TPS40211: TPS40211

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.
 

Answer from TI:

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..

So now my doubt is, say if I am driving 10 LEDs at my output, Each LEDs Vf is 3V, so my 10LED is consuming 30V. If some of my LEDs (say 3LED) gets shorted means, my LED string will consume 21V, according to your reply, the balance 9V will drop in my RIFB resistor. Am I right?

As this RIFB pin is connected to the FB pin, the voltage drop in this resistor is fed to FB pin and compared with the 260mV reference of this IC's FB pin. According to this voltage difference, the error amplifier will produce some output and according this value, compensation is provided. And I will get the constant current at output. My understanding is correct?

Accordingly my  current sense resistor in the MOSFET leg will sense the MOSFET current (or the inductor current) and not the LED output current, and act according to that. 

Here my doubt is the ISENSE pin will be triggered when? for some LED( or 4 LED) short circuit or only at my complete String short circuit. Because for some 3 LED short circuit, my understanding is, the drop will happen at RIFB resistor and current will be maintained constant and there will be no trigger on the ISENSE pin.

So please advice on this.

RISNS is set for overcurrent threshold, means that resistor is detecting the overcurrent in what means? By sensing the inductor current, means the Boost controller input current? I can assume like if this input current is exceeded too much, then definitely my output (LED output) is exceeding the limit. Thats how it is detecting the overcurrent?