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Low Intensity LED Array

Discussion in 'LEDs and Optoelectronics' started by ElectronicsStarter, Mar 10, 2016.

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  1. ElectronicsStarter

    ElectronicsStarter

    3
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    Mar 10, 2016
    Hi All,

    I am working on a low intensity LED array for medical applications. Specifically the application is transcranial photobiomodulation. I would like the device to have these specifications

    1) 100 LED of the Vishay VSMF4720-GS08 type. Ideally I would like to have a 10 x 10 array of LED.
    2) From the datasheet for that LED I will need 1.45V across each LED with a current of 100mA.
    3) Each string will have 14.5V and .1A going through it.
    4) Ability to switch on/off each string of LED to get a power output that can step up.
    5) I will need a constant current through the LED string.


    To meet these specifications I have been searching for a suitable LED driver IC. I came across the CAT4106 (I understand that this can only drive 4 channels but I can use multiple of these if need be). From the specifications in the datasheet and the design notes given in the back I think I have a fair idea of how to design the structure but I have some questions.

    *I have attached the relevant datasheets*

    Question 1) - From the datasheet in the back on the section about LED1 - LED4 it says the following

    "These provide the regulated current source for driving each of the LED strings with a tightly matched constant current. To ensure optimal performance, the bias voltage on the LED channels should be set at a nominal 0.6 V or higher. Each channel is capable of driving a current up to 175 mA. All channels immediately enter a high impedance mode whenever the EN/PWM is taken LOW. "

    - How do I set the bias voltage to .6V

    Question 2) - From the datasheet this is what is said about CTRL Pin.

    CTRL pin is a multiplexer output which selects the lowest operating voltage appearing on any of the four LED output drive channels. This control signal represents the cathode terminal voltage of the LED string with greatest forward voltage (VF). An external resistor network from CTRL to FB can be used to set the lowest operating voltage of each channel. External current loading of the CTRL pin is recommended to be less than 25uA.

    - How I can select the external resistor loading to force the current to be less than 25uA?

    Question 3) - From the datasheet this is what is said about the VFMIN pin.

    VFMIN pin uses a pair of external resistors (R6 & R7) to program the worst case, minimum LED string forward voltage (VFMIN) expected in the specific application. If any LED string enters full regulation before the output voltage reaches VFMIN, the output will continue to rise to VFMIN where it will signal an LED short−circuit and a fault condition will be flagged. The detection only works during during power−up and works best when VFMIN is set at least twice the value of VL.

    - So R6 & R7 together set the forward voltage drop of a single LED string am I right? Similar can be said about R4 & R5.
    - How do I know the maximum voltage drop allowable along each string? 10 LED will require a typical voltage drop of 14.5V and consequently how do I know what the maximum voltage drop can be allowed for the combination of strings? I mean if each string uses 14.5V and I have 4 strings in parallel will I need a more powerful power supply?
     

    Attached Files:

  2. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Look on page #10 of the datasheet for details.

    answers:
    1. With R2 and R3 "ratios"

    2. With R2 and R3 absolute values (higher for low current).

    3.
    a) NO,it is only for fault detection in a LED string.
    b) You know about the max and min from the LEDs in the string.
    For max take the 1.6V(spec-ed) from the datasheet for min assume a value since it isn't spec-ed say 1.3v.
    you will get max=16V min=13V typ=14.5V
    If you want to create more margin for a fault condition, you can enlarge the max and lower the min.
    c) The device regulates the current as you set it.
    if you set 100ma than the total current of 4 strings is 400ma
    your supply(driving the LEDs) should be able to supply this ,
    so use a 500ma one.

    BTW,
    A 100mA LED isn't a low intensity one.
    Are you planning to pulse the LEDs or have them constantly on?
     
    Last edited: Mar 11, 2016
  3. ElectronicsStarter

    ElectronicsStarter

    3
    0
    Mar 10, 2016
    Hi Dorke,

    Thanks for your help. I think I understand it better now.

    The voltage drop across each string is determined by Vl and the Lowest LED pin voltage. So if I have 10 LED in series each with a voltage drop of 1.45V then the total is 14.5V. Allowing a bit of head room the total voltage drop across each string is 15V. Thus the lowest LED pin voltage should be 1V (assuming a Vl of 16V). From the equation given in "Setting the Channel Voltage" what is VFB? Is it .6V as I quoted in my original post?

