Independent LED disconnect enables fast LED turn-on after long LED off time

LEDs used in machine automation and flash photography sometimes require fast turn-on times and long turn-off times. It is not always possible to achieve a regulated LED current turn-on of 100μs after a few seconds of turn-off, using traditional methods. Traditionally, shutdown functions are used to turn off LED drivers for long periods of time for energy saving. In shutdown mode, the output voltage will drop, and the restart after shutdown will invoke the soft-start function circuit to prevent large inrush currents. This will result in a relatively long and controlled turn-on process. PWM is responsible for turning the LED on and off, but it is often designed for repetitive and short turn-on and turn-off times (100Hz and higher). During the short off-time, the output capacitor is expected to maintain its value despite the minimal leakage. For the very high PWM dimming ratios of Linear Technology LED drivers, the ability of the output capacitor to maintain its voltage during the PWM off period is critical. In the case of a long low PWM, the output voltage may still collapse. The subsequent turn-on operation may be 10 times or more slower than the requirements of the mechanical automation. Therefore, a different method is needed to turn off the LED and prevent the output voltage from collapsing (other than using bulky and unsatisfactory output capacitors).

An alternative approach using individual LED disconnect (without shutting down the PWM or converter) is to achieve an on-time as low as 100μs after the LEDs have been disconnected for a few seconds using the LT3754 16-channel LED driver shown in Figure 1. When the LEDs are disconnected from the circuit but the IC is operational, the LED current drops to zero and the output voltage regulates up to the OVP level (above the LEDs). The output never collapses, in fact, it rises and regulates high. When the LEDs are reconnected after any length of time, the LED current quickly settles to its programmed level of 100mA (50mA per channel, two channels in parallel for 8 strings of LEDs). The high output voltage enables immediate operation of the internal 8-string ballast. The ballast LED pin voltage quickly rises to the level required to achieve 100mA current through each string of LEDs connected to the high output voltage. Then, with the LEDs on, the output voltage and ballast voltage drop to their intended operating points.

Figure 1: The LT3754 16-channel 50mA LED driver features independent LED disconnect (option one), enabling <100μs LED on-time after a long LED disconnect time. Option two is a traditional LED string connection to the output for typical PWM dimming and shutdown—but not fast enough for machine automation.

Figure 2a shows a 100μs LED current turn-on time achieved with the individual LED disconnects shown in Figure 1. When the LEDs are connected traditionally and PWM is used to turn the LEDs on and off, as shown in Figure 2b, the LED current turn-on time is as long as 1.35ms (compare to Figure 2a). In both cases, the LEDs remain off for 5 seconds before being turned on.

Figure 2a: Fast LED turn-on using LED disconnect (option one) after the LED has been off for 5 seconds.

Figure 2b: LED turn-on after 5 seconds of PWM off, using conventional PWM dimming (option 2).