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TPS92390: Can TPS92390 control small current like less than 1mA with analog dimming mode?

Part Number: TPS92390

Hello guys,

One of my customers is looking LED driver which can control small current like less than 1mA with analog dimming mode.

I think TPS92390 could drive the small current. The customer requirements are the follows.

 Input voltage: 5V
 Output voltage: 6.4V(2 LED)~12.8V(4 LED)
 Topology: Boost
 Channel #: 1ch (In TPS92390 case, They want to tie OUT1/2/3/4 together)
 Output current: 500mA(Max), ~ Less than 1mA(Min)
 Dimming method: Analog dimming (By continuous current)
 Current change period: Every 33ms (They want to change the output current every 33ms.)

Could you please give me your reply to the following questions?

Q1. Can TPS92390 meet these requirement and control small current like less than 1mA with analog dimming mode?
Q2. Can OUT1/2/3/4 be tied together?

Also if you have any other device can be recommended for their requirement, could you please tell me the device number? 

Your reply would be much appreciated.

Best regards,
Kazuya.

  • Hi Kazuya,

    Our expert is out of office for business trip and will reply you before 13th Aug. Thanks.

  • hi Kazuya

    1. TPS92390 can drive current down to 1mA

    2. it is ok to tie OUT1/2/3/4 together to get larger current rating.

  • Hi Mason, Hardy,

    Thank you very much for your reply.

    Could I ask you several additional questions as the below?

    Q1. They want to sink 500mA LED current as I wrote before. So OUT1/2/3/4 would be need to tie together. 
           When all output are tied together, Can TPS92390 drive LED current less than 1mA?

    Q2. Do you think what is the minimum current in this condition? 0.5mA? Or more less?

    Q3. Can TPS92390 turn on/off each output individually? 

    Q4. If Q3 answer is yes, which register can control each output turn on/off?    

    Thank you again and best regards,

    Kazuya.

  • hi Kazuya

    Q1, Q2: actually for analog dimming only, this really depends on required current accuracy, just as datasheet mentioned, to get good accuracy, better to let analog dimming current above 30mA. to get extreme low current, it is recommend to work in hybrid mode(analog + PWM). would you mind sharing why PWM dimming not preferred? some background information of this project will be helpful

    Q3: cannot be controlled individually. if independent control is required, LP8863 can be candidate.

     

  • Hi Mason,

    Thank you very much for the information.

    The customer wants to use analog dimming LED driver as a light source for video camera, still camera and medical scope. Those need to avoid flickering.
    So they don't want to use PWM dimming.

    Could I ask you a few questions as the below?

    Q1. When the maximum output current is set to 125mA by external resistor, RISET, I think the minimum output current is 125mA/4095=30uA
    by the minimum LED_CURRENT[11:0] register data though the current accuracy may be not good.
    Is my understanding correct?

    Q2. In case of "Current Dimming mode, are all brightness steps controlled by LED current not PWM?

    I understood 4ch output are con not be turned on/off individually.

    Thank you and best regards,

    Kazuya

  • hi Kazuya

    Q1: Actually, the understanding is not quite right, I think the 12bits register LED_CURRENT[11:0] is more of a factor reflecting resolution rather than min. current. therefore, if we set to like 10mA by external resistor, that does not mean we can get 10mA/4095=2.4us. 

    Q2: your understanding is right. Current Dimming mode means controlled by LED current.

  • Hi Mason,

    Thank you very much for your reply.

    Could I ask you the following questions as additional?

    Q1. TPS92390 datasheet (Ver.SNVSBY7) says on page 22 that ILED=(1.21V/RISET*2580)*(LED_CURRENT[11:0]/4095)
           So I think that when RISET is 24.9744kohm and LED_CURRENT[11:0]=FFFh, the output current is set to 125mA.
           Also I think that when LED_CURRENT[11:0]=7FFh, the output current is set to 62.5mA
           and when LED_CURRENT[11:0]=001h, the output current is set to 30uA as ideal number.

           Is my understanding correct?

    Q2. Your said at the top of this tread that TPS92390 can drive current down to 1mA.
          Does this mean that TPS92390 can drive current down to 1mA at one output? Or in case of four outputs tied together for 500mA driving?

    Q3. As you wrote, the device datasheet says that For high accuracy of LED current,
           the ILED current is recommended to set in range from 30 mA to 200 mA.
           Does this "30mA~200mA" mean ILED calculated by "1.21V/RISET*2580"?
           Or does it mean ILED calculated by "(1.21V/RISET*2580)*(LED_CURRENT[11:0]/4095)"?

    Thank you again and best regards,
    Kazuya.

  • hi Kazuya

    Q1: TPS92390 integrates both boost and current sink. ILED=(1.21V/RISET*2580)*(LED_CURRENT[11:0]/4095) is set for current sink, it is correct that when LED_CURRENT[11:0]=001h, the command for current sink is 30uA. real current will try to regulate at this value, but it is also limited by boost (EA offset, loop response, etc.)which can also be considered as source for current sink.

    Q2: I'm afraid this is for one output and already quite margin. customer can apply for one EVM to verify this.

