Hi Guys,

I have below design requirement.
V1 = 2.5V to 4.2V
V2 = 2.5V to 4.2V
Ilimt = 15mA fixed
Vout_ground = 4V fixed

So how do i calculate & fixed the value of R1 & R2 to limit the current for this range of input voltage i.e. 5V - 8.4V.

Could anybody help me out here ?


Thanks

 

hi m,
The design is way off being correct.
Use this asc file as a starting point.
E

 

hi m,
The design is way off being correct.
Use this asc file as a starting point.
EView attachment 214108

Thanks for your reply !

I do not think it is correct !

My requirement was :
Vin = 5V to 8.4V , Vcc_4V, ground = 4V always and at the same time current should be limited at 15mA...

Please correct if i am wrong.

 

I tried with altering the value but i am not sure why it does not worked .
Could anybody help me here.
I assume R1 can be calculated as per below:

Q2 - Vbe = 0.7V, VD1 = 0.3V
VR1 = 1V
R1 = VR1/15mA = 67R

Now i assume if input increases load current will increase thus VR1 increases and Q2 will turned ON for current limiting.
Can anyone help me here !

 

What's the purpose of D1?

 

What's the purpose of D1?

Thanks for support !
D1 should be connected as per circuit requirement.
Diode here is used for oring the circuit for lets say charger, switch, e.tc...so design here you have to consider the D1 always.

 

I Think mA is load requirement & beyond 15mA current limit comes into the picture !
Hope the requirement is clear to you all.

 

hi,
Consider the circuit when the Vbatt is 5V and you have 4V across the output load, that leaves only 1V to power the transistors and diode.!!!
Say 0.7V across the Diode., leaves 0.3V
E

 

hi,
Consider the circuit when the Vbatt is 5V and you have 4V across the output load, that leaves only 1V to power the transistors and diode.!!!
Say 0.5V across the Diode., leaves 0.3V
E

Your assumption may correct !
So you meant to say Q2 will not get turned tuned so current limiting function is not enabled right.
I am blank here as of now because i do not see current limiting function for 15mA. If i incense the input voltage 5 to 8.4 still i do not see stable output 4V with 15mA.
and why diode current is higher.

 

Still i do not understand why current flowing through R1 is 30mA which is above the set current limit.
I removed diode.

 

Tried Multiple attempts but still i did not get the concepts & results !

 

if you want the load (R5) regulation of 4V min 15mA max
then you need to consider the TL431 passing XmA in parallel to R5
for the "effective" regulation the TL 431 must see ≥ 800uA at all times /// for a "good" regulation it must be apx. ≥ 1.3mA

as already mentioned in the above replies : (the 5V) – (270mV Schottky AN_CA drop) – (380mV BJT CE drop) – (650mV BJT BE drop) -- leaves you 3.7V
you need to rework your topology . . . likely using a low threshold mos-fet that does both -- the voltage- and the current regulation

 

hi,
What is the current thru the TL431.?
E

 

if you want the load (R5) regulation of 4V min 15mA max
then you need to consider the TL431 passing XmA in parallel to R5
for the "effective" regulation the TL 431 must see ≥ 800uA at all times /// for a "good" regulation it must be apx. ≥ 1.3mA

as already mentioned in the above replies : (the 5V) – (270mV Schottky AN_CA drop) – (380mV BJT CE drop) – (650mV BJT BE drop) -- leaves you 3.7V
you need to rework your topology . . . likely using a low threshold mos-fet that does both -- the voltage- and the current regulation

Thanks you so much for useful information. I understand your calculation & proposal.

hi,
What is the current thru the TL431.?
E

thanks for your interest. Maximum current is 15mA that will be flow through TL431. A small load is connected between VCC_4V and ground.

 

Are V1 and V2 supposed to be lithium cells? If so, they should not be allowed to drop to 2.5V.

 

Are V1 and V2 are supposed to be lithium cells? If so, they should not be allowed to drop to 2.5V.

Yes, correct !

 

hi,
Measure the current thru the TL431.?
E

 

Yes, correct !

Then the cells are permanently damaged. Lucky they're only virtual cells .

 

if you want the load (R5) regulation of 4V min 15mA max
then you need to consider the TL431 passing XmA in parallel to R5
for the "effective" regulation the TL 431 must see ≥ 800uA at all times /// for a "good" regulation it must be apx. ≥ 1.3mA

as already mentioned in the above replies : (the 5V) – (270mV Schottky AN_CA drop) – (380mV BJT CE drop) – (650mV BJT BE drop) -- leaves you 3.7V
you need to rework your topology . . . likely using a low threshold mos-fet that does both -- the voltage- and the current regulation

So as per your calculation if input voltage is 4V+1.3V = 5.3V it has to work !
But i do not see any results after 5.3V as well !

 

Then the cells are permanently damaged. Lucky they're only virtual cells .

Any reason for damaging the cell !