Hello,
I was hoping I could get a little clarification on the TPS2421-1 at start up during a fault condition. I am interested in the since I am trying to do a damage analysis due to an incorrectly connected circuit that is essentially two 1.5 ohm resistors in series to ground. I am trying to calculate the voltage reached in the middle of the two 1.5 ohm resistors. The TPS2421-1 was supplying 12 volts to the two resistors. The reason for my confusion has to do with figure 7 and figure 19 on the datasheet for the TPS2421-1. Figure 7 shows a faulted start up into a 4 ohm load, and Vout trace looks like it goes to 2 volts and stays there until CT hits it's threshold voltage, the current is not shown. Figure 19 is a plot of a short circuit at start up, the current shown is only 0.1A for the duration of CT charging, Vout is zero as would be expected.
My question on Figure 19 is, why is the current only 0.1A and not higher value that the Rset would give (ie 1.1A to 5A)? Also, on figure 7, why is Vout only 2Volts? With the 5 Watt maximum limit of ((Vin - Vout) x Ivout = 5Watts), wouldn't the voltage be giving the maximum allowable current (12V- 2V)xIvout = 5W so Ivout would be 0.5Amps, with the minimum Iset condition of 1.1Amps, I would expect the voltage to be at least 4.4V with a 4 ohm load? My feeling is that I must be missing something on the fault start ups and the power limiting function. Any insight on why figure 7 and 19 have the currents and voltages at lower that expected values.
In my application I am using an Rset of 100kohms, with 12V In.
Thanks for you help,
Tom Pongo