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[Moved]: Nmos to drive LED strip

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Tizana

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Hi,

I'm about to make a circuit which should drive an LED strip from a 3.3V microcontroller using an NMOS and PWM.

the Nmos trasistor that i chose is this one IRLML2502 , https://www.infineon.com/dgdl/irlml2502.pdf?fileId=5546d462533600a401535667f44d2602

my question are as follow :

1- i want to know if i apply a Vgs>Vth(Threshold), the current Id will be imposed by Vgs or it will be imposed by the load connected to the drain ?
Because according to the transfer characteristic curve, for each Vgs we have a current Id, so for a Vgs = 3.3V the Id is equal to 32A. see the curve bellow :

1.png

and the 32A is a way above the current supported by the transistor.



For example the load may vary in my application, for example the circuit can drive 4 LEDs as he can drive also 8 or more LEDs, so the current may vary depending on the load knowing that Vgs is fixed to 3.3V .

2.png


2- another question related to the output curve Vds/Id :

normally when the NMOS work as a swtich it should be operate in the cutoff region or in the saturation region.
Cutoff = Nmos Blocked,
Saturation = Nmos ON,

and normally in the saturation region when the transistor is on, the Vds voltage should be as close as possible to zero but according to the output curve, in the saturation region the Vds is way far from Zero.

3.png

any explanation ?

thanks
 

Re: Nmos to drive LED strip

Hi,

1) the chart is for V_DS = 15V...But as soon as you connect the LED stripes the voltage will become close to zero and the current drops the same way.
Your 12V is NOT V_DS! 12V = V_DS + V_LED + V_R. with curents in the region of 80..160mA V_DS is about zero.

--> simply calculate with R_DS(ON).

The 32A are within specifiaction for short pulses.

2) again: --> simply calculate with R_DS(ON).

Klaus
 

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