LT8210 Startup Problem

Hi,

I am trying to use the LT8210 for a 24Vout 2.5A buck boost circuit, 18-42Vin while tolerant of spikes up to 80-100Vin.   I cannot get the device to start up.  This is on an eval kit that has been modified.

At 42Vin for instance...

1)  EN/UVLO is around 2.1V and the device is cool enough to touch (so the chip should be awake).

2)  Vdd is 3.3V but GateVcc is only 3.5V (not high enough to come up to 3.9V UVLO that it needs to start the charge pump to run the gates of the FETS).  I even tried adding capacitance to GateVcc which helped a little bit but not enough to come online completely. 

EXTVcc is connected to Vout which is only 2.7V because the charge pump and FETs aren't pushing charge through to the output. 

Vinp (the second way to turn on the 10V LDO and charge pump) is connected to Vin (42V), but it doesn't bring the 10V LDO online because it apparently uses the lesser of Vinp and ExtVcc rails, which is still stuck at 2.7V because there is no switching going on.  This therefor creates a situation where it is impossible to have the chicken or the egg, because you need one to have the other.

I read that below 8Vin it will not use the lesser of the two voltages to bring the 10V LDO online, but then my EN/UVLO voltage would not be high enough to wake the chip.  I am trying to have it enable at just under 18Vin.  It seems that my only hope of getting this to come on is to run the enable threshold to correspond to 8Vin.  Have you heard of anyone else running into this problem?  While the power optimized power source selection is a great feature, it seems to be making it unusable for me. 

Any ideas on how to get this part to come alive?

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