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LM3406: Biasing issues with DAC Driven Feedback

Part Number: LM3406
Other Parts Discussed in Thread: TPS92515, TPS92513

Team,

My customer is having issues with driving an LM3406. We are using it to control an LED. We are controlling the FB voltage through a resistor divider to increase and decrease the brightness of the LED.

We cannot use PWM b/c the LED is illuminating an object for a camera, and we cannot risk the shutter speed causing aliasing with LED signal. Therefore, we are turning the supply off by using the PWM pin. 

The control circuit is a DAC, Amplifier and resistor divider, which controls from 0-1Amp. When we turn the device off, if the control circuit has any residual voltage above zero, it seems to saturate something in the LM3406 such that the only way to turn on the LED is drive 0.8A or more through the LED. Any value less and the LM3406 will not drive the LED.

Monitoring the CS pin during an event such as this, we see this drive lower and lower and lower. They have observed CS voltages as low as 40mV, when nominal should be 200mV.

Once they overcome this saturation condition, the LED turns on, and they are able to control as expected from 1amp down to about 10mA. 

Do you have any comment as to what could be causing this saturation, and any advice on how to get around this?

  • Hello Carolus,

    The LM3406 is a constant on-time part that uses Vin and Vout to set switching frequency. It's really intended to be used in CCM mode. If the output has discharged it may not be getting a very large ton pulse since it's calculated by: tON = 1 x 10-11 x RON x(VO/VIN). I'm' not sure if this is what is going on since the part isn't really meant to analog dim. Perhaps using the TPS92515 or TPS92513 would be better?

    Best Regards,