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LM3409-Q1: LM3409-Q1 PGATE control Issue

Part Number: LM3409-Q1

Hi Expert

My customer need the test issue as below show:

firstly this device used in DLP LED driver and customer schematic as below:

in their test, they doesn't input PWM, just input 0.66V to IADJ

when input voltage =12V everything is okay but when their reduce input to 8.2V, the PGATE will keep contrast voltage(about 7V)

test waveform as below:

1. below picture is normal waveform:

output current is 5A the green is PGATE and the red is output current

2. this picture is show that input voltage form 12V to 8V. the green is PGATE and the red is output current.

you can see when input voltage reduce to 8V, PGATE will keep low voltage and PFET will keep open.

  

3. the picture is show output voltage and output current when input is 8V , you can see output current about 6.3A and max is about 6.6A

.

in my understand: 

customer Imax=Vadj/(5*Rsns)=6.6A: output current is about 6.3A so I think this current touched Lmax

when input reduce to 8V and the same time output is 7V, I think maybe the condition like near dropout.

so we need to change the parameters for letting circuit work in 8V or lower voltage.

could you please help to resolve this issue?

  • Hello Meng, you are hitting dropout.  It is designed to do this when the output voltage is close to the input voltage it switches over to linear mode and tries to regulate the constant current through the P-channel FET.  You need to have a higher input voltage to not loose the full constant current it was programmed to. 

    This is often the reason people have a BOOST converter in front of the BUCK to make sure it always has enough input voltage to provide current to the LED load based on the load Vf.  Some people will also use a sepic LED driver because it can drive the LED if VIN is higher or lower then Vout.  

    -fhoude

  • Hi Fhoude

    thanks for your reply.

    but what wrong with my customer circuit?

    how to resolve above issue base on this device?

    Can we make this circuit support 8V input voltage by adjusting Roff and Coff or other parameters?

      

  • Hi  Fhoude

    ignore above question.

    my customer will increase boost for Vin.

    they will increase input voltage to 24V and output voltage is 10.5V and output current is 6A

    1. do we have any tool can help to calculate roff ,coff, Rsense and so on?

    2. if we doesn't have tool could you please help to share the suitable parameters to me?

  • All the equations are in the datasheet and they should be going through the equations to understand the design.  I have Mathcad calculations if that would work for you.  Let me know.

    -fhoude

  • Hi Fhoude

    could you please share the Mathcad calculations to me?

    in addition when my customer move input voltage to 24V, their circuit thermal is very terrible because circuit  efficiency just 72%. so we want to know do we have any method to improve circuit efficiency?    

  • How are you measuring the efficiency?  Are you doing it while shunt fet dimming?

    -Francis Houde

  • I sent you the Mathcad sheet and other questions.  I am going to close this and we can take it off line.