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TPS610985: Large current draw during startup.

Aaron Clements
Prodigy 30 points
Part Number: TPS610985
Other Parts Discussed in Thread: TLV3691

I have designed an RF energy harvesting circuit which uses the TPS610985. The rectifier device supplying the input voltage to VIN can supply a maximum voltage of approximately 1.2 V, and a maximum current of approximately 1 mA.

I have prototyped the design and found that the TPS610985 will not fully turn on with this device as its supply. I am basically observing the behavior described in the datasheet as follows:

" If the input voltage is not high enough, a low voltage startup oscillator operates the switches first. During this phase, the switching frequency is controlled by the oscillator, and the maximum switch current is limited. Once the converter has built up the output voltage V(MAIN) to approximately 1.8 V, the device switches to the normal hysteretic current mode operation and the VMAIN rail starts to supply the internal control circuit. If the input voltage is too low or the load during startup is too heavy, which makes the converter unable to build up 1.8 V at V(MAIN) rail, the boost converter can't start up successfully. It will keep in this status until the input voltage is increased or removed. "

When I replaced the rectifier with a DC power supply to test the circuit, I have found that the TPS610985 will draw up to 16.5 mA while the low voltage startup oscillator is operating the switches (measured VIN=750 mV, VMAIN = 1.77 V, ISUPPLY=16.5 mA, MODE= 0 V ).

Is this behavior normal? VMAIN on the boost converter is very lightly loaded and once it enters the normal hysteretic current mode the whole circuit only draws 20 uA from the DC supply (with VIN = 1 V).

I have the recommended inductor (4.7 uH) and capacitors connected to the TPS610985 as described in the datasheet (as well as the 400 Ohm resistor and filter cap on the input), and it works very well once in the hysteretic current mode - this high current draw during startup was unexpected.

  • 0
    TI__Mastermind 40080 points
    The expert for this part will reply you soon.
  • 0
    TI__Mastermind 46905 points
    Hi Aaron
    yes, we expect this behavior if the power supply is weak. could you try to reduce the output capacitor to 2.2uF level and check if this help.
  • 0 in reply to Jasper Li
    Prodigy 30 points
    Hi Jasper,

    Thanks for the response, I will have to find a work around.
    I changed the output capacitance to 2 uF and there was no noticeable change in the behavior.

    For interest's sake I also tried changing the inductor to 2.2 uH. This made the problem significantly worse - drawing up to 25 mA during startup.

    I did disconnect the load from the boost converter completely (with the original 4.7 uH inductor and 10 uF output cap), and in this case the startup current was only around 2mA at it's peak.
  • 0 in reply to Aaron Clements
    TI__Mastermind 46905 points
    Aaron
    does the boost successfully startup after disconnecting the load? you mentioned the input current capability is only 1mA.
    what is the loading? some MCUs may sink larger current when it initialize at first power up.
  • 0 in reply to Jasper Li
    Prodigy 30 points
    Hi Jasper,

    No, the RF rectifier does not provide enough power to start up the boost even when the load is disconnected.
    The loading on VMAIN is a TLV3691 nanopower comparator (with output=low), and a 976 k/ 187 k voltage divider. Current draw should be very low.

    We are open to suggestions for alternative parts which can run from this small rectifier and provide a larger voltage for use powering a load. Self-powered from this rectifier is the goal (our current circuit works fine once in the hysteretic mode, but can't get through the startup phase).

    Aaron.
  • 0 in reply to Aaron Clements
    TI__Mastermind 46905 points
    the device consumes larger energy during startup, especially when the input voltage is low. that is the why the TPS610985 can's startup in only 1mA input current.
    and we don't have other solution