TPS61040 with wrong V_FB

Hi,

I'm using regulator TPS61040 to control the led backlight exactly as in figure 22 on page no 16 of the datasheet.
I've changed the Rs to 57R6, because I need I_LED = 20 mA, and I've fixed EN in 3,3V to test.

The problem is that when Vin changes, Vout changes too.

I've measured FB's voltage and it's changed too. To Vin = 5V => V_FB ~0,55V and to Vin = 3V => V_FB = 0,45V

I've understood that FB's Voltage would be fixed and equal 1,23V.

During my tests, I noted that if Vin <=2,2V, V_FB goes to 1,23V. (In this situation V_EN has been bigger than Vin)

I don't understand why this behavior.

In my aplication, I will work with 3,3V <= Vin <= 6V.

Can someone please help me?