Other Parts Discussed in Thread: STRIKE
Tool/software: WEBENCH® Design Tools
Here is the WEBENCH Power Designer design I am pondering:
Vout = 3.3V
Iout(max) = 250mA
Vin = 6v to 75v
This power supply will be used to power a small vehicle interface that will be mounted under the dashboard in a 12V or 24V vehicle. It is conceivable that the interior temperature on a hot summer day could approach 80°C, but the vehicle interior will not be that hot all year around. Input voltage spikes will be capped to 70V with a TVS diode, and a B2100-13-F Schottky diode will provide reverse polarity protection. (I will also use the same diode p/n for power supply D1.) Typical output current could be as low as 2mA or as high as 150mA. There will be one inductive load -- a 3V signal relay -- that will share the 3.3V voltage rail with a PIC MCU and fingerprint sensor. I may need to add 47uF to 100uF of added capacitance via aluminum electrolytic to ensure the 3.3V output voltage remains stable when the relay is energized.
Within WEBENCH Designer, when I set Max Ambient Temperature anywhere between 30°C and 75°C, the suggested schematic components and switching frequency do not change. But when I go above 75°C, Ron & Rr & L1 increase in value, and it suggests 2pcs of Cout. If I then set the Max Ambient to 85°C, those components increase again and the Frequency drops to only 50kHz.
QUESTION#1: Is it best to go with the suggested design shown at 75°C with its 128.781kHz switching frequency, or is it really best to go with the suggested design at 80°C which drops the frequency to 100kHz or 85°C with a 50kHz frequency? What are the disadvantages to running at a lower frequency that I should consider?
I am considering use of a single Cout with improved specifications over the recommended Cout:
LMK316AB7226ML-TR: 22uF, 10V, X7R, Derated Capacitance (at 3.3V): 15.4uF
QUESTION#2: Is it not better to use a single temperature-stable X7R capacitor over the suggested 2pcs of GRM188R60J226MEA0D (derated capacitance = 7.9uF -- 15.8uF combined)? Or is the 1m-ohm lower ESR of those two X5R capacitors in parallel more desirable than temperature stability? I'm trying to strike the right balance between cost and performance here while minimizing output ripple.