Hi everyone,
We currently use the LTC4357 in our design in conjunction with a power MOSFET, the TPWR6003PL. The design for the ideal circuit composed of these elements is basically the same as on the EVB. In our case, Vin (and Vdd) is 22;25 V
From my understanding of how LTC4357 should drive the gate:
- The device actively drives Vgs low enough to ensure between 10 to 55mV on Vsd. This voltage drop is used to assess if reverse polarity happens. Happens mostly when the load is low.
- When the load is big enough, Rds_on * Id will be the main factor of Vsd (so it should be above 55mV). In that case, the gate should be driven as high as Vin + 15V (range is 10 to 15 in the datasheet)
After repeated tests, I get the following:
- The gate is indeed driven to maintain some Vsd. Between 2 to 20A of load, I see Vgs being 2.6 to 3V. At 20A (Vgs=3.18V), I measure a Vsd of 50mV. So above this, I'm expecting the gate to be driven fully to have minimal resistance (fully closed)
- At 30A of load, Vsd is measured at 77mV. Vgs is only at 4.3V. Remember, Vdd is above 20V. I'm expecting the Vgs to be at least 10V.
I don't understand what I'm missing here. Vsd is above the range for which Vsd would be driven by the LTC4357 control, but Vgs is not at a level that it should be at to minimize Rds_on. Note that this MOSFET we use can have a Rds_on as low as 0.3mOhm when fully closed. For example, in the 30A scenario, if Vgs is at 10V, we would get a Vsd close to 9mV. Of course, this would be too low for the LTC to catch, it would then drive Vgs at a level which will bring Vsd to the usual range of 25mV approx. Here we go completely out of specs.
Let me know what you think or if I misunderstood something.
Thanks a lot for your help.
Best regards,
