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TPS92513 Ripple Current Requirement

Niq
Prodigy 50 points

From section 9.1.1 of the datasheet;

"The peak inductor current is used in current mode control and to maintain a good signal to noise ratio it is recommended that the peak-to-peak ripple current is greater than 75 mA for dependable operation."

For my application, I need to be able to handle nominal LED string currents down to around 100mA. A 75mA ripple seems quite significant when the nominal current is only 100mA. I was wondering if the 75mA applies only at higher currents (since this IC is rated for currents up to 2A), and can be relaxed for lower nominal currents?

Does the ripple current requirement depend on the value of the current-sense resistor that is used? For instance, a higher sense resistor value would give a larger voltage ripple at the ISENSE pin for the same current ripple. Can the ripple requirement perhaps be expressed in terms of volts at the ISENSE pin, instead of LED current?

Any clarification that can be provided on the ripple requirement would be much appreciated! Thanks.

  • 0
    TI__Mastermind 25500 points
    The 75mA ripple current is sensed in the IC so it is not related to sense resistor value. But that is switch/inductor ripple current, you can use an output capacitor and the LED ripple current can be as small as you want to make it.
  • 0 in reply to Clinton Jensen
    Prodigy 50 points

    Thanks Clinton. After looking more closely at the datasheet I now see that the IC is sensing current through the internal FET. I understand now why the ripple current is specified independently of the current-sense resistor value... and why the driver still works with large capacitors across the load :) 

  • 0 in reply to Clinton Jensen
    Prodigy 50 points

    Hi Clinton, 

    I find that when I add a capacitor to the output to smooth the ripple current, the circuit starts to whine. The whine only occurs while applying a PWM to dim the light. If I remove the output capacitor, the whine goes away. Any suggestions for dealing with this?

  • 0 in reply to Niq
    TI__Mastermind 25500 points

    Ceramic capacitors can do that, especially with high ripple currents. Sometimes just changing the case size will do the trick, or often using two rather than one works, like replacing a 1210 with two 1206 caps. They also make low noise ceramic caps but they cost a little more. You could also use a tantalum or aluminum electrolytic cap since they do not have piezoelectric properties like ceramics do.

  • 0 in reply to Clinton Jensen
    Prodigy 50 points
    I was using a 1210 ceramic capacitor. I replaced it with a tantalum, just to confirm that the noise was coming from the ceramic. With the tantalum capacitor the circuit is silent. I'll investigate using multiple smaller ceramics, as this might be more cost effective. Thanks for the info.