LTC3351 not boosting with high load

Hi Glenn8950 ,

Kindly explain what kind of power supply you are using.

Cheers, heke

24v output 100w supply.  

Hi Glenn8950 ,

Thanks. The first thing I'd do is checking the supercap voltage during discharge with an oscilloscope. 

Can you provide the part number for the inductor used?

While measuring the performance during back-up, are you using a real failure or simulated failure?

Does your system allow monitoring the ADC readings?

Cheers, heke

SC voltage is set for 10.0v and is measuring 10.0 v .  boost volt is set for 24v while Vin gd (power supply connected ) the Vout follows PS voltage.

if i create a real failure(remove PS ) with no load, boost stays at 24v. if i create a real failure (remove PS) with a load < 330ma boost stays at 24v. in both conditions above VC pin stays around 1.4v - 2v. once a real failure occurs with a load above 330ma boost drops to approx 2x SC voltage and VC pin goes to 5v -

for example: SC is measured at 10.0v and with real failure & a load of 880ma boost drops to 20.0v immediately and then follows 2x the SC voltage until SC are depleted. it seems whatever controls the boost either need more current from a certain cap on another pin that the DS isnt super clear about or ?? of course thats my best guess. the Inductor part number is IHLP6767DZER6R8M11

i use the Eval interface part number DC590B this allows me to communicate with my prototype pcb while measuring with scope. i loaded the settings i used with the Linear tech Eval pcb to my prototype so all the settings im assuming are the same as the eval pcb and the eval pcb worked flawlessly. 

SC voltage is set for 10.0v and is measuring 10.0 v .  boost volt is set for 24v while Vin gd (power supply connected ) the Vout follows PS voltage.

if i create a real failure(remove PS ) with no load, boost stays at 24v. if i create a real failure (remove PS) with a load < 330ma boost stays at 24v. in both conditions above VC pin stays around 1.4v - 2v. once a real failure occurs with a load above 330ma boost drops to approx 2x SC voltage and VC pin goes to 5v -

for example: SC is measured at 10.0v and with real failure & a load of 880ma boost drops to 20.0v immediately and then follows 2x the SC voltage until SC are depleted. it seems whatever controls the boost either need more current from a certain cap on another pin that the DS isnt super clear about or ?? of course thats my best guess. the Inductor part number is IHLP6767DZER6R8M11

i use the Eval interface part number DC590B this allows me to communicate with my prototype pcb while measuring with scope. i loaded the settings i used with the Linear tech Eval pcb to my prototype so all the settings im assuming are the same as the eval pcb and the eval pcb worked flawlessly. 

ok i fixed my issue the Rsnsc resistor for some reason was affecting boost backup. when i made the prototype the value was .033 ohms (which i thought would just limit charge current to <=1 A  ) and when I changed Rsnsc to .008 ohms and it now boosts properly and supports over 1.75A just fine. 

Hi Glenn8950 ,

That's good to hear! With such low ohmic value the layout may become an issue. One should ensure Kelvin connection (either using 4-terminal resistor or having careful PCB routing). Google "Kelvin connection" for further info.

Cheers, heke