Raveninghorde
Guest
Thu Feb 11, 2010 10:37 am
On Wed, 10 Feb 2010 22:24:52 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:
Quote:
On Feb 10, 9:29 pm, MooseFET <kensm...@rahul.net> wrote:
On Feb 10, 2:12 pm, dagmargoodb...@yahoo.com wrote:
On Feb 8, 9:19 pm, MooseFET <kensm...@rahul.net> wrote:
On Feb 8, 3:09 pm, Raveninghorde <raveninghorde_at_invalid> wrote:
I'm reviewing an existing charger design which I'm expanding to 10
outputs.
At the moment I use high side current sensing with an opamp and fet
level shifting the signal so that it is ground referred.
The disadvantage is that this circuit can not measure the current
during a short and relies on the swticher's own current limit for
protection.
I've used a current mirror circuit before to get around this but it's
too many parts.
If I low side sense then this problem disappears. But then I loose the
simplicity of a common ground.
I can't make my mind up. Time for a whisky.
How about:
----------------------------------------------------
! !
! ---------/\/\--- !
! ! ! !
+-----------------/\/\----+--!+\ ! s !
! ! >--+--/\/\--+--!! !
! ------/\/-----+--!-/ ! !!----
! ! ! ! --!! P-MOSFET
-----+---/\/\---+---Battery(+) --/\/\-- !d
--------+--/\/\--GND
!
V = K*I
It is the normal op-amp based constant current source but the op-amp
doesn't have to take
the large common mode and you don't need the 0.1% resistors.
The down side is you needenough voltage for the op-amp.
Or, if the op amp's inputs can go slightly below the rail:
R1 |\
.-------/\/\-----+---|-\ LM324
| | | >--.
| .-------|---|+/ |
| | | |/ |
| Rs | | |
>--+--/\/\--+-------|---------|---> to Battery (+)
i --> | |
'--|| |
||->---'
.--|| Q1
| Rs * R2
+----> V(i) = i * -------
| R1
\
/ R2
\
|
===
Two precision resistors instead of six.
(I used an LM324 for John. He loves them.)
If the load is shorted, the circuit doesn't work.
This was the problem that my circuit was aimed at solving.
I guess I'm not visualizing Ravenhorde's setup. I'm assuming Vcc to
the op amp higher than the raw battery charging voltage, just as you
did...
Ah, I see. So use mine ahead of the regulator, if that's feasible.
If not, yours wins.
Nice work.
Your circuit is the same as what I use at the moment except + and -
inputs of the op amp are swapped.
The op amp does come from a higher supply, but as you relaized that
doesn't help.
I'm using a switching regulator so measuring in front doesn't work.
MooseFET
Guest
Thu Feb 11, 2010 4:43 pm
On Feb 11, 1:37 am, Raveninghorde <raveninghorde_at_invalid> wrote:
Quote:
On Wed, 10 Feb 2010 22:24:52 -0800 (PST), dagmargoodb...@yahoo.com
wrote:
On Feb 10, 9:29 pm, MooseFET <kensm...@rahul.net> wrote:
On Feb 10, 2:12 pm, dagmargoodb...@yahoo.com wrote:
On Feb 8, 9:19 pm, MooseFET <kensm...@rahul.net> wrote:
On Feb 8, 3:09 pm, Raveninghorde <raveninghorde_at_invalid> wrote:
I'm reviewing an existing charger design which I'm expanding to 10
outputs.
At the moment I use high side current sensing with an opamp and fet
level shifting the signal so that it is ground referred.
The disadvantage is that this circuit can not measure the current
during a short and relies on the swticher's own current limit for
protection.
I've used a current mirror circuit before to get around this but it's
too many parts.
If I low side sense then this problem disappears. But then I loose the
simplicity of a common ground.
I can't make my mind up. Time for a whisky.
How about:
----------------------------------------------------
! !
! ---------/\/\--- !
! ! ! !
+-----------------/\/\----+--!+\ ! s !
! ! >--+--/\/\--+--!! !
! ------/\/-----+--!-/ ! !!----
! ! ! ! --!! P-MOSFET
-----+---/\/\---+---Battery(+) --/\/\-- !d
--------+--/\/\--GND
!
V = K*I
Since op-amps come as duals perhaps:
------------------------------/\/\----------+----/\/\----
! ! !
+-----------------------!+\ ---!-\ !
! ! >---- ! >-----
+----
! --!-/ ! ---!+/
! ! ! !
! --/\/\---+---/\/\---+---/\/------+-----/\/\---GND
! !
------+---/\/\---+---
It trades an op-amp for the transistor.
By making the first stage increase the voltage on the sense resistor
to
a higher value and then using the simple diff stage, we can get a
ground
referenced current sense.
You could also make the measurement at the catch diode of the bucker
if
you don't need it to be accurate.
