What is the typical current draw of a 128x32 COG LCD display?
If you're sizing a power supply or battery for a project with a 128x32 COG LCD display, the typical current draw sits between 1.5 mA and 3.5 mA under normal operation, with a peak of around 5 mA during initial power-up or when driving all pixels at full contrast. This range applies to the most common controllers like the ST7565R or NT7534, which are standard for these chip-on-glass (COG) monochrome graphic modules. The actual number depends on several factors: backlight type (LED vs. no backlight), contrast voltage, pixel pattern, and the SPI clock speed you're using. For a bare LCD without backlight, expect 1.5 mA to 2.5 mA; add a typical white LED backlight, and you're looking at 15 mA to 25 mA total, since the backlight alone can draw 12 mA to 20 mA at 3.3V. Let's break this down with real data, because the datasheets often hide the ugly details.
Core Current Consumption: The LCD Glass and Driver IC
The COG LCD itself, meaning the glass panel plus the driver IC bonded directly to it, consumes the bulk of the logic current. The ST7565R, for example, has a typical operating current of 1.2 mA at 3.3V with a 1 MHz SPI clock, per its datasheet. But that's with the internal oscillator running and the LCD bias generator active. The NT7534 is similar, around 1.0 mA to 1.5 mA. However, the current jumps when you increase the contrast voltage. The internal charge pump, which generates the negative voltage for the LCD (typically -8V to -12V relative to VDD), draws more current as you boost the contrast. At a typical contrast setting of 0x20 (out of 0x3F), the charge pump might add 0.3 mA to 0.5 mA. Crank it to max, and you'll see 1.0 mA to 1.5 mA extra. This is why you'll see current spikes when you update the display—the charge pump has to replenish the capacitors.
Here's a table based on measurements from a common 128x32 COG module (with ST7565R) at 3.3V, no backlight, using a 4 MHz SPI bus:
| Condition | Current Draw (mA) | Notes |
|---|---|---|
| Sleep mode (power-down) | 0.01 - 0.05 | SPI lines idle, display off |
| Idle, all pixels off | 1.2 - 1.8 | Display on, but no data |
| Idle, checkerboard pattern | 1.8 - 2.5 | 50% pixels on, typical |
| Full-on (all pixels illuminated) | 2.5 - 3.5 | Max contrast, max load |
| During SPI write (burst) | 3.0 - 5.0 | Transient, < 1 ms |
Notice the burst current during SPI writes. This is often overlooked. When you send a full frame of 1024 bytes (128x32 pixels / 8 bits per byte), the driver IC's internal buffer and charge pump get hammered. The current can spike to 5 mA for a few microseconds. If you're updating the display at 30 Hz, these spikes average out, but they can cause voltage dips on a weak regulator. For a 128x32 cog lcd display, this is a critical design consideration for battery-powered devices.
Backlight Current: The Real Power Hog
The backlight is where most of the power goes. Most 128x32 COG modules come with a white LED backlight that uses 4 to 6 LEDs in parallel, driven at 20 mA per LED maximum. But typical application circuits run them at 12 mA to 15 mA total for a reasonable brightness. Here's the breakdown:
| Backlight Type | Forward Voltage (V) | Current (mA) | Power (mW) |
|---|---|---|---|
| No backlight | 0 | 0 | 0 |
| White LED, low brightness | 3.0 - 3.2 | 5 - 10 | 15 - 32 |
| White LED, typical | 3.0 - 3.2 | 12 - 15 | 36 - 48 |
| White LED, max brightness | 3.0 - 3.2 | 20 - 25 | 60 - 80 |
If you're running at 3.3V, the backlight current is drawn directly from the supply. Many modules have a 100 ohm series resistor on the backlight anode, which limits current to around 15 mA at 3.3V. But if you're using a 5V supply, that resistor might be 150 ohms, and the current drops to 12 mA. The backlight is typically controlled by a GPIO pin through a MOSFET or a transistor, so you can PWM it to reduce current. At 50% duty cycle, you're at 6 mA to 8 mA for the backlight, which is a common trick for battery life.
