How to convert a wide voltage range (1 V-12 V) to 5 V?

Question

I have an unusual application where the input voltage can vary from 1-12 V DC. This must be converted to 5 V @ ~200-250 mA.

Since there are no available buck-boost converters suitable for this wide input voltage range, I came up with this (simplified) circuit:

^{simulate this circuit – Schematic created using CircuitLab}

The XC61CC5002MR-G is a voltage CMOS supervisory IC which:

• Outputs VCC when VCC > 5 V - this enables the LDO and protects the boost converter from the higher voltage.
• Outputs 0 V when V_CC < 5 V - this disables the LDO and biases the PMOS transistor to allow current to the boost converter.

The boost converter's absolute maximum voltage rating is 6 V, so it needs to be protected at the higher range of input voltage, whilst the LDO is OK up to 13 V.

The circuit worked as intended from 3-12 V, however the PMOS transistor couldn't switch when the input voltage was lower than 3 V, which shouldn't have surprised me since the V_GS threshold was about 2 V at 250 mA.

I have also looked at high-side load switches and over-voltage protection ICs, but I cannot find any that will operate over the whole range of the input voltages after browsing for hours on Mouser and DigiKey.

Lastly I have explored using a N-channel MOSFET in the circuit above with an open-drain output variant of the same supervisory IC and a charge pump to bias the NMOS transistor when voltages are low, but to my surprise I couldn't find any charge pumps that work in the 1-5 V range.

I am looking for any suggestions to either make my circuit work with the lower voltages or how else I can achieve this 1-12 V to 5 V conversion without dramatically increasing the PCB footprint or cost. Unfortunately the input voltage cannot be changed, but the power supply can provide more than enough current to run the circuit.

Answer 1

Allow me to make this circuit suggestion:

^{simulate this circuit – Schematic created using CircuitLab}

For 1 V < Vin < 6 V the boost converter converts to a 6 V internal voltage and the LDO regulates that 6 V down to 5 V

For 6 V < Vin < 12 V the voltage at the Boost converter's output will "follow" the input voltage with some voltage drop due to a (Schottky) diode, so Vin = 7 V => Vmid = 6.5 V and Vin = 12 V => Vmid = 11.5 V.

Remember that boost converters have this basic circuit:

So when Vin is higher than the configured (regulated) output voltage, the output will follow the input voltage with a voltage drop from the coil's resistance and the diode.

Answer 2

A SEPIC might be worth a shot here

This situation (wide voltage range input, fixed voltage output, low-ish current requirements) strikes me as a decent opportunity to try one of the lesser-used switching topologies: the SEPIC. In particular, I would use a LM2621 converter IC with a low-drop Schottky diode -- this gives the chip the best chance of starting up at or about 1.0V (the datasheet specifies nominal startup at 1.1V and maximum startup at 1.2V over temperature, but the startup voltage likely depends on the drop across the diode).

As to passives, I would use the coupled-inductor version of the SEPIC topology -- this gives better ripple performance while using less board space than two separate inductor cores.

Answer 3

To avoid the EXTREME switch, you need a flyback stepup with zener post-regulator, to provide the FET's gate drive.

Set the zener at 10 or 12 volts. Many huge FETS want a low max gate voltage (from what I recall).

Now you can use a LARGE FET, capable of 10 or 20 amps or whatever you need, that FET gate being driven 0/10 volts for efficient switching.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 4

There's a lot of ways to skin a cat, and this is a particularly vicious cat. Here's my thoughts:

Boost set to 5.5V or so. I'd have to go shopping for parts to be sure, but I suspect you'll need something with an external switch to handle the 8-9A input at 1V. Follow that with a buck to 5V, to handle the case when the input's above 5V. Don't mess with the linear supplies unless you need really clean power.

If you can't find a suitable boost chip or switch FET, use a lightweight boost that's just big enough to power the "real" boost controller (and by extension, the "big" switch gate). Maybe even switch the lightweight guy out of the circuit if the input voltage gets too high.