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LT4294

     Hello! Please explain to me how the resistance in the upper arm is 174k, in the lower arm 52.3 K on the example of the DC2583A debugging Board circuit.

  • Hello,

    Please see Figure 7. in the LT4294 datasheet. The top resistor sets the auxiliary mode hysteresis, the difference between when the LT4294 enters and exits auxiliary mode. Rtop x IAUXH = VAUXHYS, with IAUXH = 5uA typical. Rtop should also be sized so that the auxiliary pin does not sink too much current at the max auxiliary voltage. AUX pin max current sink is 1.4mA, and the VAUXT threshold is 6.3V typical, so Rtop < (VAUX_MAX - VAUXT) / 1.4mA. The bottom resistor sets the turn off threshold, which is Rbottom = Rtop / [(VAUX_OFF / VAUXT) - 1]. The turn on voltage is VAUX_OFF + VUAXHYS.

    Best Regards,
    Eric

  • Hello, Eric! If look at the picture top, which parameters corresponde to the voltage 37 and 57?  

    37V is this VAUX_OFF?

    57V is this VAUX_MAX?

    Rtop<(VAUX_MAX-VAUXT)/1.4mA - I got Rtop<36214R (36K)

    On the scheem is 174K

  • 57V is VAUX_MAX. This equation states Rtop must be greater than 36k. I recommend setting VAUX_HYS to greater than 0.5V. On the evaluation board, VAUX_HYS was set at approximately 0.9V, so 0.9V / 5uA == 174k Ohms rounded to the closest 1% resistor. There was a typo in the above equation for Rbottom which I have fixed, again please refer to Figure 7. in the datasheet. Rbottom on the evaluation board is 52.3k, which sets VAUX_OFF at 28.2V, and VAUX_ON at 29.1V. This accommodates the requirement that VAUX_ON must be lower than VHSOFF.

    Best Regards,

    Eric