Driving LCD Backlight with Constant Current

1. Introduction to LED Backlight

LED backlights are the standard illumination method for modern LCD modules, including character and graphic types. These backlights are inherently current-driven, meaning their brightness depends directly on the current flowing through the LEDs. Supplying LEDs with a voltage source (typically using a current-limiting resistor) is a common but suboptimal method, often leading to inconsistent brightness due to variations in supply voltage and LED forward voltage drops.

This application note presents a low-cost, efficient constant current circuit for driving LED backlights, ensuring stable, repeatable brightness across varying conditions.

2. Driving LED Backlight with Constant Current

2.1 Reason for using Constant Current

LEDs are sensitive to temperature change, exhibiting a non-linear volt-ampere characteristic that changes with heat. As temperature rises, forward voltage drop reduces, forward current will increase quite a bit. 

When LED light bulbs are serially connected. Applying constant current to LED backlight, its temperature goes up. Because the supply current remains unchanged, the voltage applied across the LED backlight will drop. This mechanism helps the LED backlight produce even brightness despite temperature fluctuations.

On the other hand, if we use constant voltage, LED's unique volt-ampere properties can lead to issues. When an working LED's temperature rises, the current flowing through the LEDs will also increase. This higher current, in turn, generates even more heat, creating a self-magnifying effect within the circuit.

This escalating current can significantly speed up an LED's degradation and shorten its lifespan. Additionally, because each individual LED has slightly different volt-ampere characteristics, they'll end up with varying current flows. This results in uneven brightness across the LED backlight.

2.2 Key Benefits of Constant Current Driving

• Brightness Stability: Unaffected by supply voltage fluctuations.
• Improved Lifetime: Avoids overdriving LEDs.
• Design Simplicity: No need for complex ICs or PWM controllers.
• Low Cost: Total BOM under $1.50 (depending on current level).

3. Basic Constant Current Circuit

3.1 Principle of Operation

The circuit uses a BJT transistor with two diodes (or one BJT used as a diode) to set a fixed emitter voltage. The current is determined by:

3.2 Design Example (100mA Output)

• Vdd: 5V
• LED Forward Voltage: ~3.2V
• Desired Current: 100 mA

Component Selection: 

• Re (emitter resistor): 0.7𝑉/0.1𝐴=7Ω
• Rb (base resistor): Sets current through diode stack (typically 4–5 mA).

4. Practical Implementation Circuit

4.1 Thermal Stability Enhancement

Use a single transistor (e.g., TIP100) in place of the diode stack by connecting the collector to base, forming a diode equivalent with matched temperature behavior.

4.2 High Current Variant (700mA)

• Re = 1Ω
• Rb can remain at 910Ω due to the high hFE of TIP100.

5. Adjustable Current Version

5.1 Design Notes:

• Use a multi-position rotary switch with multiple emitter resistors.
• Each position corresponds to a fixed current (e.g., 20mA, 50mA, 100mA).
• Label switch positions after calibration.

5.2 Avoid Potentiometers:

• Require current meter during adjustment.
• Residual resistance complicates calibration.
• Need fixed limiting resistor in series.

6. Laboratory Precision Variant

For Higher Accuracy and Versatility:

• Use separate 12V–15V supply for the backlight driver.
• Replace diode reference with a zener diode (e.g., 5.1V).
• New current equation:
  
• Use matched transistors for thermal tracking.
• Insert an ammeter in series to monitor current (optional for lab use).

7. Thermal Considerations

• Ensure proper heat sinking or airflow for high current (>300mA) applications.
• Monitor transistor temperature rise during continuous operation.
• Use transistors with adequate power handling (e.g., TIP100: 50W rated).

8. Recommended Use Cases

Character LCD modules (16x2, 20x4)
Small graphic modules (e.g., 128x64)
Panel backlights in industrial HMIs
Portable instruments needing brightness uniformity

9. BOM for 100mA Driver

Component	Value	Type	Est. Cost (USD)
Q1	TIP100	NPN Transistor	0.40
Rb	910Ω, 0.25W	Resistor	0.02
Re	7Ω, 0.5W	Resistor	0.05
Total BOM Cost			~0.47

10. Conclusion

This constant current driver circuit is a simple, scalable, and reliable method for powering LED backlights in LCD applications. By ensuring a steady LED current, you significantly enhance the display’s brightness consistency and product longevity. Whether for production, lab, or prototyping, this approach offers a low-cost, high-performance solution adaptable to a wide range of applications.