Circuit Review Controlling the Low power mode of Buck Boost converter using a Microcontroller

[newbe_always]

I am using TPS63000 in my design and I need to control PS/SYNC pin using a micro-controller.

The input to the buck boost converter is 3V to 4.3V which is coming from a battery. The output of the converter is 3.3V

The microcontroller is working at 3.3V. This 3.3V is coming from TPS6300

I am using a BJT switch to control the PS/SYNC pin. The default mode of TPS6300 is high power mode

When micro-controller output is high the BJT will turn on and PS/SYNC will be connected to GND and TPS63000 will go to low power mode.

My circuit is given below .May I know is it fine or not.

 

[aliarifat794]

How did you calculate the values of R2 and R3?

 

[newbe_always]

I just simulated a BJT Switch.I just put the values which are commonly used in BJT switch.Usually R2 is 10K and R3 is 100K

 

[Nigel Goodwin]

I just simulated a BJT Switch.I just put the values which are commonly used in BJT switch.Usually R2 is 10K and R3 is 100K

I certainly wouldn't have said so? - they aren't even in the usual places - usually R3 (if present at all) should be the other side of R2, and should be lower, and R2 should be a LOT lower.

Generally you want the transistor to be fully switched ON, and a 12K resistor is far too high for that.

But I agree there's no need to do any calculations.

 

[newbe_always]

I certainly wouldn't have said so? - they aren't even in the usual places - usually R3 (if present at all) should be the other side of R2, and should be lower, and R2 should be a LOT lower.

Generally you want the transistor to be fully switched ON, and a 12K resistor is far too high for that.

But I agree there's no need to do any calculations.

Yes R3 need s to be placed after that.I changed R2 to 1K and R3 to 10K
May I know other than that this circuit is fine or not.Below is the new circuit

 

[Nigel Goodwin]

Yes, those values (and positions) are a lot better.

Although, in this exact case, the collector current required probably isn't very high anyway - but it's good practice to drive the transistor correctly.

 

[danadak]

R3 should be on micro side of R2, otherwise it reduces base drive due to V
divider effect if its on right. Ic is a f(Vbe). Note by being on left
side that takes a little more current out of micro pin, hence its high
side Vds increases, but with values shown not significant I would posit.

https://alan.ece.gatech.edu/ECE3040/Lectures/Lecture19-BJT%20Ebers-MollModel.pdf

Tradeoffs between power, Ic, Ib, Vcesat :

Purpose of pulldown is to absorb leakage w/o turning on downstream devices. Eg. when typical
micro starts up pin is placed into inputs state so effectively floating.

Regards, Dana.

 

[newbe_always]

R3 should be on micro side of R2, otherwise it reduces Ic due to V
divider effect if its on right.

You mean my first circuit is correct where R3 comes near to Mcrocontroller?

 

[danadak]

The difference however in circuit behavior not large. Note
simulators typically use a modified Ebers Moll model, and
some DC stuff may not be accurate.

So in short I place on left, in some apps use internal GPIO pull down. But then
its effectiveness comes back under program control on startup, eg. can you config
a PD before turning GPIO into output.

Regards, Dana.

 

[Nigel Goodwin]

You mean my first circuit is correct where R3 comes near to Mcrocontroller?

No, the way I suggested is the usual method - the potential divider effect is completely insignificant due to the component values.

 

[danadak]

@OP

Sims show at low drive levels (say around 1V), eg, Vin, up to a 25% difference occurs in Ibase, but
at the typical 3.3 or higher drops to 2% or less. I attribute this to knee of emitter base junction
around its "threshold" V.

Interestingly the Ic is almost insensitive. However I found on web simulators using Ebers Moll
model may have some DC accuracy issues. So if you have time do some measurements,
otherwise I would ignore as stated in post #9

Why do you need 2N2222, why not drive the PS/Synch pin directly ?