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TPS54122 Issue

Other Parts Discussed in Thread: TINA-TI

Hi,

I have calculated the R and C values for compensation loop of TPS54122 and used these values in simulation and got totally unexpected results.

For detailed description see below. I need support for clarification of this issue.

Thanks

Sahmusa

TPS54122 Compensation Loop

I am trying to determine the value of compensation loop for TPS54122 but I am totally confused. The results of calculation and simulation do not match. I am summarizing the procedure briefly.

Application Requirements:

Uinmin = 3.2V

Uinmax=3.4V

Uout = 1.8V

Iout=140mA

f=1.8 MHz

Cout = 38 uF (Rers=2 mOhm)

L=10uH

Only SMPS is used, LDO is not used. TPS54122 SMPS is actually for 140 mA output current oversized but we have this device in our application and want to keep it.

First I have calculated the component values R and C according to document "

SLVA602–November 2013 Design Procedure for TPS54122" and got the following values for resistor and capacitors.

With fc=52 kHZ (crossover frequency of compensation loop), gmea=245uA/V, gmps=18A/V, Rout=1.8V/0.14A, Cout=38 uF, Resr=2mOhm (38uF due to tolerance,..):

R4=6.1kOhm, C8=79nF

C6=12pf (parallel to R4 und C8)

In datasheet of TPS54122 there is a model of control loop (see figure below). There are two parameters Ro and Co missing and I got the values of these parameters from TI (Ro=2MOhm, Co=5pF)

I put it in PSpice and simulated the transient analysis with calculated R4, C8, C6 values. The result was not satisfied (too much over and undershoots and long regulation time).

I got from TI a reference design for transient analysis with TI simulation tool TINA and simulated it with calculated values (R4, C8, C6).

The result was much more surprising. It did not work with these parameters (output voltage was less than 200 mV).

Now I need TI support to clarify these issue