Adding a simple PWM circuit to this circuit?

[dawhale]

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

 

[BrownOut]

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.

 

[dawhale]

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

 

[BrownOut]

Correct. The middle connection is the pot's wiper connection.

 

[ItsMike]

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 ?

 

[carbonzit]

We need a little more info. Define "big". 12 volts @ 1 amp? 3-phase 440 volts 20 HP?

 

[ItsMike]

Motor specs here.

 

[BrownOut]

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]

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]

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.