# Setting Rds(Ron) for PMOS for LDO

I am modeling LDO using ZXCL280 Datasheet. By understanding datasheet dropout voltage of the PMOS is seen as 1.46Ω(From page 5 Ron=Vdrop/Id = 220mA/150mA ). I assumed this Ron is including Rs and Rd (Source and Drain resistance). So while calculating Kp value using this formula

Ron=1/Kp(Vsg-Vt)


Considering maximum current as 150mA dropout voltage is 220mV from the dropout vs drain current plot

Vsg-Vt= Vsd = Vin-Vout = 220mV


I am assuming Vout to be 1.8V so Vin will be 2.02V

By substituting values to find Kp

Kp=1/1.46*(220mV) =3.24A/V^2


I began to simulate dropout test case schematic and waveform is as below

I this can be corrected by adding Rs and Rd in the model card to add up to Ron(Rds) But i should have got 1.46Ω without adding. Is there anything that I am missing or wrong?

Second Method:

I rearranged Id equation to find Kp (this is the only value that i get to set Ron in PSpice model card)

Kp=W*Id*2/L*(Vsg-Vt)^2


W and L is 1 default so,

Kp= (150mA)*2/(220mA)^2 = 6.198 A/V^2


But even this method didn't attain any joy

Is this the right way of setting Ron(Rds), if Vsg(Vgs) is not mentioned or given ? If Vsg is provided how should I proceed and If not provided how should proceed ? How shall I look at output and Input characteristics relation with Ron?

• Why not just RDS(ON) as a model parameter? Mar 23, 2020 at 16:09
• Since I am embedding individual characteristics so first was dropout, I belive kp is dependent on dropout. Is there any other parameters dependent?
– Pai
Mar 23, 2020 at 16:36
• Drop-out is dependent on RS and RD both of which are zero as default: seas.upenn.edu/~jan/spice/spice.MOSparamlist.html. You are operating in the triode version of the MOSFET remember. Mar 23, 2020 at 16:43
• So total resistance is Ron + Rs + Rd in which Ron is set by kp. But how shall I assume Vg?
– Pai
Mar 24, 2020 at 4:18
• How shall I view Ron for PMOS in output charateristicts? Can you share a activity or document which will help me understand?
– Pai
Mar 27, 2020 at 13:27

Assume L=0.6u, Imax=150mA

W/L  =  2*Idmax/un*Cox(Vdsat)^2

W/0.6u= 2*(150mA)/34u*(220m)^2

W/0.6u = 182304.32

W = 109.38mm

rds= Vdrop/Imax = 220m/150m = 1.46Ω (Total resistance to be offered)


Add the above calculated W,L,Kp in model card and simulate, Check the drop out and add the remaining resistance by subtracting it with total resistance.