current model buck compensation of LM3477

stevenhendry

各位大侠，在考虑loop  stability  ,用电阻和电容串联产生零点的方法，根据LM3477的补偿的计算方法，补偿电容好像都是nF的级别，如何能把补偿电容做到pF，这样能做到芯片内实现,怎么才能做小呢

happybull

R*C=constant
increase R ,decrease C,这样能做到芯片内实现

stevenhendry

是的，由于R=（Fc*Rgm)/(ADC*GM*Rgm*H*Fp1-Fc)
ADC----power stage gain  
Gm---error amplifer's gm
Rgm---error amplifier's output resistor
H---voltage divider
Fp1---the 1/(2*3.14*Rload*Cout)
Fc---crossover frequncy
对于固定的应用--Fc Fp1,H,ADC都基本上固定了，因此要增大R，Rgm基本上任何影响，所以只有减小GM，我仿真过如果C变成pF级，那么GM就要小于60e-6，不知道推断是不是正确

happybull

This equation looks too fancy. I just use this simplified one.
Fc is about GM/Cc, Cc is the Comp cap.
If GM=60e-6, Cc=60pF, Fc=1MHz, So I will set GM 10e-6 A/V, to get 166Khz Fc for 1-2MHz DC.  
Please choose GM value according to your application. GM too small means weak transient response.

stevenhendry

i still have a doubt:  set  the crossover frequency with Rcomp ,set  the compensator zero with Ccomp.

according  the equation Fc=Gm/Cc,  i think you use the Cc set the crossover frequency , can you give you some suggestions

happybull

There are many ways to stablize the Buck.
For current mode Buck, Cout*Rout is a pole, Cc*Rgm is another pole, then you just need one  Zero to compensate the circuit if you can set slope compensation pole way after the cross over.
To summarize this, the system has 3 pole 1 zero, so you have to put 2 pole and 1zero before cross over, and set 3rd pole 5 times far from cross over point to get 90degree phase margin.
Also other parasitic poles will come into play after cross over point , so you can get 60 to 70 degree pahse margin.