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PICList Thread
'Low Power Regulators'
2000\02\10@131251 by Mark Peterson

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I am running a small circuit that includes a PIC off of a 24 VDC battery
system.  I am minimizing my current draw by having the PIC sleep most of
the time.  I wake it up every few seconds, have it power up the analog
input conditioning circuitry, make its decisions, send an output signal if
required, and then have it go back to sleep.  Current draw using this
scheme is peanuts.  My problem is the quiescent current that a classic 7805
regulator uses, roughly 3.5 mA.  I saw the earlier messages about the Maxim
and TelCom regulators.  They look great but their maximum input voltage is
around 11 volts.  I've considered various dropping resistor and zener
schemes to use in conjunction with one of the low power regulators but they
all end up sucking up as much current as the original 7805.  Any clever
ides out there on this?

Thanks.

2000\02\10@131916 by M. Adam Davis

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I saw a circuit recently where the PIC was powered off a zener @ about 4.8vdc
from a higher supply (as you mention).  When the PIC woke up, it started PWMing
a transister which hooked up to the power supply and it became its own pwm
voltage regulater.  Due to another diode in the circuit, when it powered up to
5v, the zener took no current.  I don't recall how much current it took when it
was asleep through the zener...

At least, it was something along those lines...  Rather a neat idea.

-Adam

Mark Peterson wrote:
{Quote hidden}

2000\02\10@132128 by Matt Burch

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Mark,

I have had good success with the Linear LT1121 series of low-dropout linear
regulators... I-sub-q is somewhere in the 50uA range, I recall.

For really low-power, long-life battery operations, consider going to a
step-down switching regulator with an automatic shut-down under low-load
conditions... for example, the LTC1735 would fit the bill nicely.

mcb



At 12:09 PM 02/10/2000 -0600, Mark Peterson wrote:
{Quote hidden}

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    Matt Burch      |  Pinnacle Technology   | tel: (785) 832-8866
 Project Engineer   | 619 E. 8th St. Suite D | fax: (785) 749-9214
spam_OUTmburchTakeThisOuTspampinnaclet.com |  Lawrence, KS  66044   |  http://www.pinnaclet.com
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2000\02\10@133202 by Matt Burch

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In a similar vein, I once had to power a pic from a tiny lithium battery
with a puny max output current. (lithium coin cells aren't known for their
great ability to deliver current!)  Solution: trickle-charge a large
tantalum cap thru a resistor, and wake the pic up every once in a while to
do a furious bit of sampling, processing, and transmitting before going
back to sleep to let the capacitor charge back up again.

mcb



At 01:16 PM 02/10/2000 -0500, M. Adam Davis wrote:
>I saw a circuit recently where the PIC was powered off a zener @ about 4.8vdc
>from a higher supply (as you mention).  When the PIC woke up, it started
PWMing
>a transister which hooked up to the power supply and it became its own pwm
>voltage regulater.  Due to another diode in the circuit, when it powered
up to
>5v, the zener took no current.

-------------------------------------------------------------------
    Matt Burch      |  Pinnacle Technology   | tel: (785) 832-8866
 Project Engineer   | 619 E. 8th St. Suite D | fax: (785) 749-9214
.....mburchKILLspamspam@spam@pinnaclet.com |  Lawrence, KS  66044   |  http://www.pinnaclet.com
-------------------------------------------------------------------

2000\02\10@151625 by Russell McMahon

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Mark,

Some of the older devices still have their place :-)

LM2936 (again) has input Vmax of 40 volt.
TO92 or SO8 packages

Survival Vin is +80 / -40

Quiescent current is 15uA TESTED LIMIT at 100 uA load (9uA typical) and 2.5
mA tested at 50 mA load all at 24 volts in.

Iout max is 50 mA.

At 100uA PIC average load (and you probably require much lower than this)
this would take about 1000 mAH of battery per year
A switching regulator design could reduce this appreciably due to the large
voltage drop but if this level of drain is acceptable the LM2936 is a very
easy solution.
If you need MUCH higher powered up currents than the '2936 can handle you
could consider powering up a higher power regulator using a high side pnp
transistor only when the PIC is awake.



     Russell McMahon
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Help the hungry at no cost to yourself!
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(* - or woman, child or internet enabled intelligent entity :-))


{Original Message removed}

2000\02\10@155926 by paulb

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Matt Burch wrote:

> lithium coin cells aren't known for their great ability to deliver
> current!  Solution: trickle-charge a large tantalum cap thru a
> resistor,

 First consideration relates to the tantalum caps, consensus is there
are good tantalum caps and there are refuse ones.  Trick is to determine
which you are being offered by a given supplier.

 The main question here:  *Why* would you put a resistor in series,
when the lithium cell is by your own argument, the resistor?
--
 Cheers,
       Paul B.

2000\02\10@161354 by Matt Burch

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At 07:57 AM 02/11/2000 +1100, Paul B. Webster VK2BZC wrote:

>  The main question here:  *Why* would you put a resistor in series,
>when the lithium cell is by your own argument, the resistor?

IIRC, it worked better that way.  :)

mcb

-------------------------------------------------------------------
    Matt Burch      |  Pinnacle Technology   | tel: (785) 832-8866
 Project Engineer   | 619 E. 8th St. Suite D | fax: (785) 749-9214
mburchspamKILLspampinnaclet.com |  Lawrence, KS  66044   |  http://www.pinnaclet.com
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