What voltage does a 3.18 inch 128x64 COG LCD need?

If you’re working with a 3.18 inch 128x64 COG LCD, the voltage it needs isn’t a single number—it depends on which part of the display you’re powering. The logic supply voltage (VDD) typically runs at 3.3V DC, but the display driver IC, often the ST7565R or a compatible variant, can handle a range from 2.8V to 5.5V on the logic side. However, the LCD drive voltage (VLCD or VOUT) for the segment and common drivers is internally generated by a charge pump circuit, and it can go up to around 12V to 15V depending on the contrast setting and temperature compensation. For a specific product like the 3.18 inch 128x64 cog lcd display, the recommended logic supply is 3.3V, with a maximum absolute rating of 6V on VDD, and the internal boost converter needs a stable 3.3V input to generate the high voltage for the LCD glass. The backlight, if it’s an LED type, usually requires a separate 3.0V to 3.3V at 40mA to 80mA, depending on the number of LEDs and brightness level. You can’t just feed 5V into the logic pins without a regulator, because the COG (Chip-on-Glass) construction is sensitive to overvoltage—exceeding 6V on VDD can permanently damage the bonding pads. The charge pump efficiency is around 85% to 90%, so if you’re running the display at full contrast, the input current on VDD can spike to 2mA to 5mA, but the backlight current dominates the power budget. Always check the datasheet for your exact driver IC, because some clones use the SSD1305 or UC1701, which have slightly different voltage thresholds.

Let’s dig into the power architecture. The COG LCD has three main power domains: the logic interface, the charge pump, and the backlight. The logic interface, which handles SPI or parallel data, uses VDD as the reference. For a 3.18 inch 128x64 display with an ST7565R controller, the VDD minimum is 2.8V, typical is 3.3V, and maximum is 5.5V. But here’s the catch: if you run VDD at 5V, the internal logic levels shift, and the charge pump might produce a higher VLCD than intended, causing ghosting or excessive contrast. The charge pump itself is a switched-capacitor voltage doubler or tripler, depending on the mode. The ST7565R has a built-in DC/DC converter that generates VOUT from VDD, and VOUT can be programmed via software to range from 6.0V to 15.0V in 0.1V steps. The actual voltage needed for the LCD glass is determined by the liquid crystal mixture’s threshold voltage, which for a typical TN (twisted nematic) display at 25°C is around 8V to 10V. But temperature compensation is critical—at -20°C, the threshold voltage can rise to 12V, so the charge pump must be able to deliver that. The display module you’re looking at likely has a built-in temperature sensor (or uses a lookup table) to adjust VLCD automatically. The boost converter requires a 1µF to 10µF capacitor on the VOUT pin, and the ripple should be less than 50mV to avoid flicker. If you use a 3.3V supply, the charge pump can source up to 1mA at 12V, which is enough for the 128x64 matrix with 128 segments and 64 commons.

Now, what about the backlight? Most 3.18 inch COG LCDs use a white LED backlight with 4 to 6 LEDs in series or parallel. The forward voltage per LED is typically 3.0V to 3.2V at 20mA, so if they’re in series, you’d need 12V to 19V, but that’s not common for these small displays. Instead, they’re usually in parallel with a single resistor, so the backlight voltage is 3.0V to 3.3V, and the current is 40mA to 80mA total. Some modules have a dedicated backlight driver IC, like the AP5724, which can boost 3.3V to 12V for series LEDs, but that’s rare. Check the pinout: if there’s an A and K pin (anode and cathode), it’s a direct LED connection. If there’s a BL_EN or PWM pin, there’s a driver. The power dissipation for the backlight is around 0.13W to 0.26W, which is fine for a COG module because the glass itself doesn’t generate much heat. The logic power is negligible—0.01W to 0.02W. So the total power for the display is about 0.15W to 0.3W, depending on brightness. You can power it from a 3.3V rail on a microcontroller board, but if you’re using a 5V system like an Arduino Uno, you’ll need a 3.3V regulator like the AMS1117-3.3, which can supply 1A. Don’t use the Arduino’s built-in 3.3V pin if it’s only rated for 50mA, because the backlight alone can draw 80mA, and the regulator might overheat.

