Pitfalls of series pass transistor over linear regulator?

Thread Starter

johndeaton

Joined Sep 23, 2015
63
Hi all-

One of the engineers at my work is suggesting changing from a LDO to a series pass transistor for voltage regulation as a cost reduction. What are some pitfalls to doing this other than the obvious ones such as less accurate, no short circuit protection, no thermal protection, etc.

Thanks
 

ian field

Joined Oct 27, 2012
6,539
Hi all-

One of the engineers at my work is suggesting changing from a LDO to a series pass transistor for voltage regulation as a cost reduction. What are some pitfalls to doing this other than the obvious ones such as less accurate, no short circuit protection, no thermal protection, etc.

Thanks
Your linear regulator *CONTAINS* a series pass transistor. NPN emitter follower in a standard positive regulator, LDO uses a PNP with its collector feeding the load - this configuration eliminates the Vbe overhead.
 

Thread Starter

johndeaton

Joined Sep 23, 2015
63
Hi Ian,

Thanks for your reply. Yes, I understand that the LDO contains a series pass transistor. I guess my question is really why not always use only a transistor which is cheaper than a packaged LDO.

Thanks
 

ian field

Joined Oct 27, 2012
6,539
Hi Ian,

Thanks for your reply. Yes, I understand that the LDO contains a series pass transistor. I guess my question is really why not always use only a transistor which is cheaper than a packaged LDO.

Thanks
Its not cheaper after you add all the other components. Less solder joints = less man hours and less connections to fail.

The 3-terminal variety all include current limiting and thermal shutdown.
 

crutschow

Joined Mar 14, 2008
23,544
It takes added circuitry to control the series pass transistor.
How can the sum of those additional parts, plus assembly cost, be cheaper than using an LDO?

I think that "engineer" doesn't know what he's talking about.
He sounds more like a bean-counting manager than an engineer. :rolleyes:
 

ian field

Joined Oct 27, 2012
6,539
It takes added circuitry to control the series pass transistor.
How can the sum of those additional parts, plus assembly cost, be cheaper than using an LDO?

I think that "engineer" doesn't know what he's talking about.
He sounds more like a bean-counting manager than an engineer. :rolleyes:
There can be good reasons for using discretes - but I'd have to think about it...............
 

Thread Starter

johndeaton

Joined Sep 23, 2015
63
It takes added circuitry to control the series pass transistor.
How can the sum of those additional parts, plus assembly cost, be cheaper than using an LDO?
Actually, the NPN transistor and zener diode are cheaper than the LDO (by pennies, of course). To your point, assembly may add some cost, but they only focus on BOM cost.
 

crutschow

Joined Mar 14, 2008
23,544
.... the NPN transistor and zener diode are cheaper than the LDO (by pennies, of course).
It also requires at least one resistor.
And that circuit will have much poorer regulation and voltage output accuracy.
But if pennies are important and the poorer performance of the zener-transistor circuit is tolerable, and there's no possibility of accidentally shorting its output, then you may have no choice but to bow to the bean counters. :rolleyes:
 

ian field

Joined Oct 27, 2012
6,539
It also requires at least one resistor.
And that circuit will have much poorer regulation and voltage output accuracy.
But if pennies are important and the poorer performance of the zener-transistor circuit is tolerable, and there's no possibility of accidentally shorting its output, then you may have no choice but to bow to the bean counters. :rolleyes:
Every once in a while you find a chip that's cheaper than the minimalist discrete solution.

Its more luck than anything - but there's a few out there.
 
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