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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> LDO current TF anaysis help https://designers-guide.org/forum/YaBB.pl?num=1148377600 Message started by taofeng on May 23rd, 2006, 2:46am |
Title: LDO current TF anaysis help Post by taofeng on May 23rd, 2006, 2:46am Hello everyone, Hope can get you guys help. Thanks in advance, As you can see from this circuit diagram, this is basically a LDO design for the application of digital power supply. Gm: the transconductance of OTA Caux: Compensation cap 20p F Raux: Compensation resistor, around several K ohm gm: the transconductance of Pass transistor CL: Load capacitance, around several pF RL: Load capacitance, from 300-30k ohm Cdecouple: tank or decoupling cap, 100p F which is internal. No external big capacitor are allowed to use I do not want to talk about the stability issue since it is straitforward and assume you guys all know it very well. However, I want to do some interesting things: I introduce some AC current noise @ the output, and obeserve the current change @ the VDD supply. Then I define a current transfer function as H(S)=Ivdd(s)/Inoise(s). This function reveals that how the VDD can suppress the di/dt induced by the input noise. The poles and zeros are shown in the first graph. The simulation of the H(S) is shown on figure 2, basically this is a low pass filter which is apprarent. However, from simulation, I find there is always peak. This is due to the fact that the zero is tracking the dominant pole, the consequency is to make the second dominant pole the -3dB (or cut-off) frequency. Or in other word, the cut-off frequency is shifted to higher frequency, which sacrifices dynamic noise performance. !! My goal is to make the dominant pole the cut-off frequency. !! I also find that this phenomena is intrinsic to this feedback topology, by which I mean this zero and dominant pole are exactly the same after approximation(located in the GBW of the OTA). I do not know how to solve this problem. Probably there are some options to do it: 1. kick out the zero ? this leads to the stability problem 2. make the dominant zero and pole well seperated. ... But I can not figure it out yet. Hope I make myself clear, and hope some one can help me out !!! thank you in advance! taofeng |
Title: Re: LDO current TF anaysis help Post by taofeng on May 23rd, 2006, 2:47am the simulation file. |
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