MCU controlling voltage output rails

[Sarcarean]

Hey everyone!

So I wanted to ask you all for your opinion on a design change I am working on. I have 4 lines of various voltage (5v, 3.3v, 2.5v, 1.8v) each at 200-500ma. And I want to use a microcontroller (Atmel in this case) to control / switch a line out that then connects to a level shifter IC. My current board, this is done with just a SPDT switch.

So this is what I came up with:



So I have 4 lines from MCU, going through a resistor array (220 ohm), and then each to a LED and then to the enable pin on a ADP197 (High-side power switch) and the output of all 4 of those tied together that then goes to the voltage-ref of the level shifter / translator (FXMA108).

So my question, is there anything I am missing? I was thinking that there could be a problem with the boot-up, as vcc might try to enable all 4 of the power switches at once, but the level shifter is controlled via its own OUTPUT ENABLE pin, which I have a pull-up resistor (10K) so that it should remain in 3-state until the MCU is ready and can pull down the OE pin.

[Robartes_m]

Hey everyone!

So I wanted to ask you all for your opinion on a design change I am working on. I have 4 lines of various voltage (5v, 3.3v, 2.5v, 1.8v) each at 200-500ma. And I want to use a microcontroller (Atmel in this case) to control / switch a line out that then connects to a level shifter IC. My current board, this is done with just a SPDT switch.

So this is what I came up with:



So I have 4 lines from MCU, going through a resistor array (220 ohm), and then each to a LED and then to the enable pin on a ADP197 (High-side power switch) and the output of all 4 of those tied together that then goes to the voltage-ref of the level shifter / translator (FXMA108).

So my question, is there anything I am missing? I was thinking that there could be a problem with the boot-up, as vcc might try to enable all 4 of the power switches at once, but the level shifter is controlled via its own OUTPUT ENABLE pin, which I have a pull-up resistor (10K) so that it should remain in 3-state until the MCU is ready and can pull down the OE pin.

I don't think that will work - when you set the I/O pin for one of the analog switches (eg PC16) high, the LED will probably light up, but the EN pin will not go high. In fact, you might damage the ADP197, as you're sinking a current through the pull down resistor on the EN pin which it will definitely not be designed to do.

Connect the MCU I/O pin directly to the EN pin to get this to work.

[Psi]

Wouldn't it be simpler to have a precision reference on the ADC Vref then use a PWM DAC and an dual opamp to 2-order low pass filter&buffer.
It's more flexible since you can have any voltage you want.
You can use a high frequency for the pwm and heavily filter it because the shifter voltage level isn't something you need to change often or quickly.
It's also not something that has to be super accurate.

[Sarcarean]

Thanks for the response 



@Robartes_m: Would something like this work instead?

@Psi: I read somewhere else of something similar to this, do you know of a link to a schematic I can look at?

[Robartes_m]

@Robartes_m: Would something like this work instead?

That would work better in that it would pull up the EN pin of the analog switch, but you are still driving the LED directly from the MCU pin. This might or might not be a problem, depending on what current the MCU can supply. To avoid this, I would simply put the LED on the Vout of the analog switch.