Issue Related to switching of FET in Programmable Current Source

Hi,

I am John working as embedded hardware engineer in Qmax Systems USA.

I am using programmable Current Source (Part : LT3741EUF-1#PBF ) for getting a current output of 20A for battery charging application. I used the typical application circuit of "20A Super Capacitor Charger with 5V Regulated Output" in the datasheet (i have a modification in feedback resistor, so as to get a output voltage of 2.72 V ). The bottom FET is not switching . Plz help me to troubleshoot the issue.

Regards ,

John LT3741EUF-1#PBF.pdf

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  • I took a close look at your schematic.  It is best to bypass the power VIN rail with 100uF of CERAMIC capacitance.  Your schematic has a 100uF electrolytic on the power VIN rail.  Even with high-performance electrolytic capacitors like hybrid polymers, a few ceramic capacitors in addition are a good idea to bypass the power VIN rail.  Make sure the electrolytic capacitors on your output rail are high-performance (low ESR) as well.  Also, the inductor on your schematic is undersized for 20A out.  The LT3741-1 will not switch at all if the LT3741-1 thinks FB or SENSE+, SENSE- are in regulation.  The bottom FET probably stays off at start up until SS ramps up.  At higher output currents where DCM is not possible, the bottom FET should switch.

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  • I took a close look at your schematic.  It is best to bypass the power VIN rail with 100uF of CERAMIC capacitance.  Your schematic has a 100uF electrolytic on the power VIN rail.  Even with high-performance electrolytic capacitors like hybrid polymers, a few ceramic capacitors in addition are a good idea to bypass the power VIN rail.  Make sure the electrolytic capacitors on your output rail are high-performance (low ESR) as well.  Also, the inductor on your schematic is undersized for 20A out.  The LT3741-1 will not switch at all if the LT3741-1 thinks FB or SENSE+, SENSE- are in regulation.  The bottom FET probably stays off at start up until SS ramps up.  At higher output currents where DCM is not possible, the bottom FET should switch.

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