# I need more power!

Discussion in 'Electronic Design' started by [email protected], Mar 20, 2007.

1. ### Guest

Hi!

I am trying to design a printed circuit board with quite basic
functionality. (My first one!) The main part of it is taking pulse
width signals, passing it to buffers which then goes to LED's.

The system voltage is 3.3v with a power supply of 3.7v

I took the LED's to the lab and ran some tests on them. When you give
them the appropriate voltage, around 3.5v they draw 20mA of current.
So naturally I assumed at around 3.3v they would draw somewhat less,
right? WRONG! they draw upwards of 40mA.

I can't find a single buffer that will give me more then 100mA per
chip, with a max of 50 mA per pin at 3.3v. There are ton of chips
that would work if I could supply it with 4.5 source, but the max
voltage from the battery is 3.7v.

I've looked at using the buffers to power a transistor but with my
limited math skills the transistor won't turn on because I need to
give it more than 3.3v! everything seems futile without more
voltage. There has to be a way to draw more power (current) from a
source without increasing the voltage!

Any suggestions at what I should look at?

Thanks a ton!

Matt

2. ### Guest

current devices, not voltage. You *did* put a resistor in series with
the LED?

3. ### Guest

Yes and no, a resistor is in series with the red LED, though to
calculate the resistor I looked at the voltage the data sheet
required. Then using the expected current figured it out from
there... I.E. for the red part it wants a voltage around 2.0V. So I
put a 65 hm resitor in series which drops it 1.3v at 20mA. This
seemed to work.

For the blue in green they want a voltage of around 4.0v for a 20mA
current. As I only have 3.3 I just connected that straight. That
resistances for those?

Thank you for the information on the fet's I will look into that!

Matt

4. ### HawkerGuest

I assume a ULN2003 will give you what you want,
however in a similar situation I would probably find me a pile of SOT-23
logic level FETS instead. Go to Fairchild Semi and look around for
SOT-23 logic level FETS. Finding good 3.3 ones is slightly hard. Best to
find a 2.5v one and use that so you know it is fully on.

Hawker

5. ### Guest

Alright, Thank you very much! I will go back and do some more tests.
One last general question about Tri colored LED's. Is it normal for
RED to be barely visible in comparison? To make white I have to
basically not Pulse RED, and then BLUE and GREEN have to be nearly
off. It is also possible that I damaged the RED by applying to much
voltage at first.

Thanks
Matt

6. ### Noway2Guest

You need to look at the part data sheet, which will show you an I-V
(current - voltage) curve. The curve will show you what the forward
voltage drop for the device at a given current level. So if for
example, you have a 3.3V supply and want 10mA to flow through the diode,
you find the 10mA line on the chart and the corresponding part on the
curve, which will then tell you the forward voltage at that current
level. To continue the example, say at this point the forward voltage
is 1.3V. Then the resistance you need is (3.3V - 1.3V) / 10mA = 200 Ohms.

Similary, a look at the datasheet should provide some insight as to what
happened when you put 3.3V across the diode, assuming you didn't run
into a physically limiting feature.

With the diodes, you need to be sure you don't exceed the maximum
forward current, least their life will be very short.

7. ### ehsjrGuest

Since all you want to do is light LEDs at ~ 20 mA,
you can use a small boost DC-DC converter chip, like
a TPS61070. The datasheet shows you how to hook it up.

Ed