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TPS2491 Current sense threshold with power limiting trip

Dennis Kruger
Prodigy 70 points
Other Parts Discussed in Thread: TPS2491

Hello,

I don't understand the V_CL 'current sense threshold with power limiting trip'.  Does this mean that if during initial power up the 'power limit engine' throttles M1 instead of just the current limit setting, then the current limit V_SENSE threshold will go from 50mV to 25mV.  If so, this could severly limit my initial power up current capability.  Could you provide more explaination when the V_CL specification comes into play.

Regards, Dennis

  • 0
    TI__Mastermind 42795 points

    Your understanding is basically correct. The V_CL spec in the datasheet lists two specific conditions for which the current limit sense voltage is 25mV. For these conditions, tthe TPS2491 is limiting MOSFET power to that programmed at the PROG pin (EQ 2 in the datasheet solved for Plim). For example, in case 1 (Vprog = 2.4V) the sense voltage starts at 25mV when Vout = 0 (48V across the MOSFET) and increases as Vout increases until the sense voltage plateaus at 50mV.

    Output capacitor charge time is dependent on which mode (power limit or current limit) the circuit is in. EQ 8 in the datasheet calculates charge time for both conditions (8A for power limit/current limit and 8B for current limit only). During Cout ramp up, some of that time can be spent in power limit mode (V_CL < 50mV) and some in current limit mode (Vsense=50mV) depending on the PROG pin setting. Generally when the TPS2491 is programmed for some power limit, EQ 8A should be used. This EQ accounts for the charge time for both modes of ramp up time. If the TPS2491 is programmed with no power limit (Vprog = 4V for example), then EQ 8B can be used. In most application circuits, the capacitor charge time will not be severely restricted by the mode of the TPS2491 unless very low PROG voltages are set (recommend Vprog > 400mV) or an extremely large amount of output capacitance along with an undersized MOSFET is used.

    A spreadsheet tool exists (on the web linked via the TPS2491 product) for which you can input your system variables, MOSFET, and will calculate recommended PROG pin resistor and TIMER capacitor values. This spreadsheet plots the application current limit profile (including the power limiting portion) vs. MOSFET voltage (Vcc-Vout) to illustrate the operation of the power limit engine. Also calculated is the output capacitor charge time (accounts for all circuit modes). The tool provides quick feedback of in circuit operation as system variables are changed.

  • 0 in reply to Eric Wright
    Prodigy 70 points

    Eric,

    Thank you for the explanation on V_CL and other operational aspects of the TPS2491.  It now makes sense to me what the data sheet V_CL specification implies. 

    Dennis