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LED Drivers/LCD Bias
TPS61043 Vout ripple
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Q: TPS61043 Vout ripple by 1038028
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.
Q: Re: TPS61043 Vout ripple by 10449
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.
I am currently working on a particular application where using 2 white LED as backlight, powered from 3V battery.
I am just wondering if TPS61043 is appropriate for this application (as it seems to be, at least from the datasheet).
My question is basically addressed to an AFE from TI (or anyone with experience on this device): I need to know if you have characterized this chip working with 2 white LED instead of 4.
An efficiency graph will be usefull, showing TPS61043 with 2 white LED, powered from 2V to 5V.
Thank you in advance for your quick support.
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