Hello all. Does anyone have an idea on how to build a constent current (1A) driver for such a big white led (Vf=3,5v). I need to ligh it from 2 NiMh batteries (2,2v). I have already tried a TPS61020 from TI
Olivier, Italy.
such
but
According to the datasheet, the TPS61020 provides only about
350 mA. Take a look at the TPS6130, 31 or 32Ed
The Luxeon V will drop over 6 volts or do at 700 ma. average, 1 A peak. The K2's will peak around 1.5 amps.
greg
I can't find that in the datasheet. At page 2 there is a table that says OUTPUT NOMINAL SWITCH CURRENT LIMIT 1500 mA for the TPS61020DRC.
Olivier
I use the K2 white LEDs at 1A for thermal dissipation problems.
Olivier
Have you read the datasheet with respect to thermal considerations? Looks like they expect the pad to be mounted against a heat sink.
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The easy answer is here, in TI's literature:
The answer that requires a little more effort is understanding the information in the datasheets. I'll try to explain what you can find there regarding output current and how you can use the information.
First, read the description on the first page for both the 61030 family and the 61020 family. The 30 family description states "... is capable of delivering output currents of up to 1A at 5V at a supply voltage down to
1.8 V". The 20 family description states "Output currents can go as high as 200 mA while using a single cell alkaline, and discharge it down to 0.9 V. It can also be used for generating 5 V at 500 mA from a 3.3-V rail or a Li-Ion battery." If you interpolate the numbers shown, with 2 V input you get about 350 mA out using the TPS61020.There's more to be found when you look into the circuit and the datasheet numbers. You need to consider worst case (battery voltage at ~ .9 v per cell) and use that in your design, and you need to understand the whole datasheet. Look at figure 1 (page 7) on the datasheet. It shows the maximum output current vs the input voltage. Note that with lower input voltage you get lower max current, and that at
1.8 volts input, it is no where near 1 amp - you can get a maximum of about 400 mA. You want to allow a little extra, so that's about 350 mA.Another interesting specification is efficiency. The 61030 is over 80% efficient at 1.8 volts input with a 1 amp load, while the 61020 drops below 50 % at that voltage, even if the load was only about 350 mA. That means that over 50 percent of the energy taken out of the battery becomes heat. In your circuit, it is even worse, because you are boosting the voltage, while the curve is using Vin = Vout. It's no wonder your 61020 is getting hot!
Ed
OUTPUT
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