A brief discussion on the application of analog brightness adjustment and PWM brightness adjustment in LED solid-state lighting

In applications such as smartphones or portable GPS navigation system backlighting, LED brightness adjustment is necessary to allow users to see the screen clearly in strong sunlight and low light conditions at night. When using a flashlight, users consider long battery life to be more important than providing the strongest light illumination. We can use analog brightness adjustment or pulse width modulation (PWM) brightness adjustment methods in these applications. Analog designs achieve higher efficiency than PWM-based designs by using an innovative method to establish a reference voltage.

Both analog and PWM dimming methods control the LED drive current, which is proportional to the light output. Analog dimming has a simple structure, minimizes control power, and is generally more efficient than PWM dimming because the LED forward voltage is lower at low drive currents.

However, analog dimming requires the generation of an analog voltage through a separate voltage reference (perhaps using an RC filter output for a square wave input signal, or using an expensive digital-to-analog converter (DAC)). The circuit shown in Figure 1 eliminates the complexity of these methods by modifying a potentiometer, thereby achieving a simple, cost-effective analog dimming method. This overall solution is an efficient, low-cost, low-component-count LED driver suitable for single high-current LEDs, such as OSRAM's Golden Dragon, and can be used in some small battery-powered devices.

Circuit operation

Figure 1. Analog brightness adjustment LED driver circuit implemented by potentiometer R1 requires a voltage-regulated, synchronous, step-down converter that can provide up to 1A output current from a 17V supply, such as the TPS62150. In Figure 1, this step-down converter uses the feedback (FB) pin to control the voltage across the sense resistor R2 to regulate the LED current. The FB voltage is controlled by a precise internal reference voltage (typically 0.8V) and an SS/TR (slow start and tracking) external input pin.

When the SS/TR pin voltage is lower than 1.25V, the FB pin voltage is equal to the SS/TR pin voltage multiplied by 0.64, that is, VFB = 0.64 * VSS/TR. By controlling the FB voltage and thus the voltage of R2, the IC can change the current driving the LED.

The SS/TR pin has an embedded current source, which is typically 2.5 μA. This source is often used to charge the capacitor and create a smooth, linear rise in the SS/TR pin voltage. In a typical buck converter, this results in a linear, controlled rise in the output voltage while also reducing the inrush current from the input supply. When using this design, a resistor to ground produces a constant voltage on the SS/TR pin.

A potentiometer is placed on the SS/TR pin to maintain the voltage at this pin between 250mV (potentiometer = 100kΩ) and 0V (potentiometer = 0Ω). Recalling the above equation, it means that the FB pin voltage range is between 160mV and 0V. With R2 being a 0.15Ω resistor, the LED current range is 1.07A-0A. Since the FB pin voltage is linearly related to the SS/TR pin voltage, the potentiometer provides a linear analog brightness adjustment as shown in Figure 2.

Figure 2 shows the linearity of the brightness adjustment of the circuit shown in Figure 1, which uses a potentiometer to achieve brightness adjustment.

This circuit has very high efficiency because the value of the FB pin voltage is relatively low. This low voltage reduces the power dissipation in the sense resistor R2. In addition, the TPS62150 uses a power save mode at light load currents to maintain high efficiency over most load ranges. Figure 3 shows the efficiency of the circuit shown in Figure 1 using a 12V input and using TDK's VLF3012ST-2R2 inductor during the switching output. 

We can improve the efficiency of this circuit, but at the expense of increasing the circuit size. For example, you can tie the FSW (switching frequency) control pin to the output voltage to reduce the operating frequency, and/or choose an inductor with low DCR (DC resistance) and/or better AC loss characteristics. Although implementing these two methods may require more board area, efficiencies of more than 90% can be achieved. Although its efficiency is not the highest, the design shown in Figure 1 has a small solution size and good operating efficiency.

Circuit Limitations

Since this circuit uses an imprecise analog input (a manually adjustable potentiometer) to adjust the LED current, the tolerances of the sense resistor, potentiometer resistance, and SS/TR pin current and their effect on LED brightness are not that important. If the LED is too bright, the user can simply turn down the potentiometer resistance. If it is too dim, simply turn up the potentiometer resistance. Using a multi-adjustable potentiometer, we can effectively control LED brightness for many general applications such as flashlights and backlights.

One disadvantage of this design is the compensation between the SS/TR pin and the FB pin voltage. When the SS/TR pin is pulled down to 0V, 50mA of current can still flow through the LED by reducing the potentiometer resistance. Therefore, the LED cannot be completely turned off unless you add a switch to ground with a pull-up resistor connected to the EN (enable) pin.

Other analog brightness adjustment methods

The advantages of using the potentiometer circuit described in this article are its simplicity and cost-effectiveness. The analog voltage required for analog brightness adjustment is generated by a precise current source of the IC and then converted to a corresponding light output through a user-adjustable resistor. Apart from this potentiometer, no other components are required. The input to the brightness adjustment, the potentiometer, is the only component required.

Figure 3 Efficiency of the circuit shown in Figure 1 over the brightness adjustment range.

 If such a precise current source is not available, we need to consider other methods to generate the analog voltage required for analog brightness adjustment. Some traditional methods include: using an independent reference voltage IC to generate a precise analog voltage; changing the duty cycle of the microcontroller PWM output through an RC filter to generate a precise analog voltage; or using a microcontroller with a DAC to generate a precise analog voltage.

All of these methods require user input to change the light output. When using a voltage reference IC, a potentiometer is still required as an input to the IC to adjust the voltage and control the light output. The reference IC method is more expensive than the simple method highlighted in this article.

The last two methods require the use of a microcontroller, which also increases the cost of the solution. Although smartphones and GPS systems contain a microcontroller, the average flashlight does not. Which method to use depends on the application at hand, as some products require a more user-friendly interface (perhaps using a touchscreen control).

A third approach uses a larger and more expensive DAC instead of a potentiometer. A DAC has a finer granularity of output analog voltages, so it can control light output more precisely than a potentiometer. The specific application determines whether this higher cost is worth it.

Using a potentiometer on the SS/TR pin of a buck converter is a simple, small, and low-cost method to provide linear analog brightness adjustment for high-current LEDs in applications such as backlighting and flashlight lighting. When using analog brightness adjustment, efficiency can be maintained at around 85% over most of the brightness adjustment range using a 12V input supply. The entire circuit requires only six components plus the high-power LED.