Let’s talk about the interface voltage levels. The SPI interface for this display uses CS, SCK, MOSI, and optional MISO and DC pins. The logic thresholds are CMOS-level, meaning VIL is 0.3*VDD and VIH is 0.7*VDD. At 3.3V VDD, a logic high is anything above 2.31V, and a logic low is below 0.99V. If you’re connecting a 5V microcontroller, you need level shifting, because 5V on a 3.3V input can exceed the absolute maximum rating of VDD+0.3V, which is 3.6V. A simple resistor divider works for signals going to the display, but for the MISO line (if used), you need a bidirectional level shifter like the TXB0104. The clock frequency for SPI can go up to 10MHz, but the charge pump switching noise can couple into the data lines if you run it too fast, so 4MHz is a safe bet. The display’s internal oscillator runs at 2MHz to 5MHz, and it uses that to generate the frame rate of 60Hz to 100Hz. The charge pump switching frequency is about 200kHz to 400kHz, which is in the audible range, so you might hear a faint whine if you’re in a quiet room. That’s normal for COG displays with a boost converter.

Temperature effects on voltage are a big deal. The LCD glass’s viscosity changes with temperature, so the required drive voltage changes by about -0.5% per degree Celsius. At 25°C, VLCD might be set to 9.5V. At 50°C, it drops to 8.3V. At 0°C, it rises to 11.0V. The ST7565R has a temperature compensation register that can adjust the voltage gradient, but you have to set it correctly. If you don’t, the display will be too dark in cold environments or too washed out in hot ones. The module might have a thermistor on the flex cable, but not always—some cheap COG displays omit it. The datasheet for the 3.18 inch 128x64 COG LCD should specify the operating temperature range, typically -20°C to +70°C, and the storage range is -30°C to +80°C. The voltage for the LCD driver must be stable within 1% to avoid visible flicker, especially at low temperatures where the liquid crystal response time is slower. The charge pump output capacitor should be a ceramic X5R or X7R, with a voltage rating of 25V, to handle the 15V peak. Tantalum capacitors are not recommended because they can fail short under high ripple.

What about the power-up sequence? The datasheet for the ST7565R specifies that VDD must be applied before the charge pump is enabled, and the reset pin must be held low for at least 1ms after VDD stabilizes. If you turn on the charge pump before VDD is stable, the internal voltage references can latch up, drawing 10mA to 20mA and potentially damaging the IC. The recommended sequence is: apply VDD (3.3V), wait 100µs, then release reset, wait 1ms, then send the initialization commands to set the bias voltage, contrast, and charge pump enable. The charge pump has a soft-start feature that ramps up VOUT over 10ms to 20ms to avoid inrush current. The inrush current on VDD during startup can be 10mA to 15mA for 5ms, then drops to 2mA. If your power supply is weak, this can cause a voltage dip, so add a 10µF capacitor on the VDD pin near the display. The backlight should be turned on after the display is initialized, because the high current draw from the backlight can cause a voltage drop that affects the charge pump. Use a MOSFET switch for the backlight if you’re controlling it from a GPIO pin, because the GPIO can’t source 80mA directly.

Let’s look at the physical construction and how it affects voltage. COG means the driver IC is bonded directly to the glass substrate using anisotropic conductive film (ACF). The bonding pads have a pitch of 0.1mm to 0.2mm, and the contact resistance is about 10 ohms to 50 ohms. The ACF joint can handle up to 100mA per pad, but the VDD and VSS pads are often shared across multiple pins to reduce resistance. The glass itself has a sheet resistance of about 10^12 ohms per square, so the leakage current through the LCD cell is negligible—less than 1µA. The segment and common lines are made of indium tin oxide (ITO), which has a resistance of 10 to 50 ohms per square. For a 128x64 matrix, the total resistance from the driver to the farthest pixel can be 500 ohms to 2k ohms, which causes a voltage drop along the line. That’s why the charge pump voltage is set higher than the threshold voltage—to compensate for the ITO resistance. The contrast setting adjusts the VOUT voltage, and the typical range is 0x00 to 0x3F for the ST7565R, which corresponds to 6.0V to 15.0V in steps of about 0.14V. You can fine-tune it by reading the pixel brightness with a photometer, but most people just eyeball it.

Now, let’s talk about common mistakes with voltage. One big one is using a 3.3V regulator that’s too small. The 3.18 inch display’s backlight can draw 80mA, and the logic draws 2mA, so total is 82mA. If you’re also powering a microcontroller from the same regulator, you need a 250mA to 500mA regulator. The AMS1117-3.3 has a dropout voltage of 1.1V, so you need at least 4.4V input for a clean 3.3V output. If you’re using a lithium-ion battery (3.7V nominal), the regulator will drop out when the battery voltage falls below 4.4V, so you need a low-dropout regulator like the MCP1700, which has a dropout of 180mV at 250mA. Another mistake is connecting the backlight directly to a 5V pin without a resistor. The backlight LEDs have a forward voltage of 3.0V, so 5V - 3.0V = 2.0V across the resistor. At 80mA, the resistor value is 2.0V / 0.08A = 25 ohms, and the power dissipation is 2.0V * 0.08A = 0.16W, so use a 0.25W resistor. If you don’t use a resistor, the LEDs will draw unlimited current and burn out in seconds. Also, never apply a voltage higher than VDD to the reset pin, because it’s not 5V tolerant. The reset pin is usually a Schmitt trigger input with a pull-up resistor to VDD, so it’s at 3.3V when high. If you drive it with 5V, you’ll exceed the absolute maximum rating of VDD+0.3V.

