I'm going batty after spending 3 straight days trying to wrap my head around this.
I've never used a switching regulator before. My current project has users who prefer to use 3S Li-Poly batteries (it is remote control hobby related), which means a max Vin of 12.6V. My device runs on 5V and 3.3V (but let's not worry about the 3.3 for now). The max draw is probably around 250 to 300mA but I'm basing it on 500mA to be safe.
I believe that is too much dissipation for a linear. (12.6 - 5) * 0.5 = 3.8W. Without significant heat sinking, I haven't been able to find a linear reg IC that even comes close (2.3W was highest). Even with head sinking, that's ridiculously wasteful.
So that just begs for a switcher, but that has proven MUCH more complicated than I thought.
I finally settled on the Richtek RT7251A (data sheet) for a few reasons, but partly because it appeared to spell out for me what to use for externals! I mean, it straight up tells you in a chart that for 5V regulation you use a 22uH inductor, a 10uF Cin and 2x22pF Cout. Right on! But then, the devil is in the details, isn't it? I couldn't rectify those numbers with the equations they provide further down. I emailed them and finally learn that those figures assume 1.5A full-time current. Well, that's not very realistic, now is it? Plugging in the numbers, I determine I really should be using closer to a 100uH inductor so that chart's out the window.
Now I'm stuck on the capacitors and I'm pulling my hair out.
The datasheet seems to suggest that ceramic shouldn't be used due to ringing that might cause another Manhattan project if it gets out of control and gives vague-ish warnings that polymer caps can be used (try finding one that's 10uF!!) and how "critical" it is to select ceramic properly if you choose to. Well, that's clear as bloody mud!
Between capacitance, ESR, ESL, and temperature coefficients... after all this reading, I feel like a lost child. I'd like someone to hold my hand please!
As a start, I found one page that seemed like it would help me (link), but for the life of me I can't figure out what "D" is supposed to be in their equations!
Right now, D is for Dumb and that's how I'm feeling. This just shouldn't be this difficult.
I've never used a switching regulator before. My current project has users who prefer to use 3S Li-Poly batteries (it is remote control hobby related), which means a max Vin of 12.6V. My device runs on 5V and 3.3V (but let's not worry about the 3.3 for now). The max draw is probably around 250 to 300mA but I'm basing it on 500mA to be safe.
I believe that is too much dissipation for a linear. (12.6 - 5) * 0.5 = 3.8W. Without significant heat sinking, I haven't been able to find a linear reg IC that even comes close (2.3W was highest). Even with head sinking, that's ridiculously wasteful.
So that just begs for a switcher, but that has proven MUCH more complicated than I thought.
I finally settled on the Richtek RT7251A (data sheet) for a few reasons, but partly because it appeared to spell out for me what to use for externals! I mean, it straight up tells you in a chart that for 5V regulation you use a 22uH inductor, a 10uF Cin and 2x22pF Cout. Right on! But then, the devil is in the details, isn't it? I couldn't rectify those numbers with the equations they provide further down. I emailed them and finally learn that those figures assume 1.5A full-time current. Well, that's not very realistic, now is it? Plugging in the numbers, I determine I really should be using closer to a 100uH inductor so that chart's out the window.
Now I'm stuck on the capacitors and I'm pulling my hair out.
The datasheet seems to suggest that ceramic shouldn't be used due to ringing that might cause another Manhattan project if it gets out of control and gives vague-ish warnings that polymer caps can be used (try finding one that's 10uF!!) and how "critical" it is to select ceramic properly if you choose to. Well, that's clear as bloody mud!
Between capacitance, ESR, ESL, and temperature coefficients... after all this reading, I feel like a lost child. I'd like someone to hold my hand please!
As a start, I found one page that seemed like it would help me (link), but for the life of me I can't figure out what "D" is supposed to be in their equations!
Right now, D is for Dumb and that's how I'm feeling. This just shouldn't be this difficult.