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TOP256 current limit adjust

Posted by: rpaitich@gmail.com on

I'm using a current limit resistor as charted in fig 55b of TOP252-262 data sheet.
It appears that when the current limit is reached the TOPsw shuts down, then tries again. An alternate mode might have been that the current is sustained at the limit, but this does not seem to be the case.
Please tell me if i got it right.

Comments

Submitted by PI - Traveler on 04/24/2012

RPaitich -


What you are describing sounds like the auto-restart feature on TopSwitch-HX. During start-up of the controller, a feature called soft-start is activated. During soft-start, the MOSFET duty cycle and current limit is gradually increased over the course of ~17ms. At the end of soft-start, if the control pin is not receiving sufficient current (indicating that the output is not in regulation, that there is a fault condition, an open feedback loop, etc), the auto-restart is activated.


During auto-restart, the MOSFET is turned off and the control pin capacitor is slowly discharged. Once the control pin capacitor has discharged, the controller activates the soft-start feature and tries to start up again. If not feedback signal (ie control pin current) is sensed, then the controller will go back into auto-restart. Unless a latching feature is enabled, auto-restart will continue indefinitely.


With the external current limit that you're setting, it's possible that the output is not within regulation by the time soft-start finishes. What you can try doing is implementing a soft-finish circuit as described in the TopSwitch-HX datasheet and Application Note 43. The additional capacitance of the soft-finish circuit will draw additional current through the optocoupler during power supply start-up. This will trick the controller into thinking the output has reached regulation and keep the controller from entering auto-restart. This can be used in situations like yours.


Please let me know if this was helpful. Good luck!


-The Traveler