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Q: TPS61256 - EVM start up into resistive load? by 9092
I have a customer that just called me and said our TPS61256 EVM will not start up into either a 12ohm or a 24ohm resistor but will start up without any resistive load. It sounds like he is stuck in the pre-charge phase as he sees 2.4V with a 12 ohm resistor (0.2A*12ohm = 2.4V) but he see 3.3V (Vin) with a 24 ohm resistor. is this something that we expect to happen? Is there anyway to increase the pre-charge current to aid in start-up? Or is this another one of those deals where we have to tell the customer that he'll never really see a true resistive load in system and to just ignore it? If so how do I explain why his ADI and LTC EVMs start up fine into the same test up? They are rated at approximately the same load current.
Thanks!
A: Re: TPS61256 - EVM start up into resistive load? by 1169162
Allen,
yes I think what your customer sees is that he get;s stuck in pre-charge phase.
Pleae note that the min. resistive load the device can start-up is specified in the datasheet with 55Ohm.
There is no simple staright forward way to change the start-up bahavior.
As Chris pointed already out the start-up has been inplemeted in that way, to avoid large inrush current during start-up.
It i hard to say if the test condition of starting up into resistive load will be represenatice for the real application without knowing the application in detail.
In case the customer does not ned the bypass functionality, I would recommend having a lok at the TPS61252.
The TPS61252 does privide adjustable current limit. The current limt can be adjusted with an external resistor friol pin ILIM to GND.
The TPS61252 has an internal softstart circuit that controls the ramp-up of the current during start-up and prevents the converter from inrush current that exceeds the set current limit. For typical 100 μs the current is ramped to the set current limit. After reaching the current limit threshold it stays there until V
The TPS61252 has an internal softstart circuit that controls the ramp-up of the current during start-up and
prevents the converter from inrush current that exceeds the set current limit. For typical 100 μs the current is
ramped to the set current limit. After reaching the current limit threshold it stays there until V
IN = VOUT
then theconverter starts switching and boosting up the voltage to its nominal output voltage. During the complete start-up the input current does not exceed the current limit that is set by resistor R
the input current does not exceed the current limit that is set by resistor R
ILIM
Best regards,
Christian
.
Q: Re: TPS61256 - EVM start up into resistive load? by 1090425
Yes, the datasheet shows a 200 mA current limit when Vout < Vin. This gives 2 advantages:
The input bus isn't pulled down by an output short. The IC will keep pumping that 200 mA into the short but this will be less likely to drag down the input bus than using the full switch current limit and pumping that into the short. Thus, the rest of system powered from the input bus can still operate with the output shorted. This IC is thus safer to use.
The input bus isn't pulled down due to high inrush currents at start-up. This is especially true with high value output caps and fixed voltage devices that do not provide access to the FB pin (where some external SS circuits might be added).
This is rarely an issue in real world systems that don't pull full current until their supply voltage is up and within regulation.