How a Buck Regulator Becomes a Smart Dimmable LED Driver (Part 2)

If the switches are internal, the buck is called a regulator, and if the switches are external, it is called a controller. If both switches are transistors (MOSFETs or BJTs), it is synchronous, and if the bottom switch is implemented using a diode, it is asynchronous. Each of these types of buck circuits has advantages and disadvantages, but synchronous buck regulators can generally optimize efficiency, part count, solution cost, and board area. Unfortunately, synchronous buck regulators for driving high current LEDs (up to 4A) are rare and expensive. This article uses the ADP2384 as an example to show how to modify the connections of a standard synchronous buck regulator to regulate LED current.

　　The ADP2384 high-efficiency synchronous buck regulator specifies output currents up to 4 A with input voltages up to 20 V. Figure 4 shows the normal connections for regulating the output voltage.

　　Figure 4. ADP2384 connected for output voltage regulation.

　　In operation, the divided output voltage is connected to the FB pin, compared to the internal 600 mV reference, and used to generate the appropriate duty cycle for the switch. In steady state, the FB pin is held at 600 mV, so VOUT is regulated to 600 mV times the divider ratio. If the upper resistor is replaced by an LED (Figure 5), the output voltage must be whatever value is needed (within the ratings) to maintain FB at 600 mV; therefore, the current through the LED is controlled to 600 mV/RSENSE.

　　Figure 5. Basic but inefficient LED driver.

This circuit works well when a precision resistor from FB to ground sets the LED current, but the resistor dissipates a lot of power: P = 600 mV × ILEDFor low LED currents, this is not a big problem, but at high LED currents, the inefficiency can significantly increase the heat dissipated by the lamp (600 mV × 4 A = 2.4 W). Reducing the FB reference voltage can reduce power dissipation proportionally, but most DC-DC regulators do not have a way to adjust this reference. Fortunately, two tricks can reduce the reference voltage of most buck regulators: use the SS/TRK pin-or offset the RSENSE voltage.

　　Many general-purpose buck ICs include a soft-start (SS) or tracking (TRK) pin. The SS pin slowly increases the switch duty cycle at startup, minimizing startup transients. The TRK pin allows the buck regulator to follow independent voltages. These functions are often combined into a single SS/TRK pin. In most cases, the error amplifier compares the minimum of the SS, TRK, and FB voltages to a reference, as shown in Figure 6.

　　Figure 6. Operation using the soft start pin of the ADP2384.

　　For lamp applications, set the SS/TRK pin to a fixed voltage and use it as the new FB reference. Constant voltage dividers work very well as reference sources. For example, many buck regulator ICs include a controlled low voltage output—such as the VREG pin on the ADP2384. For even greater accuracy, a simple 2-pin external precision reference can be used, such as the ADR5040. In any case, a resistor divider from this supply to the SS/TRK pin forms the new reference voltage. Setting this voltage between 100 mV and 200 mV generally provides the best balance between power dissipation and LED current accuracy. Another advantage of a user-selected reference voltage is that RSENSE can be selected to a convenient standard value, thus avoiding the expense and inaccuracy of specifying or assigning arbitrary precision resistor values ​​to set the LED current.

　　Figure 7. Using the SS/TRK pin to reduce the FB reference voltage.

　　Using the SS or TRK pin approach is not feasible for all buck regulators, as some ICs do not have these pins. Also, for some buck ICs, the SS pin changes the peak inductor current, not the FB reference, so the product data sheet must be reviewed carefully. As an alternative, an RSENSE voltage offset can be created. For example, a resistor divider between a precision voltage source and RSENSE provides a fairly constant offset voltage from RSENSE to the FB pin (Figure 8).

　　Figure 8. Generating RSENSE voltage offset

　　The required value of the resistor divider can be calculated using Equation 1, where VSUP is the auxiliary regulation voltage and FBREF(NEW) is the target voltage across RSENSE.

　　Therefore, the effective feedback reference of 150 mV can be obtained using the following formula, where R2 = 1 kΩ and VSUP = 5 V:

　　The LED current is: