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TPS61220EVM-319

Other Parts Discussed in Thread: TPS61220EVM-319, TPS61220, TPS61200

We replaced the resistors on the TPS61220EVM-319 with R1=1600 ohms and R2=510 ohms. We used a Keithley 2400 as a voltage source of 2V and a compliance current of 5 mA, which should have returned an output voltage of around 2V, but  with the compliance current being 5 mA we are somehow only putting in 500 mV and getting out 650 mV. 

  • Hi Namita,

    Would you please tell me what is your output load conditon? In your applicaiton, if there is no load, only the divider resistor R1 and R2 has already consumed 1mA current (if the output voltage is around 2V as you wish). So maybe your Keithley setting 5mA compliance is too small.

    BR

    Gene

  • Hello, Gene -

    I made some more detailed measurements on a TPS61220 EVM in the factory condition (Vout = 3.3) with no load. It appears that the device requires a substantial initial current surge during startup. The Keithley was set to source 1 Volt and the current compliance limit was adjusted to estimate the startup current. Before each change the supply (Keithley) was turned off and the EVM's output capacitor was discharged via a momentary short circuit.

    At 64mA current limit the Keithley was supplying 285mV at the input of the EVM. With the compliance set to 64.5mA the Keithley was no longer in current limit and was showing a current of less than 300 microamps at 1 volt (no load on the EVM output.) With a 3.3kOhm load on the EVM output (1mA output current at 3.3V) the converter would only start up if the current limit was above 105mA.

    Our application is a digitally-coded RF beeper that runs on a very low duty cycle, powered from a small triple-junction PV cell. In the current configuration it only operates in fairly bright sunlight conditions. We wish to extend its operating regime to lower light levels with a minimal increase in the size of the PV cell, which is where adding a boost converter would seem to be an appropriate solution. Mass and physical size are tightly constrained. Are there any TI boost converters with less demanding startup characteristics? If not I'm thinking we'd need a large input capacitor plus a voltage supervisor to keep the converter turned off until the input voltage is sufficient (>0.7V?)

    Thanks

    Rich

  • Hi Rich,

    Yes, TPS61220 does  require a surge current when starting up, which can be refered to the datasheet. Though the peak current is large, the duration time is short. So a large input capacitor may help to overcome your startup prolem.

    By the way, what is your expection of the peak current during startup and what is your load condition?

    BR

    Gene

  • Hi Gene,

    I took some more measurements on our EVM with the Vout set to 2V, all with no load and with a 470uF shunt capacitor at the input to the EVM.  Steady state current with EN floating or grounded is Iin = 0.68mA with Vin = 2V, Vout = 1.43 (no load).  With EN high (2V) Iin = 1.16mA, Vout = 2.1, no load.  Both input current levels seem awfully high relative to IQ (<1uA) and ISD (<0.5uA) quoted in the data sheet.

    The load is intermittent - average current 17mA at 2V for 2.5msec every 1 or 2 seconds.

    At the light levels we're interested in, the small PV cell we're using as a power source has ISC approx.1mA and VOC approx. 1V.  Is there a way to hold the TPS61220 in the shutdown state when Vin is below 0.7V?

    Thanks

    Rich

  • Hi Rich,

    The large input current with noload is probably due to the input 470uF capacitor. If using a electrolytic capacitor, the leakage current can be very large. Please try a ceramic cap.

    When the input voltage is lower than 0.7V, it will trigger the UVLO of IC. If you want to hold the IC when Vin is very low, TPS61200 has a lower Vin of 0.3V.

    BR

    Gene

  • Hi Gene,

    I removed the 470uF tantalum cap and there was no measurable effect on the steady state input current to the EVM.  The main trouble is that I’m seeing  no-load input currents on the order of a milliamp - roughly a thousand times larger than what’s specified in the data sheet.

    Rich

  • Hi Rich,

    What is the value of feedback resistor R1 and R2? As mentioned before, if you still use R1=1600ohm and R2=500ohm, with 2V Vout, only the resistors would consume nearly 1mA current. Please use the feedback resistors with the sum larger than 1M ohm (R1+R2>1Mohm) and try again.

    BTW, datasheet only specify the quiescent current. It seems that you are more interested in no load current. It's two different concept. No load current should be larger than quiescent current.

    BR

    Gene