Hi everyone,
I'm looking for some information on how LED tail lights traditionally regulate the battery's 12 V signal or ideas on how to solve my specific problem. I'm looking to regulate the battery's voltage at a current of 1.2 Amps while still having a low dropout voltage (<=1.5 V). (My apologies if the answer is easy, I've been researching linear and switching regulators for too many hours straight so my brain is a little fuzzy, I'm going to take a break for a couple of hours to clear my mind!)
My projects involve sequential LED turn signals therefore requiring a lot (I'm using 12 pairs) of LEDs in parallel. Total current drawn (1.2 Amps) by the 12 pairs of LEDs is too high for linear regulators but most DC/DC switching regulators have too high of a dropout voltage to output 10.5 V for my LEDs when the engine is not running (my TI LM22678-ADJ buck converter needed close to 13 V to output 10.5 V but the battery's voltage is lower than that when the engine is not running). I pulled open a set of non-sequential LED tail lights and it looks like all they use is a diode, a transistor, and a capacitor without a controller..could I make something simple like this? I assumed I absolutely needed a regulator in order to always illuminate the LEDs at the same brightness and protect against voltage spikes, or is there another option?
More information: I'm designing my project to survive the absolute worst case scenario:
-ambient air temperature of 60 C (a car interior can reach this temp in the sun);
-peak current of 1.2 Amps with average of 0.6 Amps (HIGH for 350 ms, LOW for 350 ms, repeat);
-input voltage of 14.5 V (engine running);
-output voltage of 10.5 Volts (the LEDs' forward voltage);
-maximum regulator junction temp of 125 C
The thermal resistance for the regulator I use needs to be under about 30 C/W while still satisfying the characteristics I just mentioned. Don't get me wrong, I'm not asking you all to to find a regulator that works for me. Instead I'm trying to give as much detail as possible to illustrate my difficulty. Most linear regulators either can't handle the input and output voltages I'll be working with or can't handle the current.
My previous plan was to use LEDs with a forward voltage < 3 V and a forward current of 250 mA. The plan was to use 12 pairs of LEDs in parallel, each pair itself was two LEDs in series consuming 250 mA and dropping a total of about 7 V (3.5 V each LED). My DC/DC step down converter had too high of a dropout voltage and couldn't handle the power dissipation anyway despite its very good thermal resistance of 22 C/W. My new LED design cuts the power dissipation by more than half but the 2W is still too much for many linear regulators.
Thanks in advance for any help.
Tom
I'm looking for some information on how LED tail lights traditionally regulate the battery's 12 V signal or ideas on how to solve my specific problem. I'm looking to regulate the battery's voltage at a current of 1.2 Amps while still having a low dropout voltage (<=1.5 V). (My apologies if the answer is easy, I've been researching linear and switching regulators for too many hours straight so my brain is a little fuzzy, I'm going to take a break for a couple of hours to clear my mind!)
My projects involve sequential LED turn signals therefore requiring a lot (I'm using 12 pairs) of LEDs in parallel. Total current drawn (1.2 Amps) by the 12 pairs of LEDs is too high for linear regulators but most DC/DC switching regulators have too high of a dropout voltage to output 10.5 V for my LEDs when the engine is not running (my TI LM22678-ADJ buck converter needed close to 13 V to output 10.5 V but the battery's voltage is lower than that when the engine is not running). I pulled open a set of non-sequential LED tail lights and it looks like all they use is a diode, a transistor, and a capacitor without a controller..could I make something simple like this? I assumed I absolutely needed a regulator in order to always illuminate the LEDs at the same brightness and protect against voltage spikes, or is there another option?
More information: I'm designing my project to survive the absolute worst case scenario:
-ambient air temperature of 60 C (a car interior can reach this temp in the sun);
-peak current of 1.2 Amps with average of 0.6 Amps (HIGH for 350 ms, LOW for 350 ms, repeat);
-input voltage of 14.5 V (engine running);
-output voltage of 10.5 Volts (the LEDs' forward voltage);
-maximum regulator junction temp of 125 C
The thermal resistance for the regulator I use needs to be under about 30 C/W while still satisfying the characteristics I just mentioned. Don't get me wrong, I'm not asking you all to to find a regulator that works for me. Instead I'm trying to give as much detail as possible to illustrate my difficulty. Most linear regulators either can't handle the input and output voltages I'll be working with or can't handle the current.
My previous plan was to use LEDs with a forward voltage < 3 V and a forward current of 250 mA. The plan was to use 12 pairs of LEDs in parallel, each pair itself was two LEDs in series consuming 250 mA and dropping a total of about 7 V (3.5 V each LED). My DC/DC step down converter had too high of a dropout voltage and couldn't handle the power dissipation anyway despite its very good thermal resistance of 22 C/W. My new LED design cuts the power dissipation by more than half but the 2W is still too much for many linear regulators.
Thanks in advance for any help.
Tom