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Question on LM3406HV vs. LM3404HV -- Setting Forward Current

Other Parts Discussed in Thread: LM3406HV, LM3404HV, LM3404, LM3406

Hi -- a few quick questions on the differences between LM3406HV & LM3404HV (minus the obvious 1.0A If vs. 1.5A If).

First off, why does the 3406 have a Vout pin & how does this function differently than the way that they 3404 is setup?

Second, I am a bit confused in setting & calculating the output current. In the LM3404HV datasheet examples, the current is recalculated using equations 34/36 (after selecting an Rsns resistor). In the LM3406HV, this step is not done -- the current is simply calculated as .2/Rsns. Is this due to the addition of the voltage output sense pin?

Third, in regards to the Rsns resistor. What if my actual return path to ground has some non-negligible value of resistance ('outside' of the PCB layout, say on wire to/from my regulated DC source). Is this going to affect the output voltage setting?

Lastly, I see that there is a LM3406HVQ1 model available -- how does this differ from the non-Q1 variant?

Thanks!

  • 1. The LM3404 switch ON time is generated from the input voltage only. This means for a certain LED stack voltage the switching frequency remains nearly constant as the input varies (if you have a widely varying input voltage). But if you change the output voltage the frequency will change. The LM3406 also senses the output voltage to generate its switch ON time, so it will keep the switching frequency nearly constant even if the output voltage changes.

    2. They both use controlled ON time architectures. The difference is the LM3404 uses a one shot comparator so it is sensing a 0.2V peak on CS which tells the switch when to shut off. This means the average at CS will actually be lower than 0.2V with the average depending on the ripple current in the inductor (ripple voltage on CS). So you have to adjust the peak current to get the correct average inductor/LED current depending on how much ripple current you have. The LM3406 uses a transconductance amplifier and compensation (COMP) so it will regulate to an average of 0.2V at CS.

    3. Yes, if there is significant resistance it will affect output current. But there shouldn't be if you use a PCB and follow the layout guidelines. Please don't breadboard, it will never work quite right that way.

    4. The Q1 simply means it is qualified for automotive designs. There is no difference otherwise.

  • Clinton,

       Thanks again for the responses. Does the LM3404HV measure the Rsns resistor between the CS & the GND pin of the IC? So as long as I can minimize residual resistance between the two pins, I will get an accurate current output? This is for a potential PCB design, not a breadboard.

    Thanks,

       Matt

  • That is correct. You want to tie the grounds of the input capacitor, the IC, the current sense resistor, and the switching diode all together as well as possible and as close as possible. It's best to use a ground plane on the bottom layer of the board and tie all those points to it well with vias as well as getting them as close together as possible.