Question relating to current-sense amplifiers supply voltage rail

[David_]

Qestion relating to current-sens amplifiers supply voltage rail.

Hello.

I have been faced with the situation of having the need to derive a low voltage supply from a rather large voltage source(<=65Vdc) and I have been looking at the available current-sense amplifiers such as LTC6102, and they are apparently working with a large input-voltage range. They do not require a low-voltage supply and instead they seem to derive it's power supply from the high-side supply(this is in a high-side current sense situation).

The LTC6102 are sold in two versions, one ordinary 5-60V and one high-voltage(LTC6102HV) that can handle 5-100V, I'm curious about how that is happening,

is it a reasonable thought that such a amplifier would operate at a low voltage rail?,
how is such a high voltage managed without break-down, though probably that is due to which process is used while manufacturing which I don't know anything about and is probably above my head anyway.

The datasheet is showing a very simple illustration of the amplifier connected to a FET of some sort(it may say MOSFET but I can't remember that explicitly), I am interested in any information or insight into this sort of circuit.

Regards

 

[BradtheRad]

Re: Qestion relating to current-sens amplifiers supply voltage rail.

I made homebrew circuits for my 24V backup home power system. Two solar panels in series can put out 46V when unloaded. My charge controller was built around an op amp which cannot endure such a high voltage. So I regulated its supply down to a safe volt level, with a zener diode and resistor. It worked.

Perhaps a similar system is inside the IC you speak of?

 

[David_]

Re: Qestion relating to current-sens amplifiers supply voltage rail.

Hmm, it's possible I suppose. I never considered that a Zener diode would be a possibility but if the current consumption is low enough, I'll check.......

The maximum supply current listed in the datasheet for LTC6102 is 675µA, which would mean a power dissipation of:
LTC6102 - (60V-5V) * 675µA = 55V * 0,000675 = 0,064125W ≈ 64mW
LTC6102HV - (100V-5V) * 675µA = 95V * 0,000675 = 0,037125W ≈ 37mW

Last time I where looking at a Zener diode solution for a similar situation I needed to draw 100mA, which made the circuit requirements kind of unrealistic with many watts(6W) needed to be dissipated but with such a low current that seams perfectly viable.

I don't know, unrealistic might be an exaggeration but given the alternatives I doubt many people would go for a large Zener diode dissipating a massive amount of heat, 6W is a little larger than you would want to use a Zener diode for isn't it?

 

[KlausST]

Re: Qestion relating to current-sens amplifiers supply voltage rail.

Hi,

zener is a shunt solution. drawing more current than the max expectable circuit current. This maximizes power dissipation.
But with an additional resistor and a bjt you get a low quality voltage regulator. Drawing only a bit more current than your circuit actually needs.

Klaus