R
rickman
- Jan 1, 1970
- 0
We often need to power low current circuits (<100 mA) from a wide
battery voltage of 7 to 16.5 volts. It is hard to find switching
regulators that will do this efficiently. Sometimes we can just write
off the wasted power and use an LDO if the current is low enough, but I
am looking to find a way to reclaim this lost efficiency.
I am thinking of building a switched capacitor voltage converter that
can lower the input voltage over this input voltage range to something
that is suitable for an LDO. The circuit would use a CPLD (powered by
this circuit) to generate the control signals for a small army of P and
N channel FETs (or possibly analog switches) to control three flying
capacitors. I have done a lot of searching and found a 2 x 3 mm
complementary FET that has the required characteristics that should
work well. I have also found a 4 channel comparator with a built in
reference. Now I only have two remaining problems.
The first is figuring out how to start up the circuit. I have come up
with a couple of ideas that will bypass the switches until the output
voltage is up to snuff and the CPLD starts running the switches.
However this is hard to do without exposing the LDO following the
switched cap converter to the full Vin of 16.5 volts.
This is the second problem. If I try to find an LDO with Vin up to
16.5 volts, output current up to 100 mA and dropout voltage of 200 mV,
I come up short. Considering some losses in the switching circuit,
even to work with a dropout of 200 mV will realistically require the
output voltage to be lower than the 3.3 volts I would like to use. But
I can likely live with 3.2 or even 3.1 volts if the accuracy on the LDO
is good enough to keep it above 3.0 volts worst case.
I can't raise the 7 volt Vin minimum requirement, so I am stuck with a
200 mV dropout. I can get this in a low Vin device, but not a high Vin
device in a small package. So far I have tried to keep this as simple
as possible and not used anything like a "pre-regulator" or Zener
diode. But I'm not happy with my choices.
Anyone have any suggestions on a better way to improve this circuit?
battery voltage of 7 to 16.5 volts. It is hard to find switching
regulators that will do this efficiently. Sometimes we can just write
off the wasted power and use an LDO if the current is low enough, but I
am looking to find a way to reclaim this lost efficiency.
I am thinking of building a switched capacitor voltage converter that
can lower the input voltage over this input voltage range to something
that is suitable for an LDO. The circuit would use a CPLD (powered by
this circuit) to generate the control signals for a small army of P and
N channel FETs (or possibly analog switches) to control three flying
capacitors. I have done a lot of searching and found a 2 x 3 mm
complementary FET that has the required characteristics that should
work well. I have also found a 4 channel comparator with a built in
reference. Now I only have two remaining problems.
The first is figuring out how to start up the circuit. I have come up
with a couple of ideas that will bypass the switches until the output
voltage is up to snuff and the CPLD starts running the switches.
However this is hard to do without exposing the LDO following the
switched cap converter to the full Vin of 16.5 volts.
This is the second problem. If I try to find an LDO with Vin up to
16.5 volts, output current up to 100 mA and dropout voltage of 200 mV,
I come up short. Considering some losses in the switching circuit,
even to work with a dropout of 200 mV will realistically require the
output voltage to be lower than the 3.3 volts I would like to use. But
I can likely live with 3.2 or even 3.1 volts if the accuracy on the LDO
is good enough to keep it above 3.0 volts worst case.
I can't raise the 7 volt Vin minimum requirement, so I am stuck with a
200 mV dropout. I can get this in a low Vin device, but not a high Vin
device in a small package. So far I have tried to keep this as simple
as possible and not used anything like a "pre-regulator" or Zener
diode. But I'm not happy with my choices.
Anyone have any suggestions on a better way to improve this circuit?