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12v DC to 5v DC 2.5 amps

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dranobob

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I'm installing a carpc and I want to run a powered USB hub. The AC adapter converts to a 5v dc 2.5 amps. Been many years since I've built electronics, but I understand the concepts. Trying to figure out how to step down the voltage and maintain the amps. I've read several posts on here, but none seem to require such high amps. This seems a simple problem, but I'm lost on how to do it.
 

dranobob

New Member
looks perfect, just double checking my thinking since its been a while since I read a data sheet. I'd leave the inhibit open, a 21kohm resister between the Vo adjust and ground. This will allow a 12v in and 5v out at 3 amps. What is the point of the sense pin?
 

crutschow

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As the data sheet states (pg. 5), the sense pin is for remote sensing of the output voltage to cancel the voltage drop caused by the resistance of the wire between the output of the regulator and the load. If you aren't concerned with that (it usually is small for reasonable wire resistance) then just leave the sense pin unconnected.

Be sure and add the required external capacitors on the input and output. They are needed for proper operation of the regulator.
 
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crutschow

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I'm gonna print the data sheet now that I'm work, do you recall if lists what size capacitor to use?
Data sheet, pgs. 19 & 20.
 

dranobob

New Member
k, so to be sure I read correctly, I am using this to power a USB hub using the 12v dc from a car, which of course isn't the cleanest of power sources.

I am planning on following the diagram on figure 30, page 18. Or should I do the extra filter (cap/inductors) as shown on page 21?
 

dranobob

New Member
okay, I'm lost, sorry to ask, but can someone tell what cap I should order, I have been using mouser to order stuff, but any site is fine, cause I'm reading these data sheets, and lost as to what caps I should use for my application

I mean or at least help me read the sheet. What is ripple current? What should the working current be? is this the minimum current the cap should handle ?

EDIT:
This is what I think I need
for Vo:
Mfr. #: C3225Y5V0J107Z
Desc.: High Capacitance SMD Ceramic Chip Capacitors 1210 100uF 6.3volts Y5V +80-20%

and
for Vi:
Mfr. #: C3225X7R1E225K
Desc.: High Capacitance SMD Ceramic Chip Capacitors 1210 2.2uF 25volts X7R 10%
 
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crutschow

Well-Known Member
Most Helpful Member
k, so to be sure I read correctly, I am using this to power a USB hub using the 12v dc from a car, which of course isn't the cleanest of power sources.

I am planning on following the diagram on figure 30, page 18. Or should I do the extra filter (cap/inductors) as shown on page 21?
The auto electrical environment is severe where an alternator load dump can produce spikes over 50V. The 78060 can withstand a maximum of 38V so it's possible it could be blown under worst-case conditions. To be totally safe you could add an input transient suppressor such as shown here EDN PDF pg. 64.

The The extra filter is to keep the 600KHz signal out of the input power line(which is probably only important if you play your AM radio).
 
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crutschow

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I mean or at least help me read the sheet. What is ripple current? What should the working current be? is this the minimum current the cap should handle ?

EDIT:
This is what I think I need
for Vo:
Mfr. #: C3225Y5V0J107Z
Desc.: High Capacitance SMD Ceramic Chip Capacitors 1210 100uF 6.3volts Y5V +80-20%

and
for Vi:
Mfr. #: C3225X7R1E225K
Desc.: High Capacitance SMD Ceramic Chip Capacitors 1210 2.2uF 25volts X7R 10%
Ripple current is the capacitor current due to the switching current from the power supply output. It's maximum is the maximum current you draw from the supply, but typically is much less.

The caps you selected are fine. But they are chip caps so they need to be mounted on a pcb. If you don't want to do a pcb for this (and I assume you don't) then caps with leads would be better.
 

crutschow

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the only problem with this circuit is it says it has a 1a output, I need up to 3a (actually 2.5 give or take)
It has a 3A output. Where do you see 1A?
 

crutschow

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LM2734Z Thin SOT23 1A Load Step-Down DC-DC Regulator shown on page 65
How did the LM2734Z get into the discussion?
Page 65 of what?
Do you have another thread you are posting too?
 

dranobob

New Member
The auto electrical environment is severe where an alternator load dump can produce spikes over 50V. The 78060 can withstand a maximum of 38V so it's possible it could be blown under worst-case conditions. To be totally safe you could add an input transient suppressor such as shown here EDN PDF pg. 64.

The The extra filter is to keep the 600KHz signal out of the input power line(which is probably only important if you play your AM radio).
Page 65, of using this circuit to help regulate the power spikes
 

crutschow

Well-Known Member
Most Helpful Member
Sorry, I'm a little slow today. After further looking at that suppression circuit, I realize it may not work for your application. I'll have to give it some further thought and get back with you.
 

dranobob

New Member
okay well i think I can handle the voltage spikes. Alternator spikes generally only happen during heavy load conditions, aka, starting. Well I have a simple solution, a 12v automotive relay. The power supply for the PC has a built in anti-thump feature. PCs at startup have a tendency to surge the audio channels, when if run to a audio amplifier, will produce a loud thump through the speakers. To counter this, you simply run the amp ON signal to the anti-thump lead and it waits 30 secs to power on the amp. This allows the PC ample time to boot up before the speakers are turned on. Since I can't use a powered USB hub until the PC is booted anyways, I can use a 12v relay to prevent power from getting to the 78060 until clearly after the car is running smoothly.

what do you think?

Anyone have any schematics to build a 12v relay capable of handling 50-75v spikes and allowing 3A? If not I'll just buy one from an auto parts store. But building one would be more fun.

also still wondering if the switching amperage has to be as large or larger than the amps that I will be pulling for my power supply?
 

crutschow

Well-Known Member
Most Helpful Member
Any 12V coil, 2A relay should work fine (the maximum 12V current to the converter is about 1.2A). I would never try to build a mechanical relay. Just buy one from an auto store.
 

dranobob

New Member
Here's what I'm ordering. I'm assuming this is all correct.

Desc.: Monolithic Radial Lead Capacitors 2.2uF 50volts X5R 10% 5.0mm L/S

Desc.: Radial Tantalum Capacitors, Bulk Packaging 6.3V 100uF 10%

Desc.: 1/2W 1% Metal Film Resistors 21Kohms 1% 50PPM
 
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