TPS62136: Short Circuit Protection of TPS621361

Y. Ottey,

TPS621361 will turn off the high side FET and turn on the low side FET when the high side current limit is reached.  The low side will stay on until the switch current falls below the low side current  limit.  TPS621361 will operate in this mode indefinitely. TPS621361 may have reduced output voltage depending on the load current. When the TPS621361 cannot supply the required current due to current limit, the output voltage will fold back.  It is possible for it to reduce down to near 0 V.

Regards,

Amod

Hi Y.Ottey,

Yes, the inductor current will be controlled by the current limit thresholds and propagation delay and will keep running till thermal shutdown kicks in or the short circuit condition does not exist anymore.  I could not locate any information on short circuit operation other than what is in the datasheet.

Regards,

Amod

Y.,

As Amod notes, there does not seem to be any waveforms that demonstrate current limit operation.  As I had some free time, I ran some pspice simulations that show the current limit operation.  There are two different current limits, one for the high side switch and one for the low side switch.  They are not explicitly specified in the datasheet.  In the datasheet they are both specified as "DC value" and are the same for both high side limit and low side limit.  While technically correct, it is not in my opinion a good way to specify.  I would much rather the discreet peak upper high side current limit and low side valley current limit where listed.   For current limit operation, during the on time, the current is rising and the on time ends when the switch current reaches the high side limit.  Now the off time begins and the switch current is decreasing.  The off time ends when the falling current reaches the low side valley current limit and the next cycle begins.  The load current will be bonded midway between the upper peak current limit and lower vally current limit, So by definition, when stated as load current, the current limit will be the same for the high side and low side current limit.  Now on to the simulations:

I ran two simulations and show both the compete run and a zoom in for each.  Each of the four sims show inductor (switch) current on top, SW node voltage in the middle and Vout on the bottom. The simulations are for 12 V in and 3.3 V out.  Both star with a 3.3 ohm load for 1A load current.  After the soft start time is completed I with in an additional load at the 500 usec time.  The added load for the first sim is 0.5 ohm and 0.3 ohm for the second.  The simulations are as named.  For the simulation, the effective DC load current limit is 5.4 A.  Don't ask me why it is not the nominal datasheet value of 5.6 A was not used.  I did not develop the model.  For the first sim, the effective load resistance is 3.3||0.5 = 0.43 and so during over current the output voltage will fold back to approximately 2.34 V.  You can see this on the Vout traces.  The peak switch current at the end of the off time is about 5.44 A and the bottom valley current is about 5.18 A.

In the second sim, the effective total load resistance is 0.275 ohm, the average load current is still 5.4 A and the output voltage folds back to 1.49 V.  the peak switch current is 5.47 (possibly some cursor position error on my part, but close enough to make the point, practically the same as 5.44 A) and the bottom vally current is still 5.28 A.  So the only things that change are the output voltage and the resulting changes in on and off times as ton = L * dI / (Vin - Vout) and toff = L * di / Vout). 

TPS621361 CL.zip

Hope this helps clear up your confusion.