Please help me checking this design

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Hi guys,

I just did the test for my new-born sensor board, but I got a problem:

The power consumption is just too high to be correct. I brifly measured all the power supply lines resistance to GND and there is no short circuit, now I'm checking if there is any problem with my original design.

The included picture shows my power supply switching design which I suspect the most. This design can let the power supply be switched from battery to USB 5V automatically when the USB cable is pluged. The battery voltage should be <3.3V and the 5V USB power is first down regulated to 2.8V, because the 3.3V DC-DC step-up regualtors are used following this power switch design. A power N-MOSFET and a power P-MOSFET are used for switching. This is the first time I use power MOSFET.

Please help me look at this design, because I don't think the following DC-DC regulators and then loads can suck that much current.

Any ideas?

Cheng

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I might be way off, here, as my circuit analysis is far from over-developed, but...

Should the IRF7420 be connected the other way around? The source should be at the VBAT and the drain at VPROT. Also, I assume the MGSF2N02EL is there to protect the regulator from reverse voltage, but should it be a p-channel instead of n-channel?

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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Cheng,
Clancy may be on the right track, a P channel is the animal you would need. Incidentally, I was going to suggest taking a look at the Texas Instruments TL499. I have been raving about this chip. I think you might like it as it can do exactly what you are looking to do and much more.

Jim

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Question about the TL499...

What's the benefit to increasing RCL? Seems by the charts that it reduces the current capacity, but doesn't list any benefits. Or is just if you want to intentionally limit the current? Doesn't make sense to me, though, as the series regulator has a higher limit anyways, so your circuitry would only be limited if the series supply shut down.

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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hi, guys,

The IRF7420 was used for reverse battery protection. I saw this kind of P-MOSFET protection on application note of TI SLVA139. The power N-MOSFET MGSF2N02EL was used to protect TPS76728 from becoming Vout > Vin when USB power is not available. And I think it should be N-MOSFET there, because when USB_VBUS_5V is not connected to 5V, the gate of this MGSF2N02EL should be pulled down to 0V by the 10Kohm resistor, thereby blocking the V_UNREG_PROTECTED from connecting to OUT pin of TPS76728.

Please tell me if I'm wrong.

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If your battery is >2.8V, then when the USB is connected, you have 2.8V on V_PROT after the regulator and the N-Channel FET. Plus you have the battery running through the reverse protection diode (forwards) and into V_PROT as well. Do you ensure that the battery voltage is <2.8V? If not, you'll have current running backwards through the N-Channel FET into the voltage regulator I think...

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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hi, Clancy,

When USB is connected, the 5V is applied at both gates of N and P MOSFET, so IRF7420 should be cut and MGSF2N02EL should be conduct, right? So then as long as V_UNREG_BATTERY is not larger than 5V, there will be no current through IRF7420.

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hi guys,

I did some more test.

It seems this power switching is working. I tried with battery supply input 3V and USB input 5V, and I got the V_UNREG_PROTECTED the same voltage with both cases. And I also measured the voltages of the other power supply in both cases, and they are 0V. And I got 2.8V from TPS76728 when 5V supply at V_USB is used. So I think these P and N MOSFETs and TPS76728 are working.

So now I'm looking at the next stage: The DC-DC step-up regulator. I have included its schematic, and I have checked that its output now is loaded with maximum of just 1mA, so it shouldn't be the load which suck that 150mA out of the power supply.

I used 3 switching regulators, but 2 of them are almsot the same, they use TPS61070 and the only difference are the output voltage (hence the external feedback resistors difference), one is 3.3V and another 5.5V. And I have also meausred their outpus in both cases, they are always 0V, which is correct, because the 100Kohm pull-down resistors will disable them. So the only switching regulator which should work is MAX1724, it should output 3.3V in both cases, but the measurement shows that it's always a little bit below the voltage of V_UNREG_PROTECTED. In 5V USB case, it's 2.667V (< 2.8V), and in 3V V_BATTERY case it's 2.786V.

Can you guys check this DC-DC regulator?

Cheng

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Quote:

Question about the TL499...

What's the benefit to increasing RCL? Seems by the charts that it reduces the current capacity, but doesn't list any benefits. Or is just if you want to intentionally limit the current? Doesn't make sense to me, though, as the series regulator has a higher limit anyways, so your circuitry would only be limited if the series supply shut down.

Good question. I had not noticed that. I basically use the device because it offers the ability to have your choice of wal-wart and/or battery input and it will autoswitch between the two. The other nice thing is that it can step up battery voltage to some pretty high levels while supplying up to 100ma at reasonable levels from two AA cells.

I only recommended the chip based on what the OP's requirements are.

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

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Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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hehe, 1984?!!! Isn't TL499 a little bit old?

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hehe, it seems the MAX1724 can work, it's a load which suck too much current.

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just conformed that all the switching regulators are working.

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darthvader wrote:
When USB is connected, the 5V is applied at both gates of N and P MOSFET, so IRF7420 should be cut and MGSF2N02EL should be conduct, right? So then as long as V_UNREG_BATTERY is not larger than 5V, there will be no current through IRF7420.

No, because it's not acting like a gate in that configuration. It's acting like reverse polarity protection. Assume you eliminate the whole USB, N-Channel, regulator, R26, etc out of the circuit and just leave the IRF7420. Then if you connect the battery in reverse, you'll see that the gate voltage goes high in relation to the source and the P-Channel cuts off any reverse current.

However, in the forward direction, you're using the protection diode in a forward conducting manner, which is how it's designed to be used in the reverse protection circuit. That diode conducts irrespective of the gate voltage, so even when the USB is connected, it's still passing VBat (- diode drop) to VProt.

You're looking for a P-Channel connected the other way around, which would cut VBat->VProt current when the gate was high, as you described.

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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darthvader wrote:
hehe, 1984?!!! Isn't TL499 a little bit old?
If it works, don't fix it? =)

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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Quote:

darthvader wrote:
hehe, 1984?!!! Isn't TL499 a little bit old?

At least if the supply has an issue I am not looking to the internet for help in fixing it.

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Why the hell did you create a new post?

Do us a favor and stick with the one you start, and finish it. Don't start a new post because your original circuit sucked and instead of owning up to it, you instead do the political switch and reformat it into another post.

:x

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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I agree with Jim's Point of View on starting a new thread, and will merge these two threads

Nard

Edit: and done :)

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sorry, guys,

The reason for the new thread was that I don't know how to add new pics to the reply.

Clancy, you are right, The IRF7420 will indeed conduct the current from V_BATT to V_PROT if V_BATT is 0.7V higer than V_PORT, but since V_BATT shouldn't be larger than 3.3V, then 3.3 - 2.8 = 0.5V, so I think it's still ok, right?

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Quote:

The reason for the new thread was that I don't know how to add new pics to the reply.

Don't use the "Quick reply" area in the thread. Use the "new reply" button down left. You will then get a posting screen just like the one where you wrote your original post, and you can add images just as in the original post.

Also note that you can edit what you have already posted (but keep in mind that it is good "netiquette" not to edit in such a way that the thread becomes hard to understand, ie. removing text that is quoted in later posts, and indicating edited parts clearly, maybe with the label "EDIT:").

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ok, thanks

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Arr, that be a lot of capacitors...

Quote:
The power consumption is just too high to be correct.

What is the measured and expected current consumption?

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post