Replacing chip with PCF85176 from PCF8576C

[k_chan]

Hi,

We're currently looking to replace PCF8576C with PCF85176 however, I've noticed on the datasheet in section 6.3.1 for PCF85176, the LCD bias generator is different from section 7.2 for PCF8576C where in the former, it says the LCD biasing voltages needs to be between VLCD and VSS whereas the latter says it's between VLCD and VDD despite the typical system configuration being the same.

I was wondering if the VSS reference is supposed to be VDD like in PCF8576C? I just wanted to see if it can be used as a drop in replacement.

If not, I'm not that experience in electronics, from my understanding the voltage divider instead of VDD being the supply, the VLCD is now being the supplied. Is that correct in my understanding?

[JozefKozon · Accepted solution]

Hi K_Chan,

please refer to the Figure 1. in both datasheets. 

JozefKozon_0-1764049365291.png

JozefKozon_1-1764049405152.png

This looks confusing because the typical LCD driver configuration uses VDD as the positive supply and VSS as ground. If the bias generator references VSS instead of VDD, it suggests the internal voltage ladder is built between VLCD and ground, not VLCD and VDD. And it is correct, there is no typo in the in either datasheet. 

JozefKozon_4-1764050049918.png

JozefKozon_3-1764049771274.png

Your Understanding About Voltage Divider

• In PCF8576C, the bias voltages are derived between VLCD and VDD.
• In PCF85176, they are derived between VLCD and ground (VSS).

So effectively, VLCD becomes the “top rail” for the LCD bias generator, and VSS is the bottom rail. VDD is only for logic, not for LCD biasing.

The pinout of PCF85176H and PCF8576CHL is very same, but please be aware of the operational and limiting voltages. Make sure, that you do not exceed the PCF85176 limiting voltages, if exceeded, the PCF85176 will be destroyed. If operational voltage is exceeded, no functions are guaranteed. 

• If your current design ties VLCD = VDD, then PCF85176 can work as a drop-in replacement electrically, but contrast may differ slightly.
• If your design uses VLCD > VDD for better contrast, PCF85176 is actually more flexible.
• No pinout changes are required, but you should verify the LCD bias configuration and contrast settings.

With Best Regards,

Jozef