simple switcher constant current supply with high side sense resistor

[Tim Mitchell]

I'm trying to design a simple switcher constant current circuit (using
the LM2574-ADJ). It's easy enough if you put the sense resistor between
the load and ground. However the snag is that the current sense resistor
has to be on the high side of the load (there are 3 current supplies
driving 3 devices with a commoned ground).

Does anyone know of a clever way of doing this without getting into
differential amplifiers?

 

[budgie]

I'm trying to design a simple switcher constant current circuit (using
the LM2574-ADJ). It's easy enough if you put the sense resistor between
the load and ground. However the snag is that the current sense resistor
has to be on the high side of the load (there are 3 current supplies
driving 3 devices with a commoned ground).

Does anyone know of a clever way of doing this without getting into
differential amplifiers?

Are you also using the switcher to buck or boost the input voltage
substantially? If not, I wonder why you are approaching a constant current task
that way.

 

[Terry Given]

I'm trying to design a simple switcher constant current circuit (using
the LM2574-ADJ). It's easy enough if you put the sense resistor between
the load and ground. However the snag is that the current sense resistor
has to be on the high side of the load (there are 3 current supplies
driving 3 devices with a commoned ground).

Does anyone know of a clever way of doing this without getting into
differential amplifiers?

www.zetex.com

they make clever high-side current sense thingies that level-shift the
signal down to the low side. digikey even sell them.

And if they do, others will too. Bear in mind that a differential
amplifier is really a resistor mismatch detector.

Cheers
Terry

 

[Tim Mitchell]

Are you also using the switcher to buck or boost the input voltage
substantially? If not, I wonder why you are approaching a constant
current task
that way.

No (sorry should have given more details), it's a supply for luxeon
LEDs, but there can be a variable number connected in series. There's a
48V supply on one end and from 1 to 12 LEDs in series on the output. So
a switcher is the only way to go otherwise too much power has to be
dissipated if low numbers of LEDs are connected.

 

[Fred Bloggs]

I'm trying to design a simple switcher constant current circuit (using
the LM2574-ADJ). It's easy enough if you put the sense resistor between
the load and ground. However the snag is that the current sense resistor
has to be on the high side of the load (there are 3 current supplies
driving 3 devices with a commoned ground).

Does anyone know of a clever way of doing this without getting into
differential amplifiers?

There is no need for a differential amplifier if you have access to an
OA with RRIO that can work with your output voltage levels- those are
the levels too unimportant for you to mention. The OA is configured as a
simple voltage controlled current source, with input voltage IxRs, the
output current of which is used to develop the feedback voltage. R2=1K
would be in line with the 2574 fixed version value, so that IxRs/R1
should be calculated to be Vref/R2=1.23mA.

View in a fixed-width font such as Courier.




+-------------------+
| I |
==== | ----> |
sw>-----UUUU--+----+---[Rs]-+ |
| | | |
[R1] | | |
| | | |
+----------+ | |
| | | | |
e | (-)| | |
\| /|---+ | |
pnp|--< | | |
/| \|------+ |
c | (+) | |
Rs x R2 | | L |
Vfdbk=I x ------- <--+ | O ===
R1 | | A CF
[R2] | D |
| | | |
+----+ +-----+
| |
sig gnd ---
///

 

[Fred Bloggs]

No (sorry should have given more details), it's a supply for luxeon
LEDs, but there can be a variable number connected in series. There's a
48V supply on one end and from 1 to 12 LEDs in series on the output. So
a switcher is the only way to go otherwise too much power has to be
dissipated if low numbers of LEDs are connected.

Mind telling why the LEDs *have* to be grounded?

 

[Fred Bloggs]

No (sorry should have given more details), it's a supply for luxeon
LEDs, but there can be a variable number connected in series. There's a
48V supply on one end and from 1 to 12 LEDs in series on the output. So
a switcher is the only way to go otherwise too much power has to be
dissipated if low numbers of LEDs are connected.

This is a variation of an output voltage bootstrap regulation scheme
posted by WH a while ago. Since your load is relatively light, the
current can be regulated by an LM317 linear and the switcher can be used
to bootstrap the total output so as to maintain enough voltage across
the 317 for constant current source operation. The LED current is set at
1.25/42=30mA in this case.