    Q3: it means ILED calculated by "(1.21V/RISET*2580)*(LED_CURRENT[11:0]/4095).

  • hi Kazuya

    Based on my understanding, high dimming ratio is needed for video camera. would you mind sharing why? right now we have a device under development that also features high dimming ratio and fit for voltage/current requirement. Size will be smaller than this device, roughly 3*4. 

  • Hi Mason,

    Thank you very much for your reply.

    I will check the reason of the customer dimming requirement and I will inform you as soon as I get the information.
    Also I will tell the customer to evaluate the minimum analog current using EVM.

    Could I ask you one additional question as the below?

    Q.   Is the LED String current (mA) of the Figure 6-6 Current Linearity graph in TPS92390 datasheet meant the output analog constant current?
     

    Thank you again and best regards,
    Kazuya.  

  • hi Kazuya

    Figure 6-6 current linearity is for phase shift pwm mode and hybrid mode rather than pure analog mode. 

  • Hi Mason,

    Thank you for your reply.

    I think that in the both mode (phase shift PWM mode and hybrid mode), LED current is turned on/off periodically.
    But the vertical axis name of Current Linearity graph in  Figure 6-6 is "LED string current (mA)"

    What does "LED string current (mA)" mean?
    I think it mean a continuous constant current. Is my understanding correct?

    Thank you again and best regards,
    Kazuya.

      

  • hi Kazuya

    LED string current (mA) means average LED current for each channel. 

    You can see this figure shows the relationship between 16bits current command code to LED average current. 

    For  Current Dimming Mode, it is continuous constant current; for hybrid mode, continuous constant current for D>12.5%, pulse current (average current) for D<12.5%; for phase shift mode, pulse current (average current).

  • Hi Maison,

    Thank you very much for your reply.

    I understood that the LED string current (mA) means average LED current not continuous constant current.
    But the customer wants use analog dimming mode. Also they don't have TPS92390 EVM at this moment.

    So could you please measure TPS92390 continuous constant output current in analog dimming mode vs  LED_CURRENT[11:0] data?

    Is it difficult?

    Thank you again and best regards,
    Kazuya.

  • hi Kazuya

    I can check internally if there is any data available. Otherwise I can test manually a few points.  

  • Hi Mason,

    Thank you very much for your reply and your kindness.
    I'm looking forward to receive any data of output current in analog dimming mode.

    Thank you again and best regards,
    Kazuya.  

  • hi Kazuya

    no problem. I will get back to you when I have some results

  • Hi Mason,

    Could you please update the test status?

    Thank you and best regards,
    Kazuya.

  • hi kazuya

    I will share with you today or no late than tomorrow

  • Hi Mason,

    Thank you very much for your update.
    I'm looking forward to receiving the measured data.

    Thank you again and best regards,
    Kazuya.

  • hi Kazuya

    Sorry for causing misunderstanding. I've verified this on the EVM and for current dimming mode(analog dimming), the right logic is that LED current can be set by external resistor and 12 bits analog register. Based on this, current can be further scaled down by 16-bit brightness register. So the curve figure 6-6 is also valid for analog dimming. Ideally, this can be regarded as a 28-bit register for analog dimming, but limited by other factors, the min. current can not reach Imax/2^28. This only provides a flexible way to adjust analog dimming. 

  • Hi Mason,

    Thank you very much for your reply and strong supports.

    Could I ask you the follows
    Q1. Do you mean that the 12 bits analog register you wrote is LED_CURR_CONFIG Register shown as Figure 7-24?
    Q2. Do you mean that the 16-bit brightness register you wrote is BRT_CONTROL Register shown as Figure 7-23?
    Q3. Do you mean that the X axis of figure 6-6 graph is mixed with these two register data? 

    Thank you again and best regards,
    Kazuya.

  • hi Kazuya

    Q1: Yes

    Q2: Yes

    Q3: this is only for 16-bit brightness register as higher bit does not own any meanings either for resolution or min. current.   

  • Hi Mason,

    Thank you very much for your reply and strong supports.

    As the result, what is the minimum output current by LED_CURR_CONFIG Register control?
    Could you give me actual measured data?

    Thank you again and best regards,
    Kazuya.

  • hi Kazuya

    I will share with you this afternoon

  • hi Kazuya

    the minimum output current is around 0.4mA total (0.1mA per channel), the error accuracy is around 50% at this point.

  • Hi Mason,

    Thank you very much for your strong supports.

    Could I ask you one question about the result?
    Q. What are LED_CURR_CONFIG register and BRT_CONTROL register data when the minimum current is output 0.4mA(0.1mA per each CH)?
        Is the LED_CURR_CONFIG register data 004h or 003h?
       Is the BRT_CONTROL register data FFFFh?

    Thank you again and best regards,
    Kazuya.

  • hi Kazuya

    LED_CURR_CONFIG register data 002h

    BRT_CONTROL register data FFFFh

  • Hi Mason,

    Thank you very much for your strong supports and I'm sorry to be late my response.
    I'm still discussing about the minimum output current of TPS92390 with the customer base on your measurement result.
    I will ask on a new thread if we have any question about TPS92390.

    Thank you again and best regards,
    Kazuya.