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Original question:

TPS92518EVM-878: Inductor calculation

TPS92518EVM-878: Output current is not getting generated on TPS92518EVM

Guruprasad Solapurkar
Prodigy 240 points
Part Number: TPS92518EVM-878
Other Parts Discussed in Thread: TPS92518

Hello,

We are using TPS92518 EVM in below configuration

1)VIN=24V, PWM mode, EN pin 24V applied (R1 is replaced by 0 ohms)

2) Load resistor 5 ohms connected.

3) PWM frequency-100 KHz 50% duty cycle, Voltage developed on J2 is 0.2V (means 400 mA current)

4) Even we apply 3.3V 0Hz to PWM1 still 0.2V is observed on J2.

What we want is by applying PWM duty cycle we want to program output current from 0 to 1.7A. Please let us know why this EVM is not working as desired.

  • 0
    TI__Mastermind 28655 points

    Hello,

    1)  First, EN at 24V may not be enough.  The typical value is 23.4V.  Are you measuring the voltage at the EN pin?

    2)  You can test with a load resistor however you may find that the operation isn't what you expect.  For instance at 5 ohms 400 mA the output voltage would be near 20V.  The TPS92518 regulates peak current so if the design has high enough current ripple it won't switch, the MOSFET will just turn on and there will be 4.8A in the resistor.

    3)  100 KHz PWM is too high, there is not enough time for the inductor current to ramp up and down though there should be some output.  (do you mean 40 mA?, 0.2V/5ohms = 40 mA).  Have you looked at any signals on the board such as the switch node and inductor current with an oscilloscope?

    4)  Is there current actually flowing to the load?

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 240 points

    1) We have measured voltage at EN pin it is showing 24V.

    2) We have connected 5 ohms and we expect output current programmable from 0 to 1.7A, so expected  output voltage is 0 to 8.5V

    3) Yes I mean output current is 40 mA, (5 ohms*40 mA=0.2V).

    4) I have not checked yet with Multimeter how much output current is flowing, I will check it.

    5) Can you please tell me relation between PWM duty cycle and output current? Like duty cycle 0%, output current 0 mA, if duty cycle is 100% then output current is 1.7A etc.

  • 0 in reply to Guruprasad Solapurkar
    TI__Mastermind 28655 points

    Hello,

    First, I'm off by a factor of ten for output voltage on number 2 above, 400 mA should give 2V.

    EN at 24 may not be enough to put all TPS92518s in the non-SPI run mode, 23.4V is typical.  You should be able to tell by looking at the switch node to see if it is trying to run.

    You really need to look at the signals such as the switch node with an oscilloscope to see how long the switch node is high and how long the switch node is low.  You are also trying to run at low output voltage when your current is low.  This can put it into maximum off-time mode.

    Best Regards,