HELP LCD LED BACKLIGH DIMMER

Thank you matin for your help,

I think that it is the more easy and cheaper idea, I will check in the prototype board.

Can you recomend me a common MOSFET transistor?

Alberto

Martin is correct..........his method will work fine and will not require many additional components and it should not waste much power.

Although your ap is somewhat of a low power ap, you might want to investigate putting the diodes in series and using a chip to control the average current delivered to the LED's. In this method, the 12 ohm resistor is not used, so power isn't wasted with the resistor. But, you will have to build an additional circuit board to run a switching regulator. Not sure if you want to go to this amount of trouble. Although you will save power with this method, you have to evaluate whether it's worth the expense and the extra board space.

Maxim

makes special 30 to 40 volt switching regulators made to drive series LED's. There are many vendors that do the same.

Mebart

V=ir, 5v-3.3= 1.7V

1.7/.14=12 ohms So a 12 ohm resistor is needed in series with the leds to give the maximum current, from a 5V supply. You DO NOT connect the LEDS directly across the 5V supply, there must always be a current limiting resistor. I normally put the resistor between the 5v and the anode of the led

Put a fet in the ground/cathode side of the of the leds, and use the micro to pulse width modulate (PWM) the gate of the fet.

martin

"Wales is a big welsh-shaped rain collection device"

I ended up using an LM10/series pass transistor/sense resistor combo so one customer could set his LED backlight current to match whatever bargain basement display he could get that week. A trim pot gave him (close to) 0 - 200mA. I played around with a few switching designs, but all of the "one chip" solutions seemed to be for boosting from 1 lithium to several series LEDs or constant current sources that topped out at

90mA or so, and I know of one display he used often that needed 120mA. My home brew switchers always seemed to loose all efficiency at the low end of the range (below 50mA), so a linear constant current source was the cheapest/simplest route. It would be an easy matter to replace the trim pot with one of the digitally controlled pots. As usual, I'm sure there are many ways to skin this cat, that was just how I did it.

Jim

I'm Terrible at FETS's, but probably BS170, or cheaper, I think the main spec is Rdson at the current you are using. So if the Rdson is 20 Ohm, it wont work, look for something with less than 0.5 Ohm. power disipation will not be a problem. It might be worth putting a small R in series with the gate to the micro, say 100R, for RF reasons

martin

"Wales is a big welsh-shaped rain collection device"

Thanks again Martin,

I have see the BS170 from Fairchild,

and the RDSon is 1.2ohm, typical. ID max: 500mA (no problem is nice for our purpose), and the power dissipation is 802mW, I think it is fine.

See the final design: +5V | | 12ohm +3.3V | ___ |-----------|___|------------oA | 1/4w V -> Backlight LED's - 3.3V | 140mA oK o ||-+ PWM 100ohm ||