We need to design a led driver with preferably following specs:
- input voltage range: 2.4...4.5V form a single cell LiFePO4 battery (
- Continuous Led current <= 1.2A
- Dimmable (0..100%)
- 1 to 3 leds (3W) in series
What device would you suggest ?
Can the TPS6105x be used for this purpose. Apparantly the 1.2A output on these devices only functions in flash mode not in torch mode (as are most devices found in the product selection guide under this low input voltage condition). Can the flash current output be used in continuous mode and is it safe? How would that affect the efficiency and thermal dissipation of the device?
Thanks for your advice,
The TPS6105x cannot be configured for 1.2A torch mode.
For 1 LED, you can use the TPS63020 configured as Fig 2 or Fig 3 as explained here: http://www.ti.com/litv/pdf/slva419a.
For 2 LEDs in parallel not series, you can use the same configuration.
For 2 to 3 LEDs in series, I suggest the TPS43000 configured as a boost converter.
Thanks for your reply. When we use the TPS43000 as LED driver, I guess we could control the current through the leds using the FB pin, where the voltage on this pin is derived from the led current. How will thos affect the loop stability?
Secondly, we must be able to dim the LEDS by means of a microcontroller. What would be the best practice?
We also want to limit the maximum current throught the leds. What is your solution to this?
Another option that I thought of is the TPS40211 with a charge pump like TPS60150 to provide VCC for the TPS40211, i.e., the battery would provide power to the inductor of the TPS40211 and the charge pump and the charge pump would provide power to the TPS40211 IC. This combo might even be less expensive than the TPS43000. Regardless, the concepts in the TPS40211 apply to the TPS43000.
Regarding dimming, the TPS40211 datasheet figure 36 shows a way to do PWM dimming. Alternatively, you can do analog dimming as explained in the app note here: http://www.ti.com/lit/an/slva471/slva471.pdf.
Using the configuration in the TPS40211 datasheet, the circuit is designed to regulate to a maximum LED current and then dimmed. For additional protection, I recommend a fuse in series with the hi side of the battery.
Thanks for your effort.
The TPS40211 can not be used in our design, since this device operates from 4.5V where we need it to work from 1.8V on.
Using the TPS43000, I would like to 'fool' the comonent, in a way that I can generate an output voltage higher than the specified 8V (in fact I would need 9.6V = 3x the forward voltage of the LEDS). Is there a way to do that? (for instance by deviding the output voltage before it goes into the VOUT and VP pins?
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