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TPS2490: POWER LIMITING and CURRENT LIMITING

Sam Ho
Prodigy 10 points
Part Number: TPS2490
Other Parts Discussed in Thread: TPS1653, TPS2640

Dear TI team,

For the TPS2490 hot swap controller

I have 4 questions

1. In page 6 of theTPS2490 datasheet, there are two spec for "power limiting and current limiting (sense)". One is V(CL) which is said with power limiting and is 25mV.typ. The other is V(sense) which is said without power limiting and is 50mV. So when I use power limiting feature, I should the V(CL) spec of 25mV?

2. When power limit feature is used, PROG pin can be set to 0.4V to 4V through a voltage divider (R3, R4) on Vref. When power limit feature is not used, PROG should be tied to Vref through a 47kohm resistor which is 4V. So when PROG is set to 4V, how do I know whether it is a power limit or not? If I do not know it is a power limit or not, I do not know whether to use V(CL) spec of 25mV or to use V(sense) spec of 50mV for current limit.

3. In the TPS249x_8x_Design_Calculator_REV_B excel sheet. It does not have an option to choose between 25mV and 50mV of V(CL), V(sense) spec in step 2. only one 50mV for current limit calculation which is for without power limit.

4. in the cell F67, comment says compare with min. power limit on cell F46, but cell F46 is current limit. please check.

Best Regards

Sam

  • 0
    TI__Guru** 105361 points

    Hi Sam,

    Welcome to E2E!

    Sam Ho said:
    1. In page 6 of theTPS2490 datasheet, there are two spec for "power limiting and current limiting (sense)". One is V(CL) which is said with power limiting and is 25mV.typ. The other is V(sense) which is said without power limiting and is 50mV. So when I use power limiting feature, I should the V(CL) spec of 25mV?

    Rakesh-> Power limit engine uses 25mV threshold 

    Sam Ho said:
    2. When power limit feature is used, PROG pin can be set to 0.4V to 4V through a voltage divider (R3, R4) on Vref. When power limit feature is not used, PROG should be tied to Vref through a 47kohm resistor which is 4V. So when PROG is set to 4V, how do I know whether it is a power limit or not? If I do not know it is a power limit or not, I do not know whether to use V(CL) spec of 25mV or to use V(sense) spec of 50mV for current limit.

    Rakesh-> Use equation (2) in page 12 for power limit setting

    Sam Ho said:
    3. In the TPS249x_8x_Design_Calculator_REV_B excel sheet. It does not have an option to choose between 25mV and 50mV of V(CL), V(sense) spec in step 2. only one 50mV for current limit calculation which is for without power limit.

    Rakesh-> Design calculator uses 25mV for CL setting and then equation (2) in page 12 for power limit setting

    Sam Ho said:

    4. in the cell F67, comment says compare with min. power limit on cell F46, but cell F46 is current limit. please check.

    Rakesh-> can you please share the design calculator with your system inputs to review and suggest.

    Best Regards

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 10 points

    TPS249x_8x_Design_Calculator_REV_B_.for PAGA.xlsx

    The calculator is using Vsense = 50mV for current limit calculation, in the case of power limit, it should be 25mV. Then it turns out all the following calculation could have got wrong.

  • 0 in reply to Sam Ho
    TI__Guru** 105361 points

    Hi Sam,

    Correction in my last statement. Design calculator uses 50mV for CL setting and then equation (2) in page 12 for power limit setting

    I see your current is just 2.2A. so, you can use integrated solutions such as TPS2640, TPS1653

  • 0 in reply to Rakesh
    Prodigy 10 points

    If power limit is use, should we use V(CL) = 25mV instead? Then the current limit would be 1.1A, not 2.2A?

  • 0 in reply to Sam Ho
    Prodigy 10 points

    and the power limit is also cut by half?

  • 0 in reply to Sam Ho
    TI__Guru** 105361 points
    Sam Ho said:
    If power limit is use, should we use V(CL) = 25mV instead? Then the current limit would be 1.1A, not 2.2A?

    Even if power limit mode is used, you have to use V(CL) = 50mV for setting the overload current limit threshold. 

    Please note current limit is active for overload in steady state where SENSE pin voltage VSENSE = OUT pin voltage VOUT.

    Power limit is active only during startup. The current rises as per PLIM/(Vds * Id), so the drop across Rsense is not constant. 

    25mV is the value where we characterized the power limit accuracy but in actual case the V(CL) changes during startup as you see in the below test waveform.

  • 0 in reply to Rakesh
    Prodigy 10 points

    I am not sure I understand correctly:

    1, V(CL) = 25mV is for current limit on steady state, i.e. when Vds is low?

    2. use Vsense = 50mV for P.lim and I.lim at startup?

    My question:

    In section 7.4.3:

    1. Based on what information the P.lim is set to 34W? By measurement? Or by setting the I.lim to 0.7An first?

    Thanks

    Sam

  • 0 in reply to Sam Ho
    TI__Guru** 105361 points

    Hi Sam,

    Please refer Section 7.3.2 for current limit setting and Section 7.3.7 for power limit setting. same is followed for the design example in Section "8.2 Typical Application"

    Sam Ho said:

    In section 7.4.3:

    1. Based on what information the P.lim is set to 34W? By measurement? Or by setting the I.lim to 0.7An first?

    Rakesh-> There is no calculation for this 34 W. Figure 14 is captured with 34W and is example value for test result. Please refer design example in Section "8.2 Typical Application"

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 10 points

    Thanks for you prompt reply.

    My other question is when to use VCL = 25mV? It does not mention in the datasheet if I am not wrong.

  • 0 in reply to Sam Ho
    TI__Guru** 105361 points

    Hi Sam,

    VCL=25mV has no relevance in the system design. 25mV is the value (just data point) where we characterized the power limit accuracy 

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 10 points

    Could you kindly explain the spec. above? It caused a bit confusing to me. Confused

  • 0 in reply to Sam Ho
    TI__Guru** 105361 points

    Hi Sam,

    Let me explain you with an example: Vin =48V;  Current limit is 10A and power limit = 240W

    1. Using V(CL) = 50mV gives RSNS = 50mV/10A = 5 mΩ
    2. Now, using equation (2) to set power limit during startup. To set 240W PLIM, we need to set VPROG = 240W/ 10 x 10A = 2.4V

    During power into short, i.e., VOUT = 0V and Vin = 48V; the current through the FET is PLIM/ Vds = 240W / 48V = 5A

    --> this gives Vsns = RSNS * current through the FET = 5 mΩ *  5A = 25mV

    The power limit 25mV has variation from 17mV to 33mV i.e., ±32% accuracy. So, in this example, if you set 240W PLIM, it can vary 240W±32%

    As you see above, 25mV has no role in design but it gives an indication on PLIM accuracy. I have intentionally, selected 240W of PLIM to get VPROG of 2.4V spec item.

    Hope this is helpful.

    Best, 

    Rakesh