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TPS61043 Vout ripple

Dan Tooth
Mastermind 19585 points
Other Parts Discussed in Thread: TPS61043, TPS61042

Hi

What is responsible for the very different Vout ripple seen in Figures 13 & 14 of the d/s?

The operating conditions look similar in terms of LED current, pk SW voltage. The d/s doesn't have anything to say about "Burst Mode" as in Figure 14. (Figure 13 labelled PFM Mode I would say is also a sort of burst mode.) And yet one gives high Vout ripple of 1Vpkpk and the other mode gives about 50mV.

My customer has L=4u7 and Cot=1uF and gets high ripple. So he changes Cout to 10uF and it is much better. I assume that this hysteretic type converter is fine with 10uF for Cout.

  • 0
    TI__Mastermind 36110 points

    TPS61043 datasheet figure 13 and 14 most likely have different component values.  The switch on-times, output voltage, and input voltage appear to be identical, so the inductor values should be the same.  Figure 14 was probably tested with a larger output capacitor which would be consistent with lower output voltage ripple and the "burst mode" operation.  This is an LED driver, so the output voltage ripple is typically not the main concern in the design.  The preferred output capacitor is the small 1uF output capacitor because is produces the most cost effective design with acceptable output voltage ripple.  If you want lower output ripple, you can use larger capacitor values.

    The "burst mode" operation refers to the situation where a single TPS61042 or TPS61043 switch cycle does not increase the output voltage enough to get the FB pin above it's hysteresis point and requires two or more switch cycles to make this happen.  When the FB voltage drops below the nominal reference voltage, the IC starts a switch cycle.  If at the end of the switching cycle, the FB voltage is above the hysteresis value (typically 25mV-50mV above the nominal feedback voltage), the IC stops switching and waits for the FB voltage to drop below the reference voltage before starting again.  If the FB voltage doesn't go above the reference voltage plus the hysteresis, the TPS61042 immediately starts another switch cycle. 

    If you require a lower output voltage ripple, you can operate with a larger output capacitor.  This will not damage the IC.