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TPS61158: Measure LED current produced by TPS61158 with INA139

Part Number: TPS61158
Other Parts Discussed in Thread: INA139

Is it possible to attach a INA139 (Hi-Side Current Shunt Monitor) parallel to the feedback resistor of the TPS61158 WLED driver as shown below?

I'd like to use PWM for Dimming and need to measure the resulting current.But I do not know, if the INA139 sees a constant voltage or a rectangle or sawtooth waveform, when applying PWM with approx. 100kHz. I assume, that the additional resistive load from the INA139 does not disturb the regulation of the TPS61158.

  • Hi,
    The INA139 will see a constant volatge with some ripple. If you attach a INA parallel to the RFB, it will have effect on the control loop. The input impedence of INA139 will disturb the current setting.If you really want to monitor the current through the LED, you can move attach an extral resistor inserial with LED on high side.
  • INA319 has already an internal resistor of 1kohms. Does this really disturb the control loop of TPS61158? Its FB current is only 2uA and the FB resistor has 10ohms.

    An external resistor Rin+ on Vin+ input of INA139 has to be added in the calculation of the INA319, i.e. Vo=Is * Rs * RL / (1k + Rin+). On the other hand an external resistor would give me the possibility to add a low-pass filter to reduce the ripple for the measurement.

  • Hi ,

    It should be ok with this application. But You need to make sure the INA319 has enough input impedence. I suggest you to request an EVM to verify this application. Thanks.

  • I just find out, that my approach would not work, since INA139 requires at least 2.7V at its inputs.It is a hi-side monitor and thus needs a shunt between VOUT and the anode of the LED chain.