App note: Voltage generating circuits for LCD contrast control


Another application note from Hantronix, Inc. on simple to digitally controlled efficient power supply for LCD display contrast. Link here (PDF)

Many LCD display modules require a negative or positive voltage that is higher than the logic voltage used to power an LCD. This voltage, called Vl, Vee or the bias voltage, would require a second power supply in the application device. If this power source is not available the LCD bias voltage must be generated from an existing voltage, either the logic voltage (+3.0-+5v) or a battery. This application note describes circuits for generating either a negative or positive LCD bias voltage from such a voltage source.

The LCD bias voltage is used to power the circuits that drive the LCD glass. This voltage sets the contrast level of the LCD. Since any changes in this voltage will cause a visible change in the contrast of the LCD it must be regulated to better than about 200mV. Any noise or ripple on this signal may cause visible artifacts on the LCD so they must be kept below about 100mV.

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    1. Well spotted. Hantronix is pretty optimistic too, they think they can get -4V from that, well, only with the Schottky…

      1. @KH; Yes – using the (non-optimal) capacitor values shown in the crappy Hantronix AppNote with a 1 KHz ideal unipolar 5V, 50% duty-cycle, 40ns rise/fall square wave source and a 1 KOhm resistive load yields the following when I plug the AppNote parts into a little old SPICE CP-Inverter test jig I have kicking around (yes I flipped the output capacitor first). I selected decent capacitors with low ESR, Vmax, and peak current that can handle the transients (Nichicon models). The diode models do handle junction capacitance and step-recovery parms from cursory inspection:

        The Si 1N4148 circuit gives you around -3.6 Vavg out.

        The Schottky 1N5817 circuit gives you around -4.5 Vavg out.

        So you are spot-on in your comment @KH :-)

        Efficiency is (obviously) much improved with the Schottky diodes. As always with these simple diode-capacitor charge pumps, be careful to select your capacitors so they can handle the transient peak +/- currents, especially with with the input capacitor.

        Also be very careful when driving a simple charge pump like shown in the AppNote from (e.g.) a micro-controller GPIO pin that has protection diodes as you can damage the pin unless you tame the transients.

        To help snubbing transients, you can gently increase source rise and fall times with something like an RC low pass. On some microcontrollers or CPLD’s this can also be done with no additional parts by using something like an on-die timer-counter with a LUT to tailor the rise/fall times.

        Have a look at the Wikipedia page for the subject “Charge pump”. While the Wikipedia page is not particulary useful in-itself, the external reference links on the page may be useful.

        Do NOT use these charge pump inverters with high dynamic range and/or high sensitivity applications (e.g., audio, and/or sensor/ADC apps). Ripple will be an issue.

        Also without snubbing, the transients/noise WILL propagate all over the circuit unless you take care by using split analog/digital busses and careful management of the grounding nets and bypass. And then there is the issue of using the likes of MLCC caps – where is that “whining” noise coming from, you ask…

        As long as you aren’t going to production with a $2 toy design (or you are a “Maker” living in the DANGER knowledge-quadrant), avoid these simple diode-capacitor charge-pump circuits. Instead use a gum-drop charge pump inverter chip. If the cP inverter chip doesn’t include an on-die LDO (which dramatically reduces noise, and transients, and increases stability), then include an LDO externally (with care).

        Have Fun, David in Jakarta

      2. I’ll give them some credit for choosing 1N5817 for the Schottky. If they have chosen BAT85 then it would have run out of steam very quickly. With BAT85 I think it would not hit -4.0V, give it say -3.9V. I’ve tried simple boost switching comparing 1N4148 and BAT85, and BAT85 can’t handle any worthwhile loads, switcher perf was only marginally better than 1N4148…

    2. I spotted that too. The author is incompetent and should not have been allowed to write ANs. I am sick of ANs, datasheets and user guides full of mistakes and errors. Even errata sometimes contais errors!
      I do not want to live on this planet anymore…

      1. “contais”… how ironic :-p (read this in Alan Rickman’s voice)
        No, companies must be allowed to keep on publishing ANs, it’s our source of fun! ;-)

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