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LM3409HV-Q1: current varies with load

Part Number: LM3409HV-Q1
Other Parts Discussed in Thread: LM3409


The product I designed USES the LM3409 chip as the constant current drive source of the relay, which needs to provide about 3.5a constant current of 100mS after power on, and then provide about 800mA constant output current, and the load resistance is 2-3R.

The problems we encountered in debugging are as follows:

(1)LM3409  Unable to output constant current,current varies with load.

(2)When the load is below 3R, the system has no output.

Please give some advice on the above questions,thank you.



  • Hello,

    You are using an LED driver as a relay driver, which can work however certain things need to be done. The LM3409 regulates peak current, the average is controller by the off-time circuit, output voltage and the peak current setting. The ripple stays fairly constant with changes in Vout if Vout is high enough. Driving a relay that will not be the case. If the LM3409 isn't running when load is below 3R I'm guessing the Off-time threshold isn't being met. If you can handle the current variation you may be able to set the off-time by tying the off-time resistor to a fixed voltage so it runs regardless of the output voltage.

    Yes, the voltage will vary with the load in your case because Vout is so low. Increasing the switching frequency will help with this since the current ripple will become smaller, using a fixed voltage for the off-time may actually help in this regard (even though that is backward when the output voltage is higher).

    Best Regards,