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Adding a simple PWM circuit to this circuit?

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dawhale

New Member
Hi all,

Nice forum!

It has been many, many years since I did any electronic experimentation so please be gentle :)

I am developing a LED light test model. At this stage I am creating a test unit using a TI TPS61042 constant current LED driver. A smaller .pdf of the LED driver is attached.

I believe Figure 24 on Page 11 is want I want to start with however I will only be using 5 LED's, if that makes a difference.

I want to add a variable brightness control to go from 0 to 100%. Such as a rheostat. The diagram indicates I can do this via a PWM circuit to the CTRL pin. There is much more info on Pg.2 under the CTRL section.

I have been looking at dozens of PWM variations and now I am more confused.

Can anyone assist with a simple PWM I can add into this circuit?

Thx DD
 

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BrownOut

Banned
You can try this simple circuit. R1 and R3 model a variable resistor, with the junction of the two model resistors being the wiper connection. The variable resistor controlls the duty cycle. Connect the "OUTPUT" wire to your chip.
 

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dawhale

New Member
You can try this simple circuit. R1 and R3 model a variable resistor, with the junction of the two model resistors being the wiper connection. The variable resistor controlls the duty cycle. Connect the "OUTPUT" wire to your chip.
Hi BrownOut - thx for the circuit. So, to have variable control, I should replace R1 & R3 with possibly a 100k pot? If I want one with a switch on and off, same process?

Thx DD
 

ItsMike

New Member
Sorry for bumping this old thread.

According to my simulation this circuit produces pwm at about 250HZ.
Is that frequency high enough to driver a (big) motor ?
Is there any way of making a higher frequency pwm generator with a 555 ?
 

BrownOut

Banned
Sorry for bumping this old thread.

According to my simulation this circuit produces pwm at about 250HZ.
Is that frequency high enough to driver a (big) motor ?
Is there any way of making a higher frequency pwm generator with a 555 ?
Lowering the value of C1 will increase the frequency. So will lowering the value of the variable resistor, R1/R3.
 

ItsMike

New Member
What would be the maximum frequency the 555 can withstand ?
Also, I guess i'll be driving a mosfet. As little as I know about mosfets I know I gotta supply them a lot of current to switch them quickly.
How can I determine how much current a mosfet needs for any given switching speed ?
 

crutschow

Well-Known Member
Most Helpful Member
For best performance you should probably keep the maximum operating frequency of a 555 below 1MHz.

The current required to drive a MOSFET is determined by its gate capacitance, which is given on the data sheet.

For a given switching speed the approximate formula is I = (V*C)/t where V it the switching gate voltage (typically 10V), C is the gate capacitance, and t is the desired switching time.

If the gate switching charge is given on the data sheet, then the formula is I = q/t.
 
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