Add basic comparison between STUSB and TPS

README.md

@@ -39,7 +39,44 @@ standard uses it.
 
  - STUSB4500
  - STUSB4500L
- - Some Texas Instruments chips
+ - TPS25730
+ - TPS25751
+
+#### Comparison
+
+All of the PD chips can basically run in auto-run mode--also called
+dead battey mode. Since Ethy does not have any battery, the PD devices
+have to be able to auto-configure or atleast allow for the standard 5V
+output of the USB. The STUSB4500 are specc'ed for USB type-C rev 1.2
+(for STUSB4500) and type-c rev 1.4 (for STUSB4500L). Both seem to function
+with the PD rev 2.0 standard. However, according to the STUSB4500 documentation
+it is interoperable with USB PD rev 3.0
+
+Both the TPS devices are built for USB PD rev 3.1.
+
+The biggest difference, to me, seems to be the control of an external
+power path. The STUSB devices can only function with an external power path
+while the TPS devies also have an internal power path. The biggest difference
+however, is the type of switch that can be used for each chip.
+
+The STUSB4500 and STUSB4500L have active drain outputs, meaning that the
+switches should be based on P-channel MOSFETS, as uses in their reference
+design. The TPS devies have an internal charge pump which means they can
+use N-channel MOSFETS. N-channel MOSFETS generally have a smaller footprint
+and smaller R_ds(on) than the P-channel counterparts. This can be seen in
+the reference designs of PD IC's. The TPS chips use the CSD87501L dual common
+drain n-channel from Texas Instruments. This dual channel MOSFET has a 
+source-to-source R_ds(on) of 6.6 milliOhm at a Gate Source Voltage of 10 V.
+For the maximum load of 12V @ 5A, the power loss would be around 165mW.
+
+Comparing to the P-channel MOSFETS in the STUSB chips, the reference design
+uses the STL6P3LLH6 MOSFET from STM. This MOSFET has a maximum R_ds(on) of
+around 30milliOhm. at 5A this would result in 0.75 W of loss. Considering
+that both designs need two MOSFETS, the loss in the MOSFETS for the STUSB chips
+would be around 1.5W.
+
+This difference in power loss can also be seen in the package size, since
+the CSD87501L is half the size of a single STL6P3LLH6 MOSFET.
 
 #### Possible DC/DC chips for controllers