LTC1515 adjustable output voltage

Hi Dave, What is the load in this application? If VIN - IOUT*ROUT > VOUT setpoint then the LTC1515 then the part will operate in step-up mode. When this occurs the power dissipation in the part is (2xVIN-VOUT)*IOUT. This package has a ΘJA of 110°C/W. This can heat up the part quickly with even 100mA of IOUT, 8.4VIN, and 7.4V IOUT. 

I do not know what the open loop resistance is for the part in step down mode. It was difficult to extrapolated this from the efficiency graphs, maybe about 15Ω. That probably changes with VIN also. 

Regards,

Marty

Hi Marty, Our load is an LT3650 boost pin - it appears our issue (the POR output from the LTC1515 is feeding its /SHDN input) is on startup of the LT3650 it's pulling down the incomming 8.4V supply rail and re-triggering the LTC1515 POR circuit until it dies, at certain VIN values. Your point about power dissipation in the part - could it be heating up too fast for the LTC1515 thermal shutdown to kick in? Kind regards,

Hi Dave, Is this set up like the datasheet app in the LT3650? In this app the VIN to the LTC1515 is limited to 6.2V with zenor diode and emitter follower circuit. Something tells me this was done to prevent the issue you are seeing. You might be correct about the thermal shutdown. Is it possible to reduce the input like done on the LT3650 app?

Thanks,

Marty 

Hi Dave, I think I miss read your email a little. The LT3650 is pulling down the 8.4V causing a POR on the LTC1515. I was thinking the thermal shutdown was causing the LTC1515 POR was causing the shutdown. Is the 8.4V a battery?

Hi Marty, yes its a 2S LiIon feeding it. I thought that clamp was just to allow the LTC1515 to run over the full 32V VIN for the circuit - hadn't noticed it was clamping it at 6V2 - 0V7 = 5V5ish VIN to the LTC1515 - so it will only be running in boost mode. This sounds like the issue. Dave