What about using a different voltage for the logic? Some people try to run the display at 2.8V to save power, but the charge pump might not generate enough voltage for the LCD glass at low temperatures. At 2.8V VDD, the charge pump output is about 2.8V * 3 = 8.4V (assuming a tripler), but the actual voltage is lower due to diode drops, so you might get 7.5V. That’s barely enough for the LCD threshold at 25°C, and it will fail at 0°C. The minimum VDD for the ST7565R is 2.8V, but the charge pump is only guaranteed to work down to 3.0V. So stick with 3.3V unless you’re in a controlled environment. On the other hand, running at 5V VDD is possible if you use a level shifter for the SPI lines, but the charge pump will produce 15V to 18V, which can cause the LCD to be too dark and may reduce the lifetime of the liquid crystal due to electrochemical degradation. The liquid crystal material has a maximum voltage rating of 20V, but prolonged exposure to voltages above 15V can cause ion migration and permanent damage. So don’t exceed 5.5V on VDD, and set the contrast register to compensate.

Let’s get into the data sheet specifics for a common driver. The ST7565R has a built-in voltage regulator that can be configured in three modes: internal resistor, external resistor, or external voltage. Most COG modules use the internal resistor mode, where the voltage is set by a register. The typical application circuit shows a 1µF capacitor between VOUT and VSS, and a 0.1µF capacitor between VDD and VSS. The charge pump uses two external capacitors of 1µF each (C1+ and C1- pins). The boost converter efficiency is about 85% at 3.3V input and 10V output, so the input power is 10V * 0.5mA / 0.85 = 5.9mW, which is fine. The maximum output current from the charge pump is 1mA, which is enough for the 128x64 matrix because the LCD pixels are capacitive—each pixel has a capacitance of about 0.1pF to 0.5pF, so the total load is 128 * 64 * 0.3pF = 2.5nF, and at 60Hz refresh, the current is 2.5nF * 10V * 60Hz = 1.5µA. That’s tiny, but the charge pump also has to drive the segment and common line drivers, which have parasitic capacitance of 10pF to 50pF each, so the total current is about 100µA to 200µA. The 1mA rating is plenty.

Now, let’s talk about the backlight in more detail. The 3.18 inch display typically uses a white LED backlight with a brightness of 200 to 300 cd/m². The LEDs are arranged in a light guide plate, and the typical forward current is 20mA per LED. If there are 4 LEDs in parallel, the total current is 80mA. The forward voltage drops as temperature rises—at 25°C, it’s 3.0V, at 50°C, it’s 2.8V. So if you’re using a constant voltage source, the current will increase as the display warms up, which can cause thermal runaway. That’s why it’s better to use a constant current driver for the backlight, like the TPS61165 or a simple resistor with a high enough value to limit the current variation. A resistor of 10 ohms in series with the backlight will give a current of (3.3V - 3.0V) / 10 = 30mA, but if the voltage drops to 2.8V, the current rises to 50mA. So use a resistor of 22 ohms to get 15mA at 3.0V, and accept lower brightness. Or use a PWM signal from a microcontroller to control brightness, but make sure the PWM frequency is above 1kHz to avoid visible flicker. The backlight response time is about 1ms, so 1kHz is fine.

What about the contrast voltage and how it affects the display? The contrast voltage is set by the internal charge pump, and it’s applied to the LCD common and segment drivers. The ST7565R has a register called the “Electronic Volume” (EV) that sets the voltage step. The typical range is 0x00 to 0x3F, with 0x00 being the lowest voltage (6.0V) and 0x3F being the highest (15.0V). The actual voltage is VOUT = 6.0V + (EV * 0.14V). So at EV = 0x20 (32 decimal), VOUT = 6.0 + 32*0.14 = 10.48V. That’s a good starting point for most displays. But the voltage also depends on the temperature coefficient setting. The ST7565R has a register for temperature compensation that can be set to 0, 1, 2, or 3, corresponding to -0.05%/°C, -0.10%/°C, -0.15%/°C, or -0.20%/°C. For