View in a fixed-width font such as Courier.

..
..
..
..
..
.. Vin >--+
.. |
.. +------------+ Vleds+3.3V
.. | Vin |
.. | sw |--------L1--+---------------+----------+ <- Vout
.. | | | _|_+ |
.. | | GND--|>|---+ --- |
.. | | D1 | | |
.. | | | GND +-----+
.. | | | |LM317|-+
.. | | | +-----+ |
.. | | | | |
.. | UC2574-ADJ | | 42 |
.. | etc | 2.7K | |
.. | | | +----+
.. | | | D2 |
.. | | +--------+---|<|---+-------+
.. | | | | |
.. | | e\| |/e |
.. | | |--+--| -
.. | | /| | |\ V LEDs
.. | | 1.23V | | | -
.. | fb |------+--------------' '----+ |
.. | gnd | | pnp pair | |
.. +------------+ 1K bcm857bs 10k |
.. | | bc857bs | |
.. GND------+-------------+------------------------+-------+
..
..

 

[Fred Bloggs]

This is a variation of an output voltage bootstrap regulation scheme
posted by WH a while ago. Since your load is relatively light, the
current can be regulated by an LM317 linear and the switcher can be used
to bootstrap the total output so as to maintain enough voltage across
the 317 for constant current source operation. The LED current is set at
1.25/42=30mA in this case.

View in a fixed-width font such as Courier.

.
.
.
.
.
. Vin >--+
. |
. +------------+ Vleds+3.3V
. | Vin |
. | sw |--------L1--+---------------+----------+ <- Vout
. | | | _|_+ |
. | | GND--|>|---+ --- |
. | | D1 | | |
. | | | GND +-----+
. | | | |LM317|-+
. | | | +-----+ |
. | | | | |
. | UC2574-ADJ | | 42 |
. | etc | 2.7K | |
. | | | +----+
. | | | D2 |
. | | +--------+---|<|---+-------+
. | | | | |
. | | e\| |/e |
. | | |--+--| -
. | | /| | |\ V LEDs
. | | 1.23V | | | -
. | fb |------+--------------' '----+ |
. | gnd | | pnp pair | |
. +------------+ 1K bcm857bs 10k |
. | | bc857bs | |
. GND------+-------------+------------------------+-------+
.
.

woops- adjust that L1-D1 junction:

View in a fixed-width font such as Courier.





..
.. Vin >--+
.. |
.. +------------+
.. | Vin | Vleds+3.3V
.. | sw |-----------+-- L1----+------+----------+<- Vout
.. | | | | _|_+ |
.. | | GND--|>|--+ | --- |
.. | | D1 | | |
.. | | | GND +-----+
.. | | | |LM317|-+
.. | | | +-----+ |
.. | | | | |
.. | UC2574-ADJ | | 42 |
.. | etc | 2.7K | |
.. | | | +----+
.. | | | D2 |
.. | | +---|<|---+-------+
.. | | | | |
.. | | e\| |/e |
.. | | |--+--| -
.. | | /| | |\ V LEDs
.. | | 1.23V | | | -
.. | fb |------+--------------' '----+ |
.. | gnd | | pnp pair | |
.. +------------+ 1K bcm857bs 10k |
.. | | bc857bs | |
.. GND------+-------------+------------------------+-------+
..
..

 

[Mark Jones]

I'm trying to design a simple switcher constant current circuit (using
the LM2574-ADJ). It's easy enough if you put the sense resistor between
the load and ground. However the snag is that the current sense resistor
has to be on the high side of the load (there are 3 current supplies
driving 3 devices with a commoned ground).

Does anyone know of a clever way of doing this without getting into
differential amplifiers?

Check out these switchers, might give you some ideas:
http://www.romanblack.com/

 

[Ol' Duffer]

I'm trying to design a simple switcher constant current circuit (using
the LM2574-ADJ). It's easy enough if you put the sense resistor between
the load and ground. However the snag is that the current sense resistor
has to be on the high side of the load (there are 3 current supplies
driving 3 devices with a commoned ground).

