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TPS61390: Is an APD Boost Converter with 100µV Ripple Too Good To Be True?

Charles Gervasi
Expert 2270 points
Part Number: TPS61390
Other Parts Discussed in Thread: TPS61391

I am trying to make a boost converter for an Avalanche Photodiode (APD) for instrumentation applications.  The LT3482 datasheet has a graph showing 100µV ripple on the first page.  ADI/Linear Tech doesn't say what the voltage and current are.  

LT3482 Ripple

There is a TI app note by , TPS61390 Boost Converter with Low APD Bias Voltage Ripple, that shows ripple in the 5mV range at 100µA output.  

TPS51390 Ripple

I am almost certain the 100µV is too good to be true.  

My application is for instrumentation.  My APD is a multi-pixel photon counter (MPPC), an array of APDs in one package.  So my bias current will be very low.  I'm going to filter the bias voltage as much as possible.  Does TI have any app notes on low-noise boost converters, esp for low-noise APD applications?  

  • 0
    TI__Expert 5180 points

    Hi Charles,

    The APD bias voltage ripple of TPS61390 is less than 10mV with 4mA APD current. If bias current is very low, its voltage ripple will be lower. I think for your application, TPS61390, or TPS61391 if Sample / Hold function not needed, are most suitable. App note you mentioned I think is closest to application, there is one app note for APD Current sample How to Sample the Small APD Current Precisely With the TPS61390 Boost Converter

    Best Regards,

    Eric Yue 

  • 0 in reply to Eric Yue
    Expert 2270 points

    That's for responding quickly.  Why do APD bias power supplies have current mirror outputs?  Isn't the whole purpose of the transimpedance amplifier (blue arrow) to convert small currents into significant swings in voltage, i.e. detect photodiode current?

    Transimpedance Amplifier

  • 0 in reply to Charles Gervasi
    TI__Expert 5180 points

    Hi Charles,

    Current mirror output pins are used for accurately mirroring small APD current. Two pins represent two ratio options for the current proportional to APD current, MON1 (4:5) and MON2 (1:5), so the maximum sampling current through the MON2 pin is 4 times that of through the MON1 pin.

    Yes, I think the purpose of TIA is converting small currents into voltage for initial amplify.

    Best Regards,

    Eric Yue 

  • 0 in reply to Eric Yue
    Expert 2270 points
    Eric Yue said:
    Current mirror output pins are used for accurately mirroring small APD current.

    One feature of these current mirrors is the voltage at the pins is can be stored on internal capacitors, buffered, and send to the VSP pin.  

    I am still unclear why you'd want to do this instead of reading the output from a transimpedance amplifier. 

    My thought is you could use it to monitor bias current, but in this case you wouldn't need to sample it at a precise moment.

    The current mirrors have 15-25 µA of bias current on top of the current mirrored from MONIN to APD.  This 10µA of uncertainty can be significant, so even though the mirror can be sampled at a precise time, they're not a precise measurement of APD current.  

    Another question along this vein is why does Equation 6 in the datasheet say I[SHORT] = 100/R[SHORT]?  

    Isn't it a function of the resistor connected to MON1?  The diagram looks like it compares V[MON1] + V[CE] to V[ISHORT].  What is the value of the current source connected to ISHORT?  

  • +1 in reply to Charles Gervasi
    TI__Expert 5180 points

    Hi Charles,

    1. Current mirror output pins: please refer to this app notes for detail, How to Sample the Small APD Current Precisely With the TPS61390 Boost Converter. If output current cannot be measured precisely, this current mirror pins can used to measure.

    2. I think this current is dependent on external resister, the VCC is fixed instead of a constant current source and compare Ishort_ref with Isample. so this limit current is 100/R[SHORT].

    Best Regards,

    Eric Yue