I am trying to find a switch part number that allows me to select between two Vin inputs that are roughly at 1.5V (these inputs are analog power supplies). It needs to be able to pass at least 500mA current with maximum Ron = 100m ohms.
The VDD can be anywhere up to 5V. This is supposed to be a low power device because I am going to use it in low power mode to select between an available Vin1 and a backup Vin2 (battery) in case Vin1 is not available.
May I recommend you use a latching relay.
Think of this as a relay memory. In this application memory is a mechanical latching mechanism that hold the relay contacts in a particular state.
There are two types:
- single coil: The single coil toggle for every time it is pulsed. Depending on you application, you may need to monitor the relay state.
- dual coil: with two coils, you pulse a coil and the relay moves and then stays in that state. You can have reasonable confidence that the relay will stay in that state until the opposite coil is activated.
The latching relay is an energy saving device. A single pulse will move the relay into position. This is preferred to a traditional relay that requires constant coil power to hold position.
Here are a few components for your consideration. Yes, 5 VDC coils are available.
Another idea, a lot of microcontroller boards use a Diode and a MOSFET to automatically switch between Lithium battery backup and the default USB Vin.
Here’s an example of Sparkfun’s Thing Plus - RP2040
Likely a much better solution.
I should read more carefully. In my mind I interpreted “these inputs are analog power supplies” as “inputs are analog.”
A diode world work for the power supply application. However, the MOSFET solution you recommended will have less power loss.
An additional option would be something like using a pair of LM66100DCKT ideal diodes. It’s slightly more complex, but still a pretty elegant solution.
Thanks. Relays are ideal but they create mechanical noise.
Thanks. The diode in your design impose a voltage drop. My 1.5V analog supply is supposed to supply an MCU and the min supply acceptable for it is 1.425V. So I dont have much voltage to drive a diode.
Thanks. The min operating voltage for this component is 1.5V. So it is somehow on the edge for the functionality I need plus that its series resistance is high, 230mohms max at Vin = 1.8V which I assume will be worst at Vin = 1.5V. So if I drive 500mA through it, the voltage loss across the switch could be more than 100mV. This is not acceptable because my MCU can operate at Vcc minimum of 1.425V.
I found another device similar to what you suggested, FPF1321UCX, which has better series resistance, but still minimum operating Vin for it is 1.5V which is at the edge. I dont know how such device will function if power supply drops for example to 1.4V.
I see your point. Yes, I expect that the LM66100 would be in the neighborhood of 500mOhms with 1.5V. Tricky problem. That FPF1321 is a good find, but yeah, still on the edge. The Ron graph on page 7 of the datasheet shows that it won’t meet your spec.
It looks like the problem comes down to sufficient MOSFET enhancement at such a low voltage. I’ll see what else might be out there.
I haven’t yet found a single chip solution which meets your requirements, but the NCP456RFCCT2G seems to be a good possibility if you can use two separate parts for each power source. For control in selecting your input source, you would probably need an inverting circuit (possibly as simple as a transistor with a pull-up/down resistor) to enable one device while disabling the other, unless you could guarantee that the non-battery supply was always a bit higher than the battery supply, in which case they should be able to both run enabled in parallel.
This device does not have a discharge FET on the output. I don’t think you would want a discharge FET, as that would draw a lot of current from the active supply, so I think this is good. It also has reverse voltage protection, which would also seem to be desirable. It’s specified for a max current of 2A, and a worst-case Rds(on) of 41mOhms with a 1.5V Vin voltage at 125°C junction temperature. This easily meets that requirement.
From the datasheet, here’s the Rds(on) specs: