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What is the proper way to connect an external 18V supply?

What is the proper way to connect an external 18V supply to the S128240D ? Since there is no way to drive a full screen, 32 Grey image with the internal pump I want to add an external. For my test's I just connected 18 VDC to the VLCD pin and left the charge pump run. This gave me the screen I was looking for, But I don't know how good it is to run both the internal and external together.

What is the proper way to wire up the S128240D Display for use w/ an external 18V ?

Should it be 18 V or something else ?

How regulated does it need to be ?

What Current are we looking at ?

What do I need to do in software to make it work in this mode ?


Guest's picture
June 4, 2009

I though I'd let you know that I've been able to drive the S128240D full-screen 32 grey-scale from the internal charge pump.
Here's how I did it:
Vdd: 3.30V with a 10nF + 10uF ceramic decoupler
Booster: x 6 (hardware)
Booster caps 1uF 25V ceramic (0805" packages)
Booster bleed 1M resistor to Vdd
V0..V4 Caps 1uF 25V ceramic (0805" packages)
Interface 6 lines: 4-pin serial (A0, MOSI, MCLK, MCE) + reset + LED + Vdd control
I used code based on the DisplayTech source-code for this display with some small modifications (see my post in Charge Pump / Optimize Contrast).

Guest's picture
June 4, 2009

Would you prefer to use an external supply, or would the internal one be OK if it would work in your application?
If you'd want to use the internal one, could you say a little more about your application:
What symptoms did you see when you tried the internal pump?
What is your Vdd range and interface?

dtechsupport's picture
June 5, 2009

Also, you can apply the external supply to the VLCD pin, and no special instruction is required to set. My engineering team has tested this and it gave a stable contrast from densely populated to saturated images. If you would like to have DC/DC circuit and temperature compensation circuit on board, you can connect it as per the attached file.

Guest's picture
June 5, 2009

I would prefer to use the internal but, when I connect it as shown from the Displaytech schematic and use the startup sequence shown from the example code, I get about 1/2 way down the screen with a repeating grey scale and the charge pump drops down to 12V. Everything is fine if I only use one or two grey levels, but if I try to put an image up on the display, I have a problem with anything over 1/2 screen.

Guest's picture
June 5, 2009

Can you post the byte array for the image you're using?
I have a few ideas that may help, but I'd like to make sure the solution is repeatable for you.

Guest's picture
February 18, 2010

Does your DC/DC output voltage connect to the VLCD pin(30) or the V0IN(36) pin?
I assume the V1,V2,V3,V4 caps stay in-circuit.
I assume the charge pump caps are not longer installed and the internal booster circuit disabled by software, command 0x20, paramater VB=0.
I think most of the problems stem from the ITO resistances being too high on the VSS/VDD lines.
Things gets worse when the charge pump caps switch, I was using 2u2 ceramic X7R types.
I think the actual VSS/VDD differential voltage for the digital logic and memory, at the pins on the ST7529(COG) can get too low as the charge pump switches; and if you are accessing the chip at or near these edges I think the signals levels get screwed up, as seen from inside the COG.
Might not be as bad if there was a ceramic on the COG directly. Maybe. Too late now.
Thanks for access to the DC/DC circuit, will save me a lot of time.

Guest's picture
February 25, 2010

At its minimum the implementation is straight-forward - just connect an ~18V supply straight to the VLCD (pin 30), and operate as through you were using the charge-pump!
You're right that the V1,2,3,4 caps are still required.
Looking at the block-diagram of the controller, it makes sense that the charge pump capacitors can be removed, and the booster disabled - however I I have not verified this.
You can find the details for connecting a charge pump in the post "Problems with the S128240D-RGB in Bus Mode"