Does anyone know of a clever way of doing this without getting into
differential amplifiers?

Doesn't sound like you need much accuracy, so how about
a pnp transistor and about 3 resistors to form a crude
current mirror?

 

[Fred Bloggs]

Fred Bloggs wrote:
[..snip...]

Seems like this should be impervious to switching LEDs in and out while
the circuit is powered up- the main thing is to make it impossible to
blow the LEDs with excessive current- the 2.7K will have to be a 1W to
withstand continuos short-circuited output- the LM317 protects itself:

View in a fixed-width font such as Courier.


..
..
..
..
.. Vin >--+
.. |
.. +------------+
.. | Vin | Vleds+3.3V
.. | sw |-----------+-- L1----+------+----------+<- Vout
.. | | | | _|_+ |
.. | | GND--|>|--+ | --- |in
.. | | D1 | | +-----+ adj
.. | | | GND |LM317|-+
.. | | 2.7K +-----+ |
.. | | | |out |
.. | | | D2 | |
.. | UC2574-ADJ | +---|>|-----------+ |
.. | etc | | | |
.. | | | 42 |
.. | | | | |
.. | | | +-------+----+
.. | | | | |
.. | | e\| |/e |
.. | | |--+--| -
.. | | /| | |\ V LEDs
.. | | 1.23V | | | -
.. | fb |------+--------------' '----+ |
.. | gnd | | pnp pair | |
.. +------------+ 1K bcm857bs 10k |
.. | | bc857bs | |
.. GND------+-------------+------------------------+-------+
..
..

 

[Tim Mitchell]

Mind telling why the LEDs *have* to be grounded?

They don't, but it's an rgb triad with the cathodes commoned, so I can't
measure current on the cathode side.

These are luxeon LEDs which have a drive current of 350mA, not yer
normal 20mA. The blues have a Vf of just below 4 volts, pretty much the
whole 48V input is needed to drive 12 of them.

Thanks to everyone for the ideas.

 

[Rich Grise]

They don't, but it's an rgb triad with the cathodes commoned, so I can't
measure current on the cathode side.

These are luxeon LEDs which have a drive current of 350mA, not yer
normal 20mA. The blues have a Vf of just below 4 volts, pretty much the
whole 48V input is needed to drive 12 of them.

Thanks to everyone for the ideas.

If there are three diodes in one package, how do you arrange them when you
put them in series?

Thanks,
Rich

 

[bearchow]

Try a TI INA193-INA198 for your high side sense. Has more than
enough common-mode capability and bandwidth, easy to use, and cheap.

 

[Tim Mitchell]

If there are three diodes in one package, how do you arrange them when you
put them in series?

You don't, they are made in different sizes with series stacks of 2,4,8
or 12 LEDs for each colour. But always with 3 anode connections and one
return common cathode.

 

[Fred Bloggs]

They don't, but it's an rgb triad with the cathodes commoned, so I can't
measure current on the cathode side.

And you also cannot stack these triads in series because you have to
connect each anode to the common cathode of the triad above it which
places all LEDs in the triad in parallel.

These are luxeon LEDs which have a drive current of 350mA, not yer
normal 20mA. The blues have a Vf of just below 4 volts, pretty much the
whole 48V input is needed to drive 12 of them.

You seem to be having trouble with basic circuits.

 

[Tim Mitchell]

And you also cannot stack these triads in series because you have to
connect each anode to the common cathode of the triad above it which
places all LEDs in the triad in parallel.


You seem to be having trouble with basic circuits.

No, I'm just having trouble getting all the necessary information
across...

The www.romanblack.com website Mark Jones suggested has got some
amazingly simple switcher circuits which make me realise I don't need to
use a "simple switcher" at all. Impressive stuff.

 

[Terry Given]

No, I'm just having trouble getting all the necessary information across...

The www.romanblack.com website Mark Jones suggested has got some
amazingly simple switcher circuits which make me realise I don't need to
use a "simple switcher" at all. Impressive stuff.

