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TPS40211: current mode control and overcurrent protection

Albin Raj selvaraj
Prodigy 10 points

Part Number: TPS40211
Other Parts Discussed in Thread: TPS40210

I am using this IC TPS40211 in my project.

Marked R1 resistor, connecting to ISNS pin. ISNS Pin is sensing and controlling overcurrent, right?

How this happen when LED are gotten short circuited? or what is really doing R1?

For example, i am driving 400mA current through LED. This IC have Overcurrent protection.
When my all or some LED got short circuit how the IC sensing.
 

  • 0
    TI__Mastermind 19715 points

    Hello Albin,

    Thanks for reaching out to us via e2e.

    Frist of all it is important that the LED string will be connected as shown in the image you have attached.
    In a regular boost-stage you would use a feedback divider, which would supply a fraction of the output VOLTAGE to the FB pin.
    Instead, in this constellation here, there is the RIFB resistor in series with the LEDs. This way, the LED CURRENT will cause a voltage drop on the RIFB resistor which will be fed into the FB pin.
    This way, the controller will not regulate the output voltage, instead it will keep the LED current stable.

    Please note: This concept will require that the load current (LED current) will flow through this sense resistor.
    To keep the necessary voltage-drop low and consequently minimize the losses, please make sure that you are using the TPS40211 with a feedback voltage threshold of 260mV instead of the TPS40210 which would require 700mV.

    The current sense resistor R1 cannot directly see the LED current, instead it is regulating and also monitoring the current through the low side FET.

    There are two regulation mechanisms going on:
    - A fast, cycle-by-cycle current limit for the inductor current (voltage drop on R1, ISNS pin)
    - A slower regulation loop, controlled by the FB pin. (voltage drop on RIFB).
    Usually this would keep the output voltage constant. In this constellation it will keep the LED current constant.

    On top, if the current measured via the ISNS pin exceeds the overcurrent threshold, the part will enter hiccup mode as a protection mechanism.

    So, when an LED gets shorted, the output voltage will go down, to keep the LED current (voltage drop on RIFB) constant.

    If your load requires too much current (e.g. LEDs completely shorted this may shift the duty cycle so much (Low side FET turned on for a long time, inductor current ramping up a lot), that the overcurrent mechanism (voltage drop on R1) may also get triggered.
    R1 will also help to detect a case, where e.g. the inductor gets shorted.

    Best regards
    Harry

    All information in this correspondence and in any related correspondence is provided “AS IS” and “with all faults” and is subject to TI’s Important Notice (www.ti.com/.../important-notice.shtml).