LDO ABC: Noise – the 2nd part

The ABC of LDO of a Bo Wen in me: Noise–In the 1st part, I was discussed how to reduce output noise and control to press place rate, the method is to be referenced voltage (CNR/SS) is shunt-wound a capacitor. Be in a Bo Wenzhong, my general discusses another kind of method that reduces output noise: The feed before using is allowed (CFF) .

What is before capacitance of make a present of?

Capacitance of make a present of is an optional top container before, with the paralell connection of on half resistor of resistor voltage divider, if pursue,1 is shown.

LDO ABC: Noise - the 2nd part

Graph 1: The NMOS low pressure of capacitance of the make a present of before using differs manostat (LDO)

With fall capacitance of a confusion of voices (CNR/SS) is similar, capacitance of the make a present of before adding has a variety of effects. The mainest is to fall a confusion of voices, still include to improve stability, bear to answer restrain with power source than (PSRR) . (The low pressure of capacitance of the make a present of before application reports “ is used differs the actor defect of manostat, ” is detailed discussed these benefit. ) notable is capacitance of the make a present of before the ability when using adjustable LDO only is used, because resistor network is right now outer ministry.

Fall a confusion of voices

The meeting when LDO undertakes modulatory uses error amplifier, and error amplifier can use resistor network (the gain that R1 and R2) will come to heighten referenced tension, thereby the grid of drive FET, this and inphase amplifier are very similar. Will referenced volts d.c. increase? ? ? Times. Nevertheless, considering the bandwidth of error amplifier, you still can be sent look at referenced voltage the enlarge function of certain communication component.

Through be capacitor of paralell connection of resistor of on resistor voltage divider half part, you introduced a spreader in the light of specific frequency limits. Change character, you make the communication component inside this frequency limits is contributed at unit gain, right now the circumstance of R1 imitate short circuit. (Remember the impedance property that uses capacitor well please, so that define this frequency scope. So that define this frequency scope..

If the graph is shown 2 times, you can see when using different CFF to be worth, the noise of TPS7A91 drops the effect.

LDO ABC: Noise - the 2nd part

Graph 2: TPS7A91 noise Vs. Frequency and CFF are worth

Through be paralell connection of resistor of on resistor voltage divider half part capacitor of a 100nF, can fall noise from 9μVRMS to 4.9μVRMS.

Improve stability and transient state to answer

Add a CFF to still was road of LDO feedback annulus to introduce at 0 o’clock (ZFF) and the utmost (PFF) , their computation sees equality 1 with 2:

ZFF = 1 / (2 XπX R1 X CFF) (1)

PFF = 1 / (2 XπX R1 // R2 X CFF) (2)

Form before achieving the frequency that produces unit gain at 0 o’clock, can improve phasic abundant to spend, if pursue,3 are shown.

LDO ABC: Noise - the 2nd part

Graph 3: The gain of LDO of model of compensatory of the make a present of before be being used only / phasic graph

If do not have ZFF,you can see, the happening of unit gain will make an appointment with 200kHz ahead of schedule about. Through adding at 0 o’clock, unit gain frequency moved right a bit (~300kHz) , but phasic abundant is spent also increased. Be located in unit gain frequency as a result of PFF on the right side of, the impact that so it spends to phasic abundant is the least also.

After the bear transient state that improves LDO is answered, increase what will see phasic abundant is spent apparently. Spend in phasic abundant after increasing, LDO output will reduce ringing and faster stability.

Improve PSRR

Depend on mixed at 0 o’clock the setting of the utmost, you are OK still and clever reduce gain drift. Graph the 3 effects that showed to what begin from 100kHz gain dropped at 0 o’clock. Through increasing frequency paragraph the gain inside, you still will improve this frequency paragraph annulus road is answered. This meeting improves the PSRR of this specific frequency limits. Refer to a graph 4.

LDO ABC: Noise - the 2nd part

Graph 4: TPS7A8300 PSRR Vs. Frequency and CFF are worth

If plan institute is shown, raise CFF capacitance cost, meeting general chose side of towards the left at 0 o’clock. The annulus road with the much better generation inside limits of ecbolic and inferior frequency is answered and corresponding PSRR.

Of course, you must choose to CFF is worth and be added appropriately at 0 o’clock ZFF and the utmost PFF, such ability won’t be caused not stable. Abide by above the CFF that this data table gives out is restricted to be worth, can prevent what do not stabilize a situation to appear. Before CFF can be caused, value of large electric capacity relates the other issue that uses report introduction.

Express the 1 experience law that listed how concerned CNR and CFF affect noise.

LDO ABC: Noise - the 2nd part

Express 1: CNR and CFF Vs frequency

Conclusion

The article discusses no less than in that way, capacitance of the make a present of before adding can fall a confusion of voices, improve stability, bear to answer with PSRR. Of course, you must choose capacitor ability carefully to maintain stability. If use,fall capacitor of a confusion of voices, communication function will be obtained improve considerably. These are you need to so that optimize a few methods of power source,be remembered well.

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