Providing efficient current drive for high brightness LEDs

High Brightness LED Development Background

In recent years, high brightness LED (HB LED) has become increasingly popular as a light source in various lighting systems. This is because high brightness LED has a high degree of reliability and a service life of tens or even tens of thousands of hours, which is several orders of magnitude higher than the service life of traditional incandescent or halogen lamps. Based on this advantage, high brightness LED is widely used in automotive lighting, public signs and signal signs, and architectural lighting.

Figure 1. The MAX5035 LED current driver can generate an output current of approximately 350mA to 0mA at LED_A and LED_K by adjusting the control voltage (0V to 3.9V).

Figure 2. The relationship between the LED current and the control voltage in the circuit of Figure 1. The current measurement value is obtained by connecting the ammeter between the LED_A terminal and the LED_K terminal.

High-brightness LEDs are specially processed PN junction semiconductor devices that can emit white, red, green or blue light (and may also produce other colors of light) when forward biased. As PN junctions, they exhibit VI characteristics similar to traditional diodes, but with a higher junction voltage drop. When the forward voltage reaches VF (from 2.5V for red LEDs to 4.5V for blue LEDs), the current flowing through the LED is very small; once the forward voltage reaches VF, the current will rise rapidly (the same as traditional diodes). Therefore, current limiting measures must be used to limit the rise in current to prevent damage to the LED. There are currently three basic current limiting methods, and Table 1 compares these three methods.

Table 1 Comparison of current limiting methods

High brightness LED switching power supply

Figure 1 is a schematic diagram of a high-brightness LED power supply based on the MAX5035 fixed-frequency, highly integrated PWM switching converter with an output current of up to 1A. Another similar device, the MAX5033, can output currents up to 500mA. This inductor-based buck regulator can accurately control the current flowing through an LED (or several LEDs in series with a total voltage of 12V). The MAX5035 switches at a frequency of 125kHz and has an input voltage range of up to 76V (with a higher-rated input capacitor and diode). This circuit can control and maintain a constant LED current over a wide input voltage range. Table 2 summarizes the design specifications of the circuit.

Table 2 Basic parameters of the circuit in Figure 1

1Assuming a white LED: VF=4V, ILED=350mA, VIN=12V.

Using the circuit in Figure 1, a voltage is applied to the control terminal to regulate the LED current (Figure 2). Figure 3 shows the efficiency of this control architecture.

Figure 3 Regulator efficiency vs. LED current for the circuit in Figure 1 when driving one, two, or three green 350mA series LEDs

The control voltage is applied to the IC's feedback (FB) pin along with the voltage across the three parallel current-sense resistors. The IC's internal control loop maintains the voltage at the FB pin at approximately 1.22V, so a higher control voltage will produce a lower current because both the control voltage and the current-sense voltage must be maintained at 1.22V (set by resistors R1 and R5).

The following equations apply to this example and can also be used to design other output currents and control voltages:

Where: VREF = 1.22V, RSENSE is the parallel resistance of R2, R3 and R4 (= 5Ω).

In many cases, it is convenient to use low-frequency (50Hz to 200Hz) PWM to adjust the LED current, adjusting the brightness by controlling the pulse width (Figure 4). Although the LED remains at the same brightness during each pulse, the human eye can detect a brief change in brightness, but the advantage of this method of regulation is that the spectrum remains unchanged, while the spectrum changes with the current flowing through the LED when using amplitude regulation.

Figure 4. Control and LED current waveforms for low-frequency PWM brightness control of the circuit in Figure 1. Ch1: VCONTROL, Ch3: ILED. The load is three series-connected green LEDs with a total voltage of approximately 9.5V. Replacing the output capacitor with a smaller one can reduce the oscillation amplitude during shutdown.

When a 100Hz, 0V to about 3.9V square wave control waveform is used, the pulses of the LED current are shown in Figure 4. Generally speaking, the efficiency of low-frequency PWM dimming circuits is higher than that of linear LED dimming circuits (Figure 2).

Figure 5 PCB layout of the circuit shown in Figure 1

Conclusion

The ICs (MAX5035, MAX5033) shown in Figure 1 provide a cost-effective solution for driving high-brightness LEDs with constant current. This solution has the following advantages:

The high switching frequency (125kHz) allows the selection of small reactive components (L1 and C2).

Able to achieve high conversion efficiency over a wide input voltage range.

The output voltage can reach 12V, which can drive three high-brightness green LEDs connected in series.

No mechanical heat sink is required.

The voltage range can be extended to 76V, which is suitable for driving automotive high-brightness LEDs.

Can be used for 24V signal lights and building lighting.

By changing the values ​​of current sensing resistors R2, R3 and R4, the output current can reach 1A.

Built-in switching power MOSFET to simplify design.

The brightness of the LED can be adjusted using an analog voltage amplitude (linear dimming) via the control input pin.

The brightness is adjusted via the control input using a low-frequency PWM signal.