TPS40180: How do I disable UVP and allow Constant Current Mode in OCP?

Peter,

   thanks for your reply.

Just to make sure I understand it correctly:

If I wait for the voltage to reach it's output (on startup), and then short the SS-pin to ground, the controller will not go into hick-up mode if the current limit is reached, it will supply the max current set by the Ilim?

Also does that work for a multiphase configuration, when the SS-pins of the slave(s) is (are) tied to Vcc?

   Thanks

     Best regards

                     Kurt 

Kurt,

If I wait for the voltage to reach it's output (on startup), and then short the SS-pin to ground, the controller will not go into hick-up mode if the current limit is reached, it will supply the max current set by the Ilim?

if you wait for the ouptut voltage to reach regulation and then short SS-pin to ground, the converter's output voltage will be forced to 0V.  The SS-pin is a non-inverting input to the error amplifier.  The construction of the error amplifier causes the non-inverting input to be the lower of the soft-start pin voltage or the intenral 700mV reference.

I was reviewing the data for the TPS40180 and the fault enable threshold is not 1.0V, it's 0.8V,  This will require even tighter regulation of the clamp voltage, clamping the SS voltage between 750mV and 800mV.  This level of accuracy and tight reference may not be practical without an exernal op-amp clamp circuit.

This tolerance is very tight because this was not a designed in feature of the TPS40180 controller, but is instead a result of the need to disable under voltage protection during soft-start.

Also does that work for a multiphase configuration, when the SS-pins of the slave(s) is (are) tied to Vcc?

Only the master controllers under voltage protection is acive.  The slave controllers UVPs are not active or they would enter UVP on start-up since the ouptut voltage is less than the regulation voltage.

Peter,

  looks like it's a bit trickier than I thought.

So I have to get an OP-amp to supply 750mV-800mV into the SS pin after startup?

If I use multiphase configuration, doing this to the master (supplying the 750mV to SS pin), every slave controller would supply the same current? Currentsharing is still active?

Do you have a sugestion for a circuit?

Or does TI have a multiphase controller where the hickup mode can be turned off?

Thanks for your help

   Best regards

             Kurt

The TPS40140 controller is a 2-channel version of the TPS40180.  The fault enable threshold of the TPS40140 controller was specifically increased to 1.4V to allow designers this specific functionality.  The soft-start current source is 10-15uA and the fault enable threshold is 1.4V.  Adding an 86.6k resistor from SS to GND will clamp the voltage above 800mV with a 10uA source current and below 1.3V with a 15uA source current.  This will typically be the best solution when this functionality is required.

The circuit to clamp the SS voltage of the TPS40180 to 750mV is a miller integrator with a 750mV reference voltage feed into the non-inverting input (a resistor divider from Vshare to GND works well to generate this reference voltage)  Connect the inverting input to the SS voltage through a resistor, then add a capacitor from the inverting input to the ouptut.  Final a diode from SS to the output of the error amplifier prevents the integrator from sourcing current into the SS pin, but allow it to sink current to clamp the soft-start voltage.

The integrator can be replaced with a inverting amplifier, changing the capacitor to a resistor, but some regulation of the soft-start voltage will be lost  with the fixed gain.

Peter

  Thanks for youe answer. I was not sure if I got the circuit description right (does not make much sense??!!), I tried it anyway, but as I though it does not work! As soon as the SS pin goes low (after short on output), the output pin of the opamp goes high and the diode blocks the pos voltage. The hickup state is entered as before, no chnage.

Here is the circuit I used.

Then I tried the TPS40140, as you mentioned it might be more suitable. According to the data sheet the current on the SS (Track) pin is 6uA and 12uA?!!!

I attached the resistor to GND, as you suggested, but as soon as a short is on the output, the whole unit switches off and comes only on after you turn the incomming power off and on again.

Please can you advise.

Thanks, best regards

      Kurt

Kurt,

The circuit looks correct.  Verify that the OP-AMP selected can pull the ouptut low enough to hold the SS pin at 0.75V with the soft-start current forced across the diode.  We need a pretty low minimum output voltage on the Op-AMP.  If you need some extra head-room, you could look into a Schottkey diode instead of a signal diode.

Also check the bandwidth of the integrator.  I can't read the R and C values, but it needs to be fast enough to clamp the output voltage before it rises and triggers the turn-on of the fault comparators.  Put a scope probe on the TRK pin during start-up and see if it's rising up above 0.8V or not.  If it is, the comparators are being activated and don't turn-off until power is cycled.