR vs. frequency and CFF value

[color=rgb(85, 85, As shown in the figure, increasing the CFF capacitor value pushes the zero to the left. This results in better loop response and corresponding PSRR at lower frequencies. Of course, you must choose the CFF value and add the zero ZFF and pole PFF appropriately so that instability does not occur. Adhering to the CFF limits given in the datasheet above will prevent instability. Large CFF values can cause other problems as described in the previous application report. Table 1 lists rules of thumb for how CNR and CFF affect noise.

Table 1: CNR and CFF vs frequency

[color=rg b(85, 85, 85)]Conclusion

As this article has shown, adding a feedforward capacitor reduces noise and improves stability, load response, and PSRR. Of course, you must choose the capacitor carefully to maintain stability. If a noise-reducing capacitor is used, the AC performance will be greatly improved. These are a few things you need to keep in mind to optimize your power supply.

85)]After improving the load transient response of the LDO, you will see a significant increase in phase margin. With the increased phase margin, the LDO output will have less ringing and settle more quickly.

Improving PSRR

Depending on the placement of the zero and pole, you can also subtly reduce gain drift. Figure 3 shows the effect of the zero on the gain rolloff starting at 100kHz. By increasing the gain within the band, you will also improve the loop response in that band. This will improve the PSRR for that particular frequency range. See Figure 4.

85)][/size ]

Figure 4: TPS7A8300 PSRR vs. frequency and CFF value

[color=rgb(85, 85, As shown in the figure, increasing the CFF capacitor value pushes the zero to the left. This results in better loop response and corresponding PSRR at lower frequencies. Of course, you must choose the CFF value and add the zero ZFF and pole PFF appropriately so that instability does not occur. Adhering to the CFF limits given in the datasheet above will prevent instability. Large CFF values can cause other problems as described in the previous application report. Table 1 lists rules of thumb for how CNR and CFF affect noise.

Table 1: CNR and CFF vs frequency

[color=rg b(85, 85, 85)]Conclusion

As this article has shown, adding a feedforward capacitor reduces noise and improves stability, load response, and PSRR. Of course, you must choose the capacitor carefully to maintain stability. If a noise-reducing capacitor is used, the AC performance will be greatly improved. These are a few things you need to keep in mind to optimize your power supply.

Table 1 lists the rules of thumb for how CNR and CFF affect noise. Frequency

Conclusion

As this article has shown, adding a feedforward capacitor reduces noise and improves stability, load response, and PSRR. Of course, you must choose the capacitor carefully to maintain stability. If a noise-reducing capacitor is used, the ac performance will be greatly improved. These are a few things you need to keep in mind to optimize your power supply.

Table 1 lists the rules of thumb for how CNR and CFF affect noise. Frequency

Conclusion

As this article has shown, adding a feedforward capacitor reduces noise and improves stability, load response, and PSRR. Of course, you must choose the capacitor carefully to maintain stability. If a noise-reducing capacitor is used, the ac performance will be greatly improved. These are a few things you need to keep in mind to optimize your power supply.

英尚微电子

月下良缘

Thanks for sharing!

alan000345

Yinshang Microelectronics published on 2019-5-28 14:53

Thank you for your support

alan000345

月下良缘发表于2019-5-29 08:40 Thank you for sharing!

Thank you as well.