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TPS24750 TPS24751 Calculator Inputs?

LL
Prodigy 200 points
Other Parts Discussed in Thread: TPS25921A, TPS25942A, TPS24750

My Applications:

Application #1 Vin = +12V

Application #2 Vin = +5V

Nominal Load Current: 250 mA for both cases.

The IC is used to protect a system where DC is run up the center conductor of a coax cable to a remote active antenna.

Can you explain from the Excel calculators ?: Rload - start input . The default values in the high (10K) and low voltage (1 ohm) calculators are significantly different.


Locally there is bulk capacitance at the IC output, but the final  load is a significant distance from the IC. Does this  factor into component or IC settings in any way?

Can you explain what leads to "No start-up. Load to big" for the Tcap calculation. That is the case for the 5V version of the circuit with the parameters I entered.

Is there an FAE who could review the calculators and my schematic? The 5V version of the calculator for my application is pasted below.

Is the current monitor scaling fixed based on other inputs (i.e. not independent)? Ideally this would be independent to scale to the range matched with an A/D converter used to monitor the system.

Example: My Inputs

ILIM Current Limit (Recommended >10% over max load) 0.3 A
VBUS_MAX maximum input bus voltage 5.0 V
CLOAD Output load capacitance 47.0 uF
RLOAD-start Resistive load present at start up (not recommended) 1.0 Ohm
TAMB Maximum Ambient Temperature 50.0 ˚C
RJA Junction to ambient thermal resistance 40.0 ˚C/W
QB_FET Gate charge (Vg = 5.8V) of blocking FET. Enter 0 if no blocking FET is used. 000.0E+0 C
Rsns_max Maximum Sense Resistor 173.3 m-ohm
Rsense Sense Resistor 10.0 m-ohm
PLIM,MIN Minimum recommended Power Limit 1.5 W
Plim-typ Programmed power limit 3.0 W
Retry? Is this configured in retry mode? (Yes or No) Yes  
dV/dt? Gate slew rate control? (Yes or No) Yes  
Iramp Choose output capacitor charge current (<Plim/Vccmax) 0.2 A

Results:

  Basic Circuit Design Results    
Parameter Description Value Units
Tcap Time to charge output cap under Power Limiting No start up, load too big ms
T_FET Time to charge Vgs to Vt, and Vt to 5.8V once Vout is up 0.733 ms
T-dvdt Time for Vgate = Vbus+5.8, when using dv/dt start-up with C-gate 3.600 ms
T_total Typical Start Time 4.333 ms
T_fault Needed Fault time for margin to start-up time 6.500 ms
PDCFET Maximum steady state FET power dissipation 0.12 W
TJDCMAX Maximum Steady State Junction temperature 54.8 ˚C
SOA_25C Power FET can handle for T_fault @ 25C 36.366 W
SOA_der derate above for the eleveated junction temperature 27.696 W
SOA_mrg SOA margin 89.2 %
  Power Limit Pass YES  
  SOA Pass YES  

  Compute Required Circuit Values    
Parameter Description Value Units
Rset Choose Rset value 51.1 Ohm
Rimon1 Calculate Rimon value for current limit 11497.500 Ohm
Rimon Choose closest 1% resistor 11500.000 Ohm
Ilimit_act Actual I-limit value 0.300 A
       
Rprog1 Calculated Rprog value for Plim-typ 12497 Ohm
Rprog Choose closest 1% resistor 12400 Ohm
Plimit_act Actual power limit for Rprog 3.024 W
Cgatec Calculated gate capacitor to obtain inrush current = Iramp 3.3E-9 F
Cgate Gate capacitor used for dV/dt Control 10.0E-9 F
Iramp_act Actual Inrush (Ramp) Current for dV/dt Control 0.102 A
       
Ctimer1 Calculate fault timer capacitor value 48.1E-9 F
Ctimer Choose Ctimer >= Ctimer1 220.0E-9 F
Tfault_act Actual fault Time 29.7 ms
       
ENRise Circuit turn on voltage with rising Vin 4.5 V
OVRise Circuit turn off under over voltage event 5.5 V
R2 Target middle resistor 10000.0 Ohm
R3_c Calculated bottom resistor 45000.0 Ohm
R3 Choose R3 45300.0 Ohm
R1_c Calculated top resistor 129255.6 Ohm
R1 Choose R1 130000.0 Ohm
       