Oh yeah - go simulate one, and see how crap it really is. I looked into
it a while ago for a 48:9V buck converter. The transistor switching isnt
too good, and gets a lot worse at high voltage. Without the current
limit transistor the whole thing fries on an overload; but the Ilimit
BJT basically operates in the linear region - when current limiting it
pulls the base of the main switching transistor down to whatever voltage
is required to get the requisite current - needless to say this does NOT
help the switching BJT power dissipation. Half a days SPICE was enough
to convince me to abandon the thing and do it properly.

Its OK if you're just a hobbyist, and dont really care about
reliability. OTOH if you plan on selling it, be very afraid. It turns
out there are VERY good reasons why we make buck regulators as complex
as they are. Its a bit like driving really - any idiot can drive a car
on a deserted, flat, straight piece of road. Going sideways around a
corner in the pissing rain, into the path of an oncoming vehicle, is
another matter entirely. Dont say I didnt warn you

Cheers
Terry

 

[Winfield Hill]

Fred Bloggs wrote...

Seems like this should be impervious to switching LEDs in and out while
the circuit is powered up- the main thing is to make it impossible to
blow the LEDs with excessive current- the 2.7K will have to be a 1W to
withstand continuos short-circuited output- the LM317 protects itself:

. Vin >--+
. |
. +------------+
. | Vin | Vleds+3.3V
. | sw |-----+-- L1----+------+----------+<- Vout
. | | | | _|_+ |
. | | _|_ | --- |in
. | | /_\ D1 | | +-----+ adj
. | | | | GND |LM317|-+
. | | GND 2.7K +-----+ |
. | | | D2 |out |
. | UC2574-ADJ | +---|>|-----------+ |
. | etc | | | |
. | | | 42 |
. | | | | |
. | | | +-------+----+
. | | | | |
. | | e\| |/e |
. | | |--+--| -
. | | /| | |\ V LEDs
. | | 1.23V | | | -
. | fb |------+--------' '----+ |
. | gnd | | pnp pair | |
. +------------+ 1K bcm857bs 10k |
. | | bc857bs | |
. GND------+-------------+------------------+-------+

Without an LED load the voltage soars to near Vin, which could be as
high as 45V. What happens the instant the LEDs are attached? First
a short high-current spike, followed by excessive LM317 voltage (if
only one LED is used), etc.? Hmm, I wonder how the LM317 responds
to a high voltage across its Vout and ADJ terminals for a short time?
I see there's a 6V zener in series with 50 ohms inside the chip, but
that resistor isn't a power resistor... Time for some experiments.

 

[Fred Bloggs]

Fred Bloggs wrote...



Without an LED load the voltage soars to near Vin, which could be as
high as 45V. What happens the instant the LEDs are attached? First
a short high-current spike, followed by excessive LM317 voltage (if
only one LED is used), etc.? Hmm, I wonder how the LM317 responds
to a high voltage across its Vout and ADJ terminals for a short time?
I see there's a 6V zener in series with 50 ohms inside the chip, but
that resistor isn't a power resistor... Time for some experiments.

Too much current error with the dual PNP- make it:

View in a fixed-width font such as Courier.


..
..
..
..
.. Vin >--+
.. |
.. +------------+
.. | Vin | Vleds+3.3V
.. | sw |-----------+---L1----+------+----+-----+ <- Vout
.. | | | | _|_+ | |
.. | | GND--|>|--+ | --- | |in
.. | | D1 | | | +-----+ adj
.. | | | GND | |LM317|-+
.. | | 2.7K | +-----+ |
.. | | | | |out |
.. | | | D2 | | |
.. | LM2574-ADJ | +---|>|-----------+ |
.. | etc | | 1N4148 | | |
.. | | | | 42 |
.. | | | \| | |
.. | | | BC846 |---+----+
.. | | | e/| |
.. | | e\| | |
.. | | BC856 |---------+ -
.. | | /| | V LEDs
.. | | 1.23V | | -
.. | fb |------+--------------' | |
.. | gnd | | | |
.. +------------+ 1K 10k |
.. | | | |
.. GND------+-------------+--------------------------+-----+
..
..