1
\$\begingroup\$

I am trying to make a power up sequence for a sensor that requires 3.3V, 5V and 10V. 5V and 10V power should be applied after 3.3V. My PCB is powered by 12V, and I use STM32L4R5, connected to the EN pins of the 5V and 10V convertors. My main concern is that 12V is simultaneously powering all convertors and there is undefined state until the MCU powers up to take control of the 5V and 10V enable pins. I use tps54202 for 5V output. It is active high, but it has internal pull up, so floating the pin also enables it. For 10V I use tl751l10, active low.

All works perfectly once the MCU is on, but I want to make sure I will not damage the sensor on initial power up before MCU sets the correct values at the GPIO pins that control the EN inputs. I was considering to put pull-up resistor on tl751l10 EN pin (RESET actually) to assure it is off until MCU GPIO pin drives it down. Similarly, I want to put pull-down resistor to tps54202 EN pin. The question is I have no idea what values to choose so that GPIO pin can control the pins correctly.

Any ideas are most appreciated.

PS. I was also considering using sequence control like LM3880, but I still have doubts what would be the state before the chip is powered. (I have 12V -> 5V conversion by TPS, then 5V->3.3V by LDO that I use to power up LM3880 or STM32, meanwhile, a separate 5V TPS for the sensor and tl751l10 are directly powered by 12V).

\$\endgroup\$

1 Answer 1

Reset to default
2
\$\begingroup\$

One crude solution is to place some semiconductor switches (easily implemented with two transistors) in the 10v and 5v lines to the sensor. Then use either: 1) two pins on the microcontroller to control those switches, or 2) a simple RC delay circuit at the input to each switch. Here is an example of the latter (see diagramme). When C1 charges sufficently (to around .6v or .7v for a standard silicon NPN transistor), Q1 will turn on; Q2 will then also turn on, seeing current flow at its base. You will have a .2v to .3v voltage drop across Q2, so you might want to set the power supply voltages a fraction of a volt higher than 10v or 5v. Also, you will need to determine good values for the resistors and capacitor (I did not spend the time working this out, but the process is very straightforward). Of course there are lots of other implementations I can think of that will work; there might even be ICs for this purpose. You can easily test this in NGSpice or other simulator.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
1
  • \$\begingroup\$ Thank you for the answer! My main concern is the time before MCU takes over the control. I am considering just to put an external pull down resistor at enable input and not to touch the power lines. I guess is better to make another question about how exactly to decide the resistor value. \$\endgroup\$ Commented May 3 at 12:46

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.