Designing high-power lighting solutions using high dimming ratio LED drivers

One of the reasons why LED lighting solutions are so popular is that LEDs can achieve a wide dimming range through simple current control. For example, applications such as automotive dashboards and aircraft cockpits where ambient illumination may be very low require a very wide PWM dimming range. Linear Technology's LT3478 and LT3478-1 are single-chip step-up DC/DC converters that can drive high-brightness LEDs with constant current over a wide settable range. In addition to the optional 10:1 analog dimming range, the LT3478 and LT3478-1 also have a 3000:1 PWM dimming range to maintain the color of the LED.

The LT3478 and LT3478-1 are easy to use and have programmable features designed to optimize performance, reliability, size and total cost. These devices can operate in boost, buck and buck-boost LED driver topologies. The LED current they can provide depends on the topology and can reach up to 4A. The LT3478 and LT3478-1 are ideal for high-power LED applications (including automotive and avionics lighting) and are available in a 16-pin thermally enhanced TSSOP package with E or I temperature ratings.

Figure 1: Boost LED driver circuit for automotive TFT LED backlight applications.

The LT3478 and LT3478-1 operate similarly to conventional current-mode boost converters, but they use the LED current (rather than the output voltage) as the primary feedback source for the control loop. Figure 2 shows the main functions of each part. Both devices use high-side LED current sensing so that they can operate in both buck and buck-boost modes. The LT3478-1 saves space and cost by integrating a current sense resistor and limits the maximum LED current to 1.05A. The LT3478 uses an external sense resistor, allowing a maximum programmable LED current of 4A.

Figure 2: LT3478 and LT3478-1 functional block diagram.

Setting the maximum LED current

Current control for dimming is an important feature, but it is equally important to avoid overdriving the LEDs beyond their maximum current rating. The LT3478 and LT3478-1 make it easy to set the maximum current and reduce it based on temperature.

Figure 3: Connection diagram for setting the maximum LED current.

The LT3478 and LT3478-1 use the CTRL1 pin voltage to control the maximum LED current unless the device is set to reduce the maximum LED current based on temperature (accomplished using the CTRL2 pin). The CTRL1 pin voltage can be set using a simple resistor divider from VREF (see Figure 3) or an external voltage source, or CTRL1 can be connected directly to the VREF pin to provide the maximum current. Figure 4 shows the LED current versus the CTRL1 pin voltage.

Figure 4: LED current versus CTRL1 pin voltage.

Reduce maximum LED current according to temperature

To ensure optimal reliability, LED manufacturers specify a maximum allowable LED current versus temperature curve (Figure 5). If the maximum LED current is not adjusted based on temperature, permanent damage to the LED may occur.

Figure 5: LED current drop curve vs. ambient temperature.

Figure 6: Setting the LED current derating curve vs. temperature.

The LT3478 and LT3478-1 use the CTRL2 pin to reduce current. Simply connect the CTRL2 pin to VREF through a temperature-dependent resistor divider, as shown in Figure 6. As the temperature rises, the CTRL2 pin voltage decreases, and when the CTRL2 pin voltage drops below the CTRL1 pin voltage, the maximum LED current is set by the CTRL2 pin voltage (Figure 7).

The temperature at which the LED current starts to fall and how fast the current falls is selected by the resistor network/resistance value used. Table 1 lists the websites of NTC resistor manufacturers Murata Electronics, TDK, and Digi-Key. Murata Electronics specifically provides an online simulator for selecting the desired resistor combination (as shown in Figure 6), which includes a product catalog that shows the specifications of NTC resistors. Figure 5 shows an example of the LT3478-1 programmed LED current fall-off vs. temperature curve using Option C of Figure 6, where: R4 = 19.3k, RY = 3.01k, RNTC = 22k (NCP15XW223J0SRC). For a more detailed description of how to determine these values ​​by hand calculation, please refer to the LT3478 and LT3478-1 data sheets.

Table 1: NTC resistor manufacturers/distributors.

Figure 7: CTRL1 and CTRL2 pin voltage vs. temperature.

Analog dimming

Many LED applications require accurate brightness control. LED brightness can be reduced simply by reducing the LED current, a method known as "analog dimming," but reducing the LED operating current will change the color of the LED. The LT3478 and LT3478-1 can achieve 10:1 dimming by reducing the CTRL1 pin voltage from 1V to 0.1V. If color preservation is important, PWM dimming is a better option.

Figure 8: PWM dimming is achieved via the PWM pin.

Figure 9: PWM dimming waveform. When the PWM pin is effectively high or low, the LED current is maximum or zero, respectively.

PWM dimming

PWM dimming (Figures 8 and 9) produces very high dimming ratios without causing current-dependent LED color changes. PWM dimming of the LT3478 and LT3478-1 is achieved through the PWM pin. When the PWM pin is effectively high (TPWM(ON)) or low, the LED current is at its maximum or zero, respectively. The LED on-time (or average current) is controlled by the duty cycle of the PWM pin. Since the LED always operates at the same current (the maximum current is set by the CTRL1 pin) and only the average current changes, dimming does not cause the LED color to change.

