Q: TPS61030 soft start switch current limit by 9312
I want to use the TPS61030 as the switcher IC for a constant current boost converter. Application is as a 2xAA (mainly NiMH) LED flashlight driver. Forward current envisioned is 1000mA. Vf of my LED can be as low as 3.2V at that drive current, or as high as 3.7V. I am somewhat confused regarding the effect of the 61030's soft start feature on such a boost converter running in full regulation. For my configuration, Vbatt is nominally 2.4V, Vout is 3.2V or 3.7V (depending on LED Vf), and Iout is 1A. If we assume that the 61030 efficiency under these conditions is 85% to 90%, then the current draw from my 2xAA cells could range from a best case (assuming Vf of 3.2V and 90% efficiency) of about 1.5A to a worst case (assuming Vf of 3.7A and 85% efficiency) of about 1.8A. During soft start, the 61030 datasheet states that the switch current is limited to 40% of nomimal. When the output voltage is reached, the regulator takes control and the switch current limit is set back to 100%. If I assume that the nominal 100% switch current limit is 3.6A (the min value, to be conservative), then the soft start switch current limit is 1.44A. If the nominal 100% switch current limit is the typical value of 4A, then the soft start limit is 1.6A. Finally, if the nominal 100% switch current limit is 4.5A (the max value), then the soft start limit is 1.8A. Comparing these these various potential soft start switch current limits to the estimated input currents of about 1.5A to 1.8A, what is the effect of soft start, if any at all, on my constant current driver's ability exit soft start and to reach full regulation? It would seem that for the switch current to get set back to the 100% nominal value, the output voltage must be reached. But to do so, it seems that the required current will exceed the soft start switch current limit, and thus the 61030 will not exit soft start mode. Am I missing something? Thank you. ]
A: Re: TPS61030 soft start switch current limit by 1090425
The TPS61030 cannot start up into a full load. If it could, it would put a huge load on the battery which when placed across the internal impedance of the battery would sag the input bus and cause bigger issues. Figure 12 in the datasheet shows the highest load that the TPS61030 can start up into. Your LED is a larger load than this.
Finally, your output voltage will be the LED's Vf + the drop across some current sensing circuit, usually a resistor. This voltage drop is usually about 200-500 mV, so the total Vout would then be higher, maybe over 4V. This is additional output power that must be delivered from the battery.
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