    In selecting Vl I assume I must be considerate of the voltage drop across the inductor (L) and the schottky diode D. Is this correct?

    What do you mean when you say "c) The device regulates the current as you set it. if you set 100ma than the total current of 4 strings is 400ma your supply(driving the LEDs) should be able to supply this, so use a 500ma one" I thought the IC can provide this?

    To answer your question - I will be having the LED's constantly on. That being said I will also add a surface mount switch across each string so I can switch on/off each string as I see fit.
     
  4. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    "From the equation given in "Setting the Channel Voltage" what is VFB? Is it .6V as I quoted in my original post?"

    VFB is 0.3V(typ.) an internally regulated voltage.
    you use it to set the 0.6V needed(see below)

    "In selecting Vl I assume I must be considerate of the voltage drop across the inductor (L) and the schottky diode D. Is this correct?"

    No,
    You set the Vout of the LED lowest LED chains to be +0.6 above what is needed.
    With setting the FB(feadback) pin resistors and forget about the inductor and diode voltages.
    In order to get Vout being 0.6V above what is needed,
    (R2+R3)/R3 is set to be 2 (i.e 2*VFB=0.6 ;VFB=0.3V)
    A value of R2=R3=20K will satisfy this,
    See page 10 bottom left.

    "What do you mean when you say "c) The device regulates the current as you set it. if you set 100ma than the total current of 4 strings is 400ma your supply(driving the LEDs) should be able to supply this, so use a 500ma one" I thought the IC can provide this? "

    No,
    the IC doesn't supply the LEDs current it only regulates it.
    For 100mA current per led chain you need to set R1=1.24Kohm.

    The LEDs current is supplied by the VL power supply.
    Look at page 6 bottom right graph.

    BTW,
    What voltages do you intend to use for VL and VI?
    Do you have any limitations, like if you already have a P.S in your design?
    You need to take into account that the efficiency is dependent on both of them.
     
    Last edited: Mar 12, 2016
  5. ElectronicsStarter

    ElectronicsStarter

    3
    0
    Mar 10, 2016
    Hi Dorke,

    Thanks again. So I understand that V(FB) is an internal signal that is set tightly at .3V.

    When you say Vout of the LED chains to be +0.6V above what is needed you do mean the voltage at the cathode terminal of the LED that is closest to the LED pin am I right?

    So from looking at page 6 bottom right hand graph. I can see that if VL is about 10.5V I can get a channel current of 100mA. This suggests that Vout will be about 20V. Then if I want a 14.5V voltage drop across the string allowing .6V headroom this comes to 15.1V. What happens to the extra voltage in the string? The LED can tolerate about 1.6V maximum. I can set Vout (in the original equation) to be a lot higher than .6V can't I?

    I guess I would use 5V for VI and approx 12V for VL.
     
  6. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    The voltages Vi=5 and VL=12 are fine.

    But,
    you do have a problem with the LED here power disipation:
    You are too close to the PD=160mW,
    if you work the led constanttly on with 100mA!


    "When you say Vout of the LED chains to be +0.6V above what is needed you do mean the voltage at the cathode terminal of the LED that is closest to the LED pin am I right?"

    NO,
    I think this should be at the SW-pin from there a schottky diode drop to the LED chain.
    This isn't very clear from the datasheet but this is the way I interpret this.

    You need to remember that the device is a DC to DC boost converter not a linear regulater!
    i.e. It operates in Switched Mode.
    The internal Switched Mosfet is operated only in the on and off states, allowing for the 90% efficiency.

    "The LED can tolerate about 1.6V maximum."
    No,
    It can tolerate more,look at fig-4 of the LED data.


    The way this device works is somewhat
    "inverse than normal thinking":
    You set the current through the LEDs to be constant.
    100ma in your case.
    For that specific current each led in the chain will develop a corresponding voltage VF on it.


    About the VL vs LED current,
    well the graph has no data for the case of VOUT=14.5v.
    You can assume that it is "higher line" than that for the 20V case. effectively lowering the VL needed for a14.5V output.
    But,
    since you are going to use 12V for VL you should have no problem anyways.
     
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