max757 maximum voltage

[jeff1500]

The max757 chip is a voltage regulating chip. You give it one or two batteries and it gives you constant output voltage between about 3 and 5 volts according to the spec sheets.

LEDs like to run in series so they all get the same amount of current. Is there a way to get higher output voltage from a max757 or similar chip?

If you could do that, wouldn't that be a voltage regulated constant current device?

Five leds in series is about 5 x 3.6 = 18 volts.

How about a different inductor and re-setting the dividing resistor?

 

[ElektroLumens]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
The max757 chip is a voltage regulating chip. You give it one or two batteries and it gives you constant output voltage between about 3 and 5 volts according to the spec sheets.

LEDs like to run in series so they all get the same amount of current. Is there a way to get higher output voltage from a max757 or similar chip?

If you could do that, wouldn't that be a voltage regulated constant current device?

Five leds in series is about 5 x 3.6 = 18 volts.

How about a different inductor and re-setting the dividing resistor?<HR></BLOCKQUOTE>


If you want to get more voltage than 5 or 5.5 volts out, you need to go to another chip, as the MAX757 has a limit. I have used the LT1302, as it can put out higer voltage and current. The LT1302 has a higher dropout of 2 volts, and it can handle more current. Check out the specs on it.

You could also run multple LED's at lower voltage, in parallel, which is what I usually do.

The outstanding feature of the MAX757 is it is big, and easy to use and design circuits with. It had an upper limit in the specs of 200mA, but more current can be obtained by using more inductance and capacitance.

Wayne

 

[remuen]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:

LEDs like to run in series so they all get the same amount of current. Is there a way to get higher output voltage from a max757 or similar chip?

If you could do that, wouldn't that be a voltage regulated constant current device?
<HR></BLOCKQUOTE>

Hi again Jeff
A higher output voltage does not say which kind of regulation a booster has. It is the chip and its design. A voltage regulator does always try to keep a certain voltage without 'looking' at the current.

A current controlled booster looks only on the current and does only care for the voltage as a certain voltage is required to push the needed current through the load.

Look for the thread of bikeNomad with the LT1308B chip. There you will find an explanation on how to modify a voltage regulator into a current regulator. But it works only with a chip which is designed for a higher output voltage as eg. the LT1308B and the LT1613 from Linear Technology. As far as I know the MAX757 and most other Maxim voltage regulator chips are limited to an output voltage of about 5.5 volts. So you have for sure to use another chip.

Do you you remember my booster with the LT1613 chip in the Minimag thread? The LT1613 is also a voltage controller but in this case modified to a current controller in a similar way bikeNomad does it with the LT1308B. In my case the loss over the 'Rsense' was too high means a too a low efficiency. This can happen if you're modifying the circuit design.

 

[jeff1500]

LT1302

This is the LT1302 data sheet. Available in 8-pin PDIP or 8-pin SO packaging.

LT1308B thread

This is the LT1308B thread.

The spec sheet for max757 says absolute maximum output voltage of 7 volts.

The thing I really like about the max757 is that it's happy to run on just about any battery. The thing that annoys me is that I have six leds. Four of them like 3.5 volts and two of them like 3.7 volts.

I guess if I were to hook them all in parallel I could use a balancing resistor with a group of four and a group of two.

Let's see, V=IR, dV=IR, R=dV/I=.2/(4)(.02)=2.5 ohms.

Or I could build two lights, one with four leds and one with two.

 

[remuen]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
[QB
The thing that annoys me is that I have six leds. Four of them like 3.5 volts and two of them like 3.7 volts.

I guess if I were to hook them all in parallel I could use a balancing resistor with a group of four and a group of two.

Or I could build two lights, one with four leds and one with two.[/QB]<HR></BLOCKQUOTE>

Using resistors means wasting energy.

You could avoid the problem you're facing here with a current controlled booster where you are going to switch the LEDs in series. Then each LED would be driven with exactly the same current.