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[moved] Can someone explain this Voltage Regulator

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xmen_xwk

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I have been looking for a 5V voltage regulator for <3A load. I have found some but one of them is LM1085IS-5.0. I do not understand this regulator, it is marked 3A output with 1.3V drop out, lets say I give it 7V as input. Now the 2V drop makes a 6W heat at 3A load. And since its Junction-to-ambient thermal resistance is 40.6 degree C/W. At 3A load it will be hot to 243.6 degree C. Thats way too hot, even over max Junction Temperature which is 150 C.


How is this voltage regulator suppose to work ?
 

You will use PCB copper area as a heat sink. See figure-29 (Heat sink thermal resistance vs area) of the datasheet.
 
You will need to use a heat sink- see page 21 of the data sheet. The junction-to-ambient thermal resistance of 40C is for without heat sink and with heat sink you need to recalculate the thermal resistance. If you are using the PCB copper as the heat sink, you can use their graph (Fig 29).
 

The junction-to-ambient (j-a) thermal resistance figure you cited is for a device that relies on its own body surface to dissipate heat to the surrounding air. The small body is rather inefficient for the job. Any device that has to dissipate appreciable power is supposed to be mounted on a heatsink. Depending on the power level, the heatsink may be a small area of copper on the PCB or a separate piece of aluminium or copper with a large surface area.

The device can transfer heat much more easily to the metal heatsink than to the air, and the large heatsink surface is more efficient at transferring heat to the air. In other words, the device case-to-heatsink thermal resistance is much lower than case-to-air, and the heatsink-to-air thermal resistance is also low due to its large surface area.
 

ahsan_i_h said:
You will use PCB copper area as a heat sink. See figure-29 (Heat sink thermal resistance vs area) of the datasheet.
Click to expand...

c_mitra said:
You will need to use a heat sink- see page 21 of the data sheet. The junction-to-ambient thermal resistance of 40C is for without heat sink and with heat sink you need to recalculate the thermal resistance. If you are using the PCB copper as the heat sink, you can use their graph (Fig 29).
Click to expand...

Pjdd said:
The junction-to-ambient (j-a) thermal resistance figure you cited is for a device that relies on its own body surface to dissipate heat to the surrounding air. The small body is rather inefficient for the job. Any device that has to dissipate appreciable power is supposed to be mounted on a heatsink. Depending on the power level, the heatsink may be a small area of copper on the PCB or a separate piece of aluminium or copper with a large surface area.

The device can transfer heat much more easily to the metal heatsink than to the air, and the large heatsink surface is more efficient at transferring heat to the air. In other words, the device case-to-heatsink thermal resistance is much lower than case-to-air, and the heatsink-to-air thermal resistance is also low due to its large surface area.
Click to expand...
True but which one I should use to calculate values if I'm using PCB board as heat sink. There are 6 of them at Page 4 > 6.4
Thermal Information.

Figure 29, also is bit confusing. How do I know my PCB heat sink thermal resistance value ?
 

The formula is given in pages 20-21. I guess it is bit confusing so I shall work out one example case. I will use the same variables.

1. total power: (Vin-Vout) -> 7-5 -> 2V; max I -> 3A; Ground current -> 0.01A (from datasheet); 2x3+7x0.01=6.07W (say 6 for calculations). This is Pd in eq 5 page 21.

2. Calculate max allowable thetaJA: -> Tjmax=150 (from datasheet); Tamax=40 (ambient temp within the box); Pd=6 (calculated above) -> (150-40)/6 -> 110/6 -> 20 (say).

3. Calculate the heat sink thetaHAmax: -> thetaHAmax=thetaJA (calculated above) -(thetaJC+thetaCH)=20-(thetaJC+thetaCH) -> 20-(take the value from section 6.4) -thetaCH (take as zero)

4. Use Fig 29 to calculate the PCB copper area: about 5000 mm2 for max value
 
c_mitra said:
The formula is given in pages 20-21. I guess it is bit confusing so I shall work out one example case. I will use the same variables.

1. total power: (Vin-Vout) -> 7-5 -> 2V; max I -> 3A; Ground current -> 0.01A (from datasheet); 2x3+7x0.01=6.07W (say 6 for calculations). This is Pd in eq 5 page 21.

2. Calculate max allowable thetaJA: -> Tjmax=150 (from datasheet); Tamax=40 (ambient temp within the box); Pd=6 (calculated above) -> (150-40)/6 -> 110/6 -> 20 (say).

3. Calculate the heat sink thetaHAmax: -> thetaHAmax=thetaJA (calculated above) -(thetaJC+thetaCH)=20-(thetaJC+thetaCH) -> 20-(take the value from section 6.4) -thetaCH (take as zero)

4. Use Fig 29 to calculate the PCB copper area: about 5000 mm2 for max value
Click to expand...

Thank you very much :thumbsup: and I have few more questions.

TA(MAX) is Junction-to-ambient thermal resistance. Right ?
ThetaJC has 2 values in section 6.4, top and bottom. Since the PCB heat sink is at below the bottom one value will be used ? what about the top value ?
 

xmen_xwk said:
TA(MAX) is Junction-to-ambient thermal resistance. Right ?
ThetaJC has 2 values in section 6.4, top and bottom. Since the PCB heat sink is at below the bottom one value will be used ? what about the top value ?
Click to expand...

TAmax is the max ambient temperature. Thermal resistances are denoted by theta.

It appears that the person who wrote the section is not the same one who compiled the data sheet values because the same symbols have not been used.

There is one place where I think a parenthesis is missing. Poor proof-reading / carelessness / or I am dumb.

I have noticed that the thetaJC (thermal resistance from junction to case) has two different values. I have no idea why two values are given but I suspect that they may be needed if you want to calculate the effect of air cooling...
 
Hi,

Did you read document SPRA953recommended in the datasheet.

Two thermal junction-case resistance:
One os for the back side. Itis a metal plate soldered to a PCB. But you could also mount it on a special heat sink pcb (ceramics and/or aluminum core, or water cooled).
The other is for the front side plastics. You may glue a heatsink on it. But the plastics thermal resistance is big, so it is not that effective.

Klaus
 
That makes sense. Thank you both.
 

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