Foreword:
Nowadays, from home to industry, people's environmental awareness has been widely improved. Therefore, LED lighting lamps with energy-saving effects are rapidly becoming popular as replacements for original lighting fixtures (incandescent bulbs, fluorescent lamps, etc.).
As LED lighting, it needs to have a high-power product power source capable of driving small power products such as small lamps such as light bulbs to large-scale lighting such as street lamps. Moreover, this kind of power supply should be compatible with the "controllable silicon dimming" used in the original lighting fixtures, the "PWM dimming" dedicated to LED lighting, and the "linear dimming" which can be dimmed by bulk resistance. compatible.
In order to illuminate and dim the LED stably, an LED driver IC is required, and the driver IC is required to be compatible with various power sources such as power and dimming methods.
Features of ROHM's LED lighting driver IC.
In order to meet the above market demand, ROHM is developing driver ICs for LED lighting. In order to keep the LEDs at a certain brightness, an LED driver IC for constant current control is required. Normally, constant current control is started from the AC power supply, so the step-down switching regulator method is used (Fig. 1).
Figure 1 Common lighting power module
In the conventional method, the input voltage variation and the output voltage fluctuate greatly, and the power module must be redesigned according to the application conditions. If the input voltage fluctuates greatly, the rippled input voltage in the AC voltage frequency (100 Hz to 120 Hz after full-wave rectification) causes the brightness of the LED to flicker. In order to stabilize the input voltage, measures must be taken when designing the power module. If the output voltage fluctuates greatly, it is necessary to change the constant of the external component for each output voltage of the light source (the number of LEDs), and design a variety of power supply modules, thereby increasing the design workload (Fig. 2).
Moreover, due to the varying Vf of the LEDs, the brightness of the LED lighting fixtures may be inconsistent.
Input voltage characteristics
Output voltage characteristics
The switching regulator uses the ratio of the on-time to the off-time of the switch to control the LED current value. The conventional method of general control is to use the peak current of the coil to determine the on-time of the switch.
This method uses the change in input and output voltages to change the gradient of the coil current. As a result, the LED current value also changes.
(Figure 3-1)
Figure 3-1 LED current waveform (peak detection control)
Figure 3-2 LED current waveform (average control)
To solve this problem, the peak current should not be used to control the LED current, but must be controlled by the average value of the LED current (average current control).
However, the average current control is difficult to implement with the analog power control method generally used, and the number of external components tends to increase.