Negative power supply for ICs

Discussion in 'General Electronics Chat' started by parawizard, Jul 23, 2009.

  1. parawizard

    Thread Starter Member

    Jul 23, 2009
    10
    0
    I am looking to build a high amperage LED Driver circuit like the examples in this datasheet below. I would be looking to make a 2A -> 20A driver so I will need a decent power supply to go AC/DC first. I do not understand how/where the drivers draw the 20 Amps. I am just starting the ebook now l;)

    Is it off -12v? I don't think its supposed to be off the logic voltage?

    If I were to run off a 60A 12v Switching power supply -> LED Driver I would need a -12v rail with sufficient amps? Or a different IC with postive voltage?



    http://www.national.com/ds/LM/LM3433.pdf
     
  2. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    The -12V rail is propably used for biasing internal transistors in such a way that they can be switched on/off fast enough. Since this chip uses a step-down (buck) switching mechanism. This chip is switching at a rather high frequency, 200-500khz.
     
    Last edited: Jul 23, 2009
  3. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    You would need a 12V supply capable of 20A. You need to connect the positive output of the 12V supply to ground and the negative output to -12V in the LM3433 schematic. The main current is sourced by the 12V supply from ground, to LED, to sense resistor to Q3 and Q4 pair and then to -12V. Q1 and Q2 conduct when Q3 and Q4 are off. All these transistors should be capable of conducting 20A.

    The circuit is a current source (sourcing a negative current), with the output transistors being switched by a controller with a frequency of 200-500 kHz. Q5 and Q6 pair in Figure 2 is used to dim the LED. They are switched with a low frequency signal. This pair should also be capable of conducting 20A.

    The 5V supply in Figure 2 is a low power supply, which is used for logic and internal circuitry (see the block diagram on page 3).
     
  4. parawizard

    Thread Starter Member

    Jul 23, 2009
    10
    0
    So Even though its documented inverse to what I am used to seeing. All the amperage still comes from the +12v rail?
     
  5. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    Yes.

    Ground is just a convention. You can set it to the negative rail or to the positive one, as in this example. Since this circuit uses a power supply with the ground on the positive rail and another power supply with the ground on the negative rail, make sure these power supply outputs are isolated from chassis.
     
  6. parawizard

    Thread Starter Member

    Jul 23, 2009
    10
    0
    1) Would a single power supply with both rails not work then?

    2) Also I think some of the confusion came from the switching power supply I was going to use having a +3 +12 -3v and -12v. The negative rails had very very low amperage.
     
  7. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    It would, but it has to be -12V/20A and +5V some tens of milliamps. Quite unusual, right? Better have two power supplies.
     
  8. parawizard

    Thread Starter Member

    Jul 23, 2009
    10
    0
    It looks like I would be better off using a Computer power supply and a different IC with a positive 12v polarity then I guess! :)

    Thanks for all the help. Good forums. Hopefully I can help others also ;)
     
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