Author Topic: lt3080: how did they achieve 350mV dropout?  (Read 1619 times)

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Offline exeTopic starter

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lt3080: how did they achieve 350mV dropout?
« on: January 30, 2016, 11:47:36 am »
Hi!

I'm curious how can lt3080 have just ~350mV dropout under full load(1.1A). Any ideas? The datasheet says it's a npn-regulator with a separate Vcontrol pin that consumes up to 30mA (needs to be pulled +1.2V above Vin or dropout will be much bigger). According to the datasheet, it supplies internal opamp that drives BJT. So... Does this mean they use some good NPN transistor with very low Vce(sat)?

A 30mA consumption for the opamp and 1.1A max output gives me ~37 hFE (assuming all this current drives the pass element). I tried to find a similar part in digikey, but there are just a few parts with such a low saturation dropout.... Did I miss something? Or may be I'm wrong on how lt3080 works inside?
 

Offline MagicSmoker

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Re: lt3080: how did they achieve 350mV dropout?
« Reply #1 on: January 30, 2016, 12:42:58 pm »
The key is in this blurb from the datasheet:

Quote
The LT3080 has the collector of the output transistor connected to a separate pin
from the control input. Since the dropout on the collector (IN pin) is only 350mV,
two supplies can be used to power the LT3080 to reduce dissipation: a higher
voltage supply for the control circuitry and a lower voltage supply for the collector.

Vcontrol is the supply terminal for the error amplifier and reference and if it is at least ~0.6V higher than the voltage at the pass transistor collector ("In") then the minimum dropout at Vout is Vce[sat].

 

Offline exeTopic starter

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Re: lt3080: how did they achieve 350mV dropout?
« Reply #2 on: January 30, 2016, 08:34:57 pm »
The key is in this blurb from the datasheet:

Quote
The LT3080 has the collector of the output transistor connected to a separate pin
from the control input. Since the dropout on the collector (IN pin) is only 350mV,
two supplies can be used to power the LT3080 to reduce dissipation: a higher
voltage supply for the control circuitry and a lower voltage supply for the collector.

Vcontrol is the supply terminal for the error amplifier and reference and if it is at least ~0.6V higher than the voltage at the pass transistor collector ("In") then the minimum dropout at Vout is Vce[sat].

Thank you, that what I was looking for.
 

Online T3sl4co1l

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Re: lt3080: how did they achieve 350mV dropout?
« Reply #3 on: January 31, 2016, 07:56:11 pm »
Vce(sat) goes down to < 0.1V for sufficiently large transistors.  It's likely the transistor takes up more area than, say, the main output device in an LM317, for this reason.

Tim
Seven Transistor Labs, LLC
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