Re: LED backlight dimmer for battery operation


rickman wrote:
linnix wrote:
rickman wrote:
linnix wrote:
PeteS wrote:
rickman wrote:
I am designing an LED backlight for a keypad and an LCD display that
will operate from a battery pack with a voltage range of 7 to 14 volts.
I have found few devices that will operate from an input voltage range
this high as most are actually boost circuits designed for driving
white LEDs from lower voltages.

I need the circuit to conserve as much power as possible, so I expect
to need a buck switcher. It also has to be very small and low cost.
There are 9 keypad LEDs, each about 2 volts. The LEDs built into the
LCD are not well known at this point. The manufacturer does not
provide info on the LEDs, they just say to drive it from 5 volts. So I
will have to characterize this before I can use it in the same circuit
for equivalent brightness.

You would need two adjustable switching regulators.

Thanks for your reply. I only need one voltage regulator. The other
application is an LED driver/dimmer. They sell for about $1.

I have a candidate circuit that uses a few transistors and a few
passives. This is driven by a PWM output on the exisisting MCU. Most
of the circuit is there to prevent damage if the software goes south.
It is a bit Rube Goldberg and will take some work with a simulator and
lab bench to get working correctly. I'd rather use a chip if I can
find one that will do the job.

Otherwise, I have found one device from Intersil IIRC, but I don't have
a price on it yet so I don't know if it will meet my $1 goal for the
chip.

$2 is more reasonable.

If I were selling them I would say so, but I am buying and I can make
the circuit for under $1 using three or four transistors. We will be

You still need coils & caps for higher efficiency, or pay for the
battery cost.

Yes, there will be a coil and caps. What is your point?


building 100,000 of these, so a dollar saved is $100,000 earned. :)

Perhaps $1.5 per 100K.



Anyone know of higher voltage buck switchers that can be used in this
application? I also need a switching PSU for about 200 mA, but they
dont' seem to exist. At these input voltages they are all either high
current requiring a larger inductor or are otherwise a poor choice
requiring diodes/too large/expensive.

You can scale down the L&C if you don't need the maximum current.

BTW, what is it with Linear Tech
requiring a couple of external diodes on so many of their switchers?
These diodes can double the size of a small switching design.

They can't dissipate all the heat internally.

What heat? The TI part has no external diodes.

The regulators are build for 1.5A, whether you need it or not. They
have to allow for worst case condition.

The diodes add very little power to the part. LIke I said, TI does not
use external diodes and the package is just as small, so clearly power
is not the reason LTI leaves the diodes out. Could it be that LTI was
being honest with me when they said they thought ti was cheaper to make
the die smaller and let you buy separate diodes?


The power is not the
issue. LTI has told me they keep their die smaller (and cheaper) by
leaving the diodes off. Problem is that TI includes the diodes (or
uses a PFET which doesn't need the boost circuit) and goes toe to toe
with them on price while saving me the space.


As to the inductor and diodes - in a very space sensitive design I use
a coilcraft DO1605T-472MLB for a 4.7uH (on the output of a LT1767)

Good solution if $4 to $5 is acceptable.

I don't see $4 for any of these solutions. The only pricey part is the
chip and they are running $2 to $2.50 at 1k qty for the voltage
regulators that can put out 1.5 Amps. The sum of the rest of the cost
is under a buck.

I mean total cost. So, you end up with $3.50/K.

Why do you add a dollar for a couple of caps and a coil? I can get all
that for about $0.50.

The 4.7uH coil alone would cost around $0.50. We use a 10uH coil for
same price but larger.


The cheapest solution is a switching controller and external MOSFET.
Can probably do $2 in Ks.

Or an LED driver for $1. I just need to find one that runs with 14
volt input which is where this thread started.

To store the energy in voltage differentials, you need external L&C.
There's no way to build a single chip or to build a module for less
than $1. We are ordering some regulator modules for $2/K. The module
is tall, but the PCB footprint is small. All we care about is the PCB
footprint.

You need to talk to Enpirion. If they had a unit that would work with
14 volts input, I would be paying under $2 for the total solution and
would only be adding a pair of $0.15 caps. :)

If your regulator has a 5 volt input, you can save some money and board
space with a 4x5 mm chip from Enpirion, EN3512, IIRC. $1.60 plus two
0603 caps and you are done!

No, we need 20V input, 1.5V, 2.5V, 3.3V and 5V outputs between 1A to
3A.


I can buy a coil, three transistors and a small number of caps and
resistors and get a solution for under $1.

You still need a controller for another $1.

The down side is the design
time which is always too short. If I can find a chip for $1, I will
use it instead of spending my time on the transistor based solution.

Linear regulator? That would be outlawed in California soon, for
certain range of power supplies.

.

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