PWM dimming is not a new technology, but achieving high PWM dimming ratios (which require very low PWM duty cycles) is challenging. The LT3478 and LT3478-1 use a patented architecture to achieve PWM dimming ratios of over 3,000:1 (100Hz). The application circuit of Figure 10 can achieve PWM dimming ratios of over 3,000:1, provided that the PWM on-time is reduced to 3 switching cycles (when fPWM = 100Hz, TPWM(ON) < 3.3μs). Figures 11 and 12 are the corresponding relationship curves and waveforms of Figure 10.

Figure 10: Boost LED driver circuit optimized for high PWM dimming ratios.

The maximum PWM dimming ratio (PDR) achieved by using the PWM pin satisfies the following relationship:

PWM dimming ratio = 1/minimum PWM duty cycle = 1/(TPWM(ON)MIN·fPWM)

The simplified waveforms in Figure 10 and the guidelines given below illustrate the relationship between PWM duty cycle, PWM frequency, PWM dimming ratio, and LED current:

1. For a PWM frequency (fPWM) of 100Hz, a PDR of 3,000 means a PWM on-time of 3.3μs.

2. For a fixed PWM on-time, the lower the PWM frequency, the higher the PWM dimming ratio. However, there is a limit to the lowest level to which the PWM frequency can be controlled, because the human eye can perceive flicker at a frequency lower than 80Hz.

3. Increasing the programmed switching frequency (fOSC) can improve PDR, but will result in reduced efficiency and increased internal heat generation. Generally speaking, TPWM(ON)MIN=3×1/fOSC (approximately 3 switching cycles).

4. The leakage current of the output capacitor should be minimized. When the PWM pin is low, the LT3478 and LT3478-1 will shut down all circuits that obtain operating current from VOUT.

5. To obtain a wider dimming range, PWM dimming and analog dimming functions can be combined. In this case, TDR = PDR·ADR, where TDR = total dimming ratio, PDR = PWM dimming ratio, and ADR = analog dimming ratio. A PDR of 3000:1 and an ADR of 10:1 (CTRL pin voltage is 0.1V) will produce a TDR of 30,000:1.

Figure 11: LED current vs. PWM dimming ratio for the circuit of Figure 10.

Open LED protection

The output voltage has a programmable maximum value to prevent damage to the LED due to disconnection (open LED) and then reconnection. During LED disconnection, the converter can become an open loop and drive the output voltage to a very high level, causing damage to the internal power switch. Most LED drivers have a fixed maximum output voltage to protect the switch, but this voltage may be too high for the reconnected LED string. The LT3478 and LT3478-1 provide a programmable overvoltage protection (OVP) level to limit the output voltage based on the number of LEDs in series. The OVPSET voltage is responsible for limiting the maximum output voltage, maximum output voltage = OVPSET voltage x 41.

Figure 12: PWM dimming waveform for the circuit in Figure 10.

The OVPSET voltage is derived from VREF using its own resistor divider or by adding a resistor to the divider used to determine the CTRL1 voltage. The OVPSET programming level should not exceed 1V to ensure that the switch voltage does not exceed 42V.

High reliability: fault detection and soft start

To achieve reliable performance during hot-swap, startup, or normal operation, the LT3478 and LT3478-1 monitor system parameters for any of the following faults: VIN < 2.8V, SHDN < 1.4V, inductor inrush current greater than 6A, and/or output voltage above the programmed OVP voltage. Once any of these faults are detected, the LT3478 and LT3478-1 immediately stop switching and discharge the soft-start pin (Figure 13). When all faults are removed and the SS voltage is discharged below 0.25V, the internal 12μA supply charges the SS pin at a rate set by the external capacitor CSS. The gentle rise of the SS voltage is equivalent to a ramp-up of the switch current limit until the SS voltage exceeds the VC voltage.

High efficiency: independent inductor and IC supplies, programmable fOSC, 60mΩ switch

The LT3478 and LT3478-1 can use separate IC and inductor supplies to optimize efficiency and switch duty cycle range. Inductor inrush current is sensed using the VS and L pins, independent of the VIN supply (Figure 2), which enables VIN to be powered from the system's lowest available supply (at least 2.8V) to minimize efficiency losses in the power switch driver. This allows the inductor to be powered from a supply (2.8V to 36V) that is more suitable for the duty cycle and power requirements of the LED load. The switching frequency of the power switch can be adjusted to achieve the optimal inductor size and efficiency performance required by the system. The 60mΩ on-resistance further improves efficiency by minimizing switching losses (for high duty cycle operation).

Figure 13: LT3478/LT3478-1 fault detection and SS pin voltage timing diagram.

Conclusion

The LT3478 and LT3478-1 are ideal for boost, buck or buck-boost LED driver applications that require high LED current and high PWM dimming ratios. The high peak switch current limit of 4.5A and the new PWM dimming architecture enable the LT3478 and LT3478-1 to provide high PWM dimming ratios at LED currents up to 4A. (Linear Technology Corporation)

Figure 14: Buck-boost LED driver circuit for portable photoflash applications.