UVLO-R Actual turn on with rising input voltage (low voltage) 4.524 V
UVLO-F Actual turn off with falling input voltage (low voltage) 4.356 V
OVLO-R Actual turn off with rising input voltage (overvoltage) 5.522 V
OVLO-F Actual turn on with falling input voltage (overvoltage) 5.277 V
       
IMONsf Current monitor scale factor (I = Vimon x IMONsf) 0.444 A/V
  • 0
    TI__Intellectual 2800 points
    Hi,

    I would like to understand your system requirements before we can discuss about the xl sheet calculator. Can you please provide your system block diagram along with required features in eFuse. Looks like your nominal load current requirement is only 250mA.

    If that is the case, we have devices like TPS25921A, TPS259241, TPS259231.

    For low input voltage support, current monitor feature we have TPS25942A.

    Please let me know if any of these devices solve your purpose.

    Regards,
    Venkat
  • 0 in reply to Venkata Nandam
    Prodigy 200 points

    Hi Venkat:

    Since the nominal load current is 250 mA I headed toward the TPS24750 because it has a current limit min. that is low and permits setting the limit at, say, 300 mA. The other parts listed appear to have current limit min. values that are too high. The TPS24750 also has the most features - which we like. For example we like to have a built-in current monitor to feed out to an A to D.


    Again, the application is in line with a DC feed to a remote active antenna up a tower. The protection circuit would reside at ground level in the primary electronics assembly. The DC and RF are multiplexed onto a coax cable and that runs up a tower to an active antenna. If you wanted to prototype this or duplicate what might happen worst case in the real world, you get a 100 foot roll of coax, feed DC down the cable and put the load at the end. Then short the load and make sure the protection circuit did its job.

  • 0 in reply to LL
    TI__Intellectual 2800 points

    Hi LL,

    Please take a took at the TPS25942A, which has all the features required.

    Minimum programmable current limit is 0.45A. Let me know your opinion. Meanwhile I will take a look at your XL sheet questions.

    Regards,

    Venkat

  • 0 in reply to Venkata Nandam
    Prodigy 200 points

    Why would weuse the TPS25942A with a current limit minimum above the target  for our application when the TPS24750 fits the target current limit?

    I wish there was an EVB for the TPS24750 too.

    Do you have any more input for us?

  • 0 in reply to LL
    TI__Intellectual 2800 points

    Hi LL,

    I thought  the TPS25942A would be more relevant device for your application. Please go ahead and use the TPS24750. We do have EVM for this device.

    Links: www.ti.com/.../technicaldocuments

    www.ti.com/.../slvu968.pdf

    Sorry for the delay in reply, please see my feedback here:

    Can you explain from the Excel calculators ?: Rload - start input . The default values in the high (10K) and low voltage (1 ohm) calculators are significantly different.

    [VN] Design calculators are optimized for typical low voltage and high voltage applications. In low input voltage application, few customer have asked for load at start-up. You can enter your load resistance at start-up. For example if you wish to start the device with 250mA load, Rload-start = 12/0.25 = 48 for 12V application. Road-start = 5/0.25 = 20 in your low voltage application.

    Locally there is bulk capacitance at the IC output, but the final  load is a significant distance from the IC. Does this  factor into component or IC settings in any way?

    [VN] Is should not affect the device operation. IC can start charge the bulk cap without any issue. Depends upon distance to load, final output voltage decreases. If total wire resistance is 1ohm, final output voltage would be (Vin-IR drop)

    Can you explain what leads to "No start-up. Load to big" for the Tcap calculation. That is the case for the 5V version of the circuit with the parameters I entered.

    [VN] Error pops-up if set power limit or Rload-start is too low.

    Is there an FAE who could review the calculators and my schematic? The 5V version of the calculator for my application is pasted below.

    [VN] I can support until completing your project. Please send me your calculators to my email venkata.nandam@ti.com

    Is the current monitor scaling fixed based on other inputs (i.e. not independent)? Ideally this would be independent to scale to the range matched with an A/D converter used to monitor the system.

