Level-shifter IC connected to microcontroller

Question

I am trying to do a power assessment of a microcontroller (TMS320F28379D, datasheet link: https://www.ti.com/lit/ds/sprs880o/sprs880o.pdf?ts=1702312460730).

The output pins of the microcontroller is connected to a level-shifter IC's \$ 3.3V \$ side, for shifting to \$ 5V \$ (SN74LVC4245A, datasheet link: https://www.ti.com/lit/ds/symlink/sn74lvc4245a.pdf?ts=1702310500407).

A simple schematic is given below:

The electrical characteristics of the microcontroller is this:

And the electrical characteristics of the level shifter is this:

Is the supply current (current going into \$ V_{CCB} \$) of the level-shifter \$ I_{CCB} \$ in the datasheet? Or, is \$ I_{CCB} \$ the current that all input pins will take?
From an earlier question (Current into level shifter IC from microcontroller), I understood that the input pins of the level-shifter IC can take +/- 5 uA. Does this mean that the microcontroller's output (connected to the level-shifter) will also be the same?
Is the +/- 4 mA (\$ I_{OH}, I_{OL} \$) mentioned in the microcontroller's datasheet just the maximum current that uC output pins can supply?

Hearth · Accepted Answer · 2023-12-12 15:29:08Z

1

\$I_{CCB}\$ is indeed the current drawn from the B-side supply. However, it is specifically the current at no load; any load on the output will increase this, because the output current has to go somewhere. Also note the quantity \$ΔI_{CCB}\$, which is the change in \$I_{CCB}\$ per high input.
Yes, unless there is something else on the microcontroller's output that pulls additional current.
Yes and no. It's the maximum current they are rated to supply. It may be possible to pull more current from them if you short them to ground, but this runs the risk of damaging your microcontroller and should not be done.

answered Dec 12, 2023 at 15:29

Hearth

36.5k3 gold badges65 silver badges151 bronze badges

\$\begingroup\$ "per high input" - sorry if this is trivial, but what does this mean? I can see that the input pins are connected to the microcontroller whose output voltage is the V_OH and V_OL specifications (I think). \$\endgroup\$
– curious_direwolf
Commented Dec 12, 2023 at 15:34
\$\begingroup\$ @curious_direwolf For every B-side input that reads a logic high, the B-side supply current increases by no more than \$ΔI_{CCB}\$. \$\endgroup\$
– Hearth
Commented Dec 12, 2023 at 15:35
\$\begingroup\$ The B-side inputs are going to be PWM signals (for my current application). Would it be right to say that the supply current will rise by no more than 5mA (for 10 inputs to the level-shifter B-side)? \$\endgroup\$
– curious_direwolf
Commented Dec 12, 2023 at 15:39
\$\begingroup\$ @curious_direwolf Considering that would be ten times what the datasheet's listed maximum would imply, yes, that's a safe assumption. \$\endgroup\$
– Hearth
Commented Dec 12, 2023 at 15:40
\$\begingroup\$ Thanks! I meant 5mA instead of 50mA (I have edited the comment to reflect the same) \$\endgroup\$
– curious_direwolf
Commented Dec 12, 2023 at 15:42

Add a comment |

InBedded16 · Accepted Answer · 2023-12-12 15:30:25Z

1

I_CCB is the supply current that the V_CCB sinks. So it is the supply current for the low-side of the level-shifter. My understanding is that when all I/O pins are at either V_CCB or GND, I_CCB is 50uA. For every low-side pin that is at a voltage below V_CCB-0.6V, I_CCB increases by 0.5mA.
Yes, the microcontroller will source the same amount of current that the level shifter sinks. This is only if the connection is a direct connection between the pins without any other connections that may cause the microcontroller to source more current.
Yes, that is the maximum current that the pins can safely source and the minimum current that the pins can safely sink as an output.

answered Dec 12, 2023 at 15:30

InBedded16

1,0963 silver badges12 bronze badges

\$\begingroup\$ Are the voltages to the low-side pins governed by V_OH and V_OL specification of the microcontroller? \$\endgroup\$
– curious_direwolf
Commented Dec 12, 2023 at 15:57
\$\begingroup\$ Yes the SN chip will accept whatever voltage the microcontroller pins are outputting. \$\endgroup\$
– InBedded16
Commented Dec 12, 2023 at 16:09
\$\begingroup\$ " direct connection" - if it isnt direct, and there is some resistor pulled to ground in the connection, the V_OH/V_OL specs can be used to find the current throught that resistor right? In addition to the 5uA current, the uC will supply this current as well. \$\endgroup\$
– curious_direwolf
Commented Dec 12, 2023 at 16:14
\$\begingroup\$ That is basically correct yes. I would recommend doing a bit of research into pull-up/down resistors if you want to understand this more. There are a lot of resources out there that will give you an in depth explanation of how these parts will play with each other and affect stuff like the voltage and current levels. \$\endgroup\$
– InBedded16
Commented Dec 12, 2023 at 16:17
\$\begingroup\$ Yes, I understood how the pull-down resistor works, but my doubt was on what the voltage that the microcontroller output pin will give if it is connected to the SN chip's input. \$\endgroup\$
– curious_direwolf
Commented Dec 12, 2023 at 16:41

| Show 1 more comment

Enrico Migliore · Accepted Answer · 2023-12-12 15:36:14Z

1

Iccb = 50 uA is the recommended current sunk by Vccb per pin.
No. Microcontroller's outputs have their own ratings. It's up to the designer verifying the electrical compatibility between the microcontroller and the level shifter.
No. Output pins can give you more than that but the output voltage will drop.

Ciao

answered Dec 12, 2023 at 15:36

Enrico Migliore

4,7671 gold badge10 silver badges12 bronze badges

Add a comment |

3 Answers 3