That's probably good advice, and he seems to be asking for that anyway. Still, it's not a clear choice if other system requirements are factored in.I"m going to infer/suggest that you want to avoid a linear regulator with currents up to 8A... unless of course you want to have this circuit double as a space heater.
That one one will be very difficult to build: About 60 - 70W drawn off a 3V source? Huge peak currents on the input devices. One of those cases where it's theoretically possible but ridiculously difficult to build. It's not a feasible design, you need to get a higher source voltage to work from.I need a Dc-dc converter with:
2 input voltage : 3.3-4.2V
output voltage : 6V.
O/p current - 8A.
You definitely want to use a linear reg on this one, a discrete LDO using parallel N-FET devices to get the on resistance low enough. Max power dissipation is about 18W (8.2V in) which is manageable with a decent heatsink. You will need to use some kind of charge pump circuit to generate a bias rail of maybe 9-10V to run the control circuitry and be able to bias up the N-FET gates.I need a Dc-dc converter with:
1 input voltage : 6.6-8.2V
output voltage : 6V.
O/p current - 8A. (max i need)
Buck reg efficiency goes to crap when the input voltage drops near to the output. In those cases, the switch is on almost all the time but you still have switching power losses and bias/driver circuits eating power. I have actually seen buck regs with efficincy LOWER than an LDO when the input voltage is near the output.steveb,
Yes, you are right, he is not looking for a large voltage drop and definitely could go the LDO route (for 1). My intention was to be humerous with the space heater.
But more importantly, I was under the impression many buck regulators were well over 90% efficient? Perhaps this isn't true.
Good points.But more importantly, I was under the impression many buck regulators were well over 90% efficient? Perhaps this isn't true. That said, I've only seen a few linear regulators that handle that kind of current, which usually implies a meaning, hence my suggestion.
It's because die size drives the price out of sight. There used to be a decent selection of 5A LDO regs around and one 7.5A which was REALLY pricey. Not many high current linears around anymore because nobody will pay $10 for a linear reg when you can get discrete FET's for about $.25 and a controller IC for maybe about the same ballpark and build your own reg. Check out the LP2975 from NS.You are probably correct about available high power regulators being not too common.
by Jake Hertz
by Jake Hertz
by Jake Hertz