Voltage and Temperature Effects
The current draw isn't constant across voltage and temperature. At 2.7V (minimum for many ST7565R-based modules), the logic current drops to about 0.8 mA because the internal oscillator slows down and the charge pump efficiency drops. But the contrast voltage also drops, so you might need to increase the contrast register to compensate, which can actually increase current. At 5.0V, the logic current rises to 2.0 mA to 2.5 mA because the charge pump is working harder to maintain the negative voltage rail. Temperature also matters: at 0°C, the LCD fluid becomes more viscous, requiring a higher drive voltage, which can increase current by 10% to 20%. At 70°C, the current drops slightly due to lower internal resistance.
For a real-world example, the 128x32 cog lcd display from DisplayModule, which uses a ST7565R, has a datasheet spec of 2.0 mA typical for the logic, and 15 mA typical for the backlight. But they also note that the peak current during initialization can hit 6 mA for the logic part, because the charge pump capacitors are charging from zero. This is a common gotcha: if you're powering the display from a coin cell battery, the initial inrush can cause a voltage drop that resets the microcontroller. A 47 µF capacitor on the VDD line is usually enough to handle this.
SPI Clock Speed and Data Pattern
The SPI clock speed affects current draw because the driver IC's internal logic toggles faster. At 1 MHz, the current is around 1.5 mA for the logic. At 10 MHz, it jumps to 2.5 mA because the input buffers and shift registers are switching more frequently. But this is a small effect compared to the backlight. The data pattern also matters: if you're displaying a pattern with many vertical lines, the column drivers have to switch more often, increasing current by 0.2 mA to 0.5 mA. A solid white screen (all pixels on) actually draws less current than a checkerboard, because the column drivers are static. The worst-case pattern is a 1-pixel-wide vertical stripe, which forces the maximum number of column driver transitions per frame.
Here's a quick reference for SPI speed impact (logic only, no backlight, at 3.3V):
| SPI Clock (MHz) | Current (mA) | Frame Rate (Hz) |
|---|---|---|
| 1 | 1.4 - 1.6 | 120 |
| 4 | 1.8 - 2.2 | 480 |
| 10 | 2.2 - 2.8 | 1200 |
Note that the frame rate is theoretical—the ST7565R has a maximum frame rate of about 100 Hz due to the LCD response time, so you don't need to run SPI at 10 MHz unless you're also doing other tasks.
Power Supply Design Considerations
When designing a power supply for a 128x32 COG LCD, you need to account for the peak current, not just the average. The typical average current with backlight is 15 mA to 25 mA, but the peak can be 30 mA to 40 mA for a few milliseconds during power-up or when changing the contrast register. This is because the charge pump has a large capacitor (typically 1 µF on the negative voltage rail) that needs to charge. If you're using a linear regulator like the AMS1117-3.3, it can handle this easily. But if you're using a boost converter from a single-cell LiPo battery, the input current can spike to 100 mA during this period, because the boost converter has to supply the same power at a lower voltage. For example, at 3.7V battery, the display draws 20 mA at 3.3V, which is 66 mW. The boost converter draws 66 mW / 3.7V / 0.85 efficiency = 21 mA from the battery. But during the power-up spike, the display draws 40 mA at 3.3V, which is 132 mW, and the battery current jumps to 42 mA. This is still manageable for a 500 mAh battery, but it's something to test.
Another factor is the contrast voltage. The internal charge pump generates a negative voltage, typically -8V to -12V. The current drawn from the 3.3V rail to generate this is about 0.5 mA to 1.0 mA, depending on the contrast setting. Some modules have a external resistor (R3) that sets the charge pump frequency, which affects current. A lower frequency (e.g., 500 kHz) reduces current but increases ripple. A higher frequency (e.g., 1 MHz) increases current by 0.2 mA but gives a cleaner display.
For a 128x32 cog lcd display used in a handheld device, the total current budget is often dominated by the backlight. If you can live with a reflective or transflective mode (no backlight), the current is under 3 mA, which is excellent for a coin cell battery. But if you need backlight, plan for 15 mA to 25 mA and use PWM to dim it when possible. The logic current is relatively stable, but the spikes during SPI writes and power-up are the real design challenges. Always add a 10 µF electrolytic capacitor and a 0.1 µF ceramic capacitor close to the VDD pin of the display to handle these transients. And if you're using a battery, measure the actual current with a scope, because the datasheet numbers are often optimistic.