LED brightness adjustment solution based on analog technology

Portable devices using LED-based solid-state lighting (SSL) require efficient driver circuits to extend battery life, and also require some method of dimming to adjust the light output to suit the surrounding lighting conditions. In applications such as smartphones or portable GPS navigation system backlighting, LED dimming is necessary to allow users to see the screen clearly in both bright sunlight and low light conditions at night. When using a flashlight, users value long battery life more than providing the strongest light. Analog dimming or pulse-width modulation (PWM) dimming methods can be used 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 is simple in structure, minimizes control power, and is generally more efficient than PWM dimming methods because the LED forward voltage is lower at low drive currents.
However, analog dimming requires the analog voltage to be generated from a separate voltage reference (which may use an RC filter output for a square wave input signal, or an expensive digital-to-analog converter (DAC)). The circuit shown in Figure 1 eliminates the complexity of these methods by modifying a potentiometer, thus achieving a simple, cost-effective analog brightness adjustment 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, which can be used in some small battery-powered devices.
Circuit Operation

Figure 1 Analog Dimmable LED Driver Implemented by Potentiometer R1
The circuit requires a voltage-regulated, synchronous, step-down converter that can provide up to 1A of 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 less 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 across R2, the IC can vary 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 form a smooth, linear SS/TR pin voltage rise. 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. Using this design, a resistor to ground produces a constant voltage at the SS/TR pin.

A potentiometer is placed at the SS/TR pin to maintain the voltage at that pin between 250mV (potentiometer = 100 kΩ) and 0V (potentiometer = 0Ω). Recalling the above equation, this means that the FB pin voltage ranges between 160 mV and 0V. With R2 being a 0.15Ω resistor, the LED current ranges from 1.07A to 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 linear case of the circuit 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-saver mode at light load currents to maintain high efficiency over most load ranges. Figure 3 shows the efficiency of the circuit in Figure 1, which uses a 12V input and uses 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 connect 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
Because 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 are not critical and how they affect the LED brightness. If the LED is too bright, the user simply adjusts the potentiometer resistance lower. If it is too dim, simply adjust the potentiometer resistance higher. Using a multi-adjustable potentiometer, we can effectively control the 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 ground switch with a pull-up resistor 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 by a user-adjustable resistor. No other components are required other than the potentiometer. The input for the brightness adjustment, a potentiometer, is the only component required.

Figure 3 shows the efficiency of the circuit in Figure 1 over the dimming range.
Without this precise current source, we need to consider other methods to generate the analog voltage required for analog dimming. Some traditional methods include using a separate reference voltage IC to generate the precise analog voltage; using an RC filter to change the duty cycle of the microcontroller's PWM output to generate the precise analog voltage; or using a microcontroller with a DAC to generate the precise analog voltage. All of these methods require user input to change the light output. When using a reference voltage 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 touch screen control).

The 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 will determine whether this high 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, using a 12V input supply maintains efficiency of about 85% over most of the brightness adjustment range. The entire circuit requires only six components plus the high-power LED.