Two question:
In the figure below of the LTC3868 on page 28, what is the use of the 1uf Ceramic cap and the schottky Diode D1 (highlighted in Green). Is the Cap used for filtering the input voltage for the MOSFET and D1 providing a path for draining the current ?
And am i right in assuming that the CB1 and the schotkky dode (Highlighted in yellow) in series be placed on the same side as the inductor and mosfets as close as possible?
Hello anishkgt,
Good points. It took me awhile to figure this out.
The โschematicโ referenced on page 28 of the datasheet for the LTC3886 is the recommended circuit layout. Itโs akin to fritzing where the physical location of the parts are shown.
For example, the 1 uF ceramic capacitor is located as close as possible to the drain of M1 and the source of M2. This provides a low impedance path. Observe that the 1uF ceramic capacitor is in parallel with the Cin input capacitor. This is a common practice to bypass a larger capacitor, presumably electrolytic, with a smaller capacitor. Also note Figure 11 that describes the branch current waveforms. As an aside, if you are not already familiar with the concept, please research star ground.
Diode D1 is classified as option. Recall that a typical MOSFET has a body diode that would normally โcatchโ the inductor when the primary switch M2 is turned off. Here is the description from the datasheet:
The optional Schottky diodes D1 and D2 shown in Figure 10 conduct during the dead-time between the conduction of the two power MOSFETs. This prevents the body diode of the bottom MOSFET from turning on, storing charge during the dead-time and requiring a reverse recovery period that could cost as much as 3% in efficiency at high VIN. A 1A to 3A Schottky is generally a good compromise for both regions of operation due to the relatively small average current. Larger diodes result in additional transition losses due to their larger junction capacitance.
As for the yellow highlighted components, install them between the LTC3868 controller and the MOSFETS.
And finally, your are correct, keep all paths as short as possible.
Please share your results with the community. A description of your lessons learned would be especially appreciated.
Best Wishes,
APDahlen
