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I would like to design an LED driver circuit that allows for simultaneous analog and pwm control. That is, I would like to set the the LED current using an analog signal and then be able to switch using another pwm signal. I have a design with the analog control shown below. Is there a simple way to add the pwm control to this circuit?

LED driver with analog control

I realize that there are numerous high level components that solve this problem but I am using this as a test case to learn basic electronics and would like to solve it using low level components.

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  • \$\begingroup\$ if the OP-AMP you choose allows suficient inout range ypu couldl just put pwm input onto the non-inverting input of the op-amp. \$\endgroup\$ – Jasen May 22 '16 at 6:43
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You can just add the control to the amplifier:

schematic

simulate this circuit – Schematic created using CircuitLab

Some notes:

  1. The input common mode range of the amplifier must go all the way to ground.

  2. The PWM control is inverted: A high on the PWM input will turn the LED off.

  3. The PWM rate will be limited by the transient performance of the amplifier.

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  • \$\begingroup\$ I tried the proposal but I have som problems in the off state. The voltage on the + input on the op-amp goes to 20 mV and not to zero. The current through the LED is around 1 mA when the analog signal is at its maximum and it is enough to get light. I wonder if it would be possible to have the pwm-input between the LED and the npn-transistor... \$\endgroup\$ – Freja Dev May 23 '16 at 13:31

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