Series LED for backlighting

[davepfz]

We are planning on using the NHD-C12864WC-FSW-FBW-3V3 for a new design. However, the ability to control the backlight level in firmware is important. Would it be possible to get this in a series configuration? This would increase the LED voltage to some multiple of 3V and then we could use switching technology like the FAN5333 to control the current.

Note that we chose the display based on it's overall size and it fits the selected case perfectly.

Thanks

[Michael_L]

We would be able to design this as a custom product if you like, however, the FAN5333 can be used with parallel LEDs as well (see the Typical Application diagram on page 1 of the FAN5333 datasheet).  I would think the latter would be more appealing to you than paying for the tooling of a custom display, and dealing with a non-stocked, custom part.  If your company is interested in pursuing the custom design, we are more than happy to provide you a quote, including tooling cost, unit price, and lead times.  For this inquiry, please email nhtech@newhavendisplay.com.

[davepfz]

The FAN5333 circuit is designed for boost applications where Vout > Vin. The display specification for the LED voltage is 3.0V. Our application will be running off 3.3V. With a BAT54 forward drop of 0.4V, this is just marginal and efficiency would be very poor - nowhere near the 80% or 90% shown for Vout = 9 or 15V on page 5. I don't like the idea of boosting it to a higher voltage just to loose a lot of energy in a series resistor.

Just a note, Ideally we'd be running the display right off battery rather than a regulated 3.3V. If this is a Li chemistry, that voltage would be 4.6V and the diode would be forward biased all the time.

Since the project is low volume, we can't justify s custom design. I guess I'll be looking into alternatives - but I'll still be using the NHD-C12864WC-FSW-FBW. I really like the display!

Thanks

[Michael_L]

Ah ok, thanks for the explanation I do understand what you mean.  As another option, could it be possible for you to use any built-in PWM on the microcontroller you are using?
Also, thanks for sharing your interest in our display, that is one of our best graphic COG LCDs!

[davepfz]

Sorry to get back to you so late on this, but since my question was answered, I wasn't looking!

Yes, you can use any standard PWM that is designed to handle the 35mA, but there are two problems with this. The first problem is the 35mA is just beyond the 25mA that a lot of uC's can handle. Sure you could just reduce the current with the limiting resistor, but this would need to be evaluated for each application to determine if the brightness at 25mA is enough. Very often some additional driver circuitry is required to bet full intensity.

The other problem is that switching 35mA at higher frequencies can produce some rather offensive RF emissions. If you look at some of the LED driver application circuits, you will see some methods of using the PWM to generate a control voltage that is used to determine the LED current. It's a little obscure, but this method keeps RF emissions down as the only high current path is through the LED(s) and the control chip itself. This means that the switching frequency is determined by the LED driver chip and not the uC.