Electronic – Question about LDOs and the practicality of this application

dcldomicrocontrollertexas instrumentsvoltage-regulator

I'll try and keep this as organized as possible so everyone can understand my thought process as easy as possible.

  1. I will be using a 12V 5A switching power supply (available on eBay) to convert my wall outlet power down to a workable voltage. That will connect to a DC socket on my board

  2. From there I need two different voltages: 9V at up to 3A for special LED's, and 5V at probably less than 1A for normal SMD LED's and a couple MCU's

  3. I was thinking of using TI's LM1804 (http://www.ti.com/lit/ds/symlink/lm1084.pdf) LDO to regulate the 12V DC input to 9V for the special LED's, and then the ON Semi NCP1117 to get it down to 5V for the MCU's and LED's.

  4. One of the other reasons I need an 8.5V+ supply is for the TI TPL7407LDR (http://www.ti.com/product/tpl7407l/description) to use as an LED driver for the special LED's. The IC datasheet makes it sound like it requires an input of 8.5V+ on the COM pin to be able to function at optimal speed and sink max current.

So here's what I'm curious about:

  1. Is using an LDO like this a smart idea?
  2. Would I be better off getting a 9V 4A switching DC adapter instead of a 12V 5A one (efficiency wise). I do need an excess of 3A as these LED's draw about 100mA's a pop, and I'll be running about 3A's worth currently. But I would prefer as much available current as possible due to future expansions that will actually draw up to 4-5A.
  3. Can I use the NCP1117 5V LDO in series after the LM1084 9V LDO?
  4. Should I need to use a rectifier diode before the inputs of the LDO's (for instance, like the Arduino does)?

I understand LDO's work by dumping the extra voltage and such as heat. I intend on using thermal vias and heatsinks. But in the case of the Arduino, for example, it takes a 9V to 12V 0.25A to 1A adapter and can safely and efficiently drop that down to 5V. So I feel like I've done a good job selecting components to stay as efficient as possible, and for not being a power expert, I feel confident this will work. I'd just like an outside opinion and some thoughts.

However, that TPL7407LDR is a tad confusing. If someone could clarify the COM pin requirements, I'd greatly appreciate it.

Thank you guys!

Best Answer

As I read the datasheet, the TPL7407LDR requires at least 8.5 volts on the COM pin - higher voltages are fine.

I would suggest a 9 volt switching regulator (or 12V->9V DC-DC converter) rather than a linear LDO. With an LDO, you will be dissipating about 12 watts in the 9 volt regulator if you start with a 12 volt power supply.

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