    [VN] No load to full load voltage range on current monitor output is limited to 0V to 0.675 (typ).  I understand the situation, flexible range selection is only possible with our other device TPS25942A. In TPS24750, current monitor output is also used for current limiting.

    See the updated XL sheet. I have only changed Rload-start to 20ohm instead of 1ohm.

    Parameter Description Value   Units
    ILIM Current Limit (Recommended >10% over max load) 0.3 A
    VBUS_MAX maximum input bus voltage 5.0 V
    CLOAD Output load capacitance 47.0 uF
    RLOAD-start Resistive load present at start up (not recommended) 20.0 Ohm
    TAMB Maximum Ambient Temperature 50.0 ˚C
    RJA Junction to ambient thermal resistance  40.0 ˚C/W
    QB_FET Gate charge (Vg = 5.8V) of blocking FET. Enter 0 if no blocking FET is used.  000.0E+0 C
    Rsns_max Maximum Sense Resistor 173.3 m-ohm
    Rsense Sense Resistor  10.0 m-ohm
    PLIM,MIN Minimum recommended Power Limit 1.5 W
    Plim-typ Programmed power limit 3.0 W
    Retry?  Is this configured in retry mode? (Yes or No) Yes  
    dV/dt? Gate slew rate control? (Yes or No) Yes  
    Iramp Choose output capacitor charge current (<Plim/Vccmax) 0.2 A
           
      Basic Circuit Design Results    
    Parameter Description Value   Units
    Tcap Time to charge output cap under Power Limiting 1.705 ms
    T_FET Time to charge Vgs to Vt, and Vt to 5.8V once Vout is up 0.733 ms
    T-dvdt Time for Vgate = Vbus+5.8, when using dv/dt start-up with C-gate 3.600 ms
    T_total Typical Start Time 4.333 ms
    T_fault Needed Fault time for margin to start-up time 6.500 ms
    PDCFET Maximum steady state FET power dissipation 0.12 W
    TJDCMAX Maximum Steady State Junction temperature 54.8 ˚C
    SOA_25C Power FET can handle for T_fault @ 25C 36.366 W
    SOA_der derate above for the eleveated junction temperature 27.696 W
    SOA_mrg SOA margin 89.2 %
      Power Limit Pass YES  
      SOA Pass YES  
           
      Compute Required Circuit Values    
    Parameter Description Value   Units
    Rset Choose Rset value 51.1 Ohm
    Rimon1 Calculate Rimon value for current limit 11497.500 Ohm
    Rimon Choose closest 1% resistor 11500.000 Ohm
    Ilimit_act Actual I-limit value 0.300 A
           
    Rprog1 Calculated Rprog value for Plim-typ 12497 Ohm
    Rprog Choose closest 1% resistor 12400 Ohm
    Plimit_act Actual power limit for Rprog 3.024 W
    Cgatec Calculated gate capacitor to obtain inrush current = Iramp 3.3E-9 F
    Cgate Gate capacitor used for dV/dt Control 10.0E-9 F
    Iramp_act Actual Inrush (Ramp) Current for dV/dt Control 0.102 A
           
    Ctimer1 Calculate fault timer capacitor value 48.1E-9 F
    Ctimer Choose Ctimer >= Ctimer1 220.0E-9 F
    Tfault_act Actual fault Time 29.7 ms
           
    ENRise Circuit turn on voltage with rising Vin 4.5 V
    OVRise Circuit turn off under over voltage event 5.5 V
    R2 Target middle resistor 10000.0 Ohm
    R3_c Calculated bottom resistor 45000.0 Ohm
    R3 Choose R3 45300.0 Ohm
    R1_c Calculated top resistor 129255.6 Ohm
    R1 Choose R1 130000.0 Ohm
           
    UVLO-R Actual turn on with rising input voltage (low voltage) 4.524 V
    UVLO-F Actual turn off with falling input voltage (low voltage) 4.356 V
    OVLO-R Actual turn off with rising input voltage (overvoltage) 5.522 V
    OVLO-F Actual turn on with falling input voltage (overvoltage) 5.277 V
           
    IMONsf Current monitor scale factor (I = Vimon x IMONsf) 0.444 A/V

    Regards,

    Venkat