Datasheet LT3590 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 48V Buck Mode LED Driver in SC70 and 2mm × 2mm DFN |
| Pages / Page | 16 / 10 — APPLICATIONS INFORMATION. Direct PWM Dimming. Figure 6. Direct PWM … |
| File Format / Size | PDF / 237 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Direct PWM Dimming. Figure 6. Direct PWM Dimming Waveforms
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LT3590
APPLICATIONS INFORMATION Direct PWM Dimming
48V input voltage. The achievable dimming range for this application and 100Hz PWM frequency can be determined Changing the forward current fl owing in the LEDs not only using the following method. changes the intensity of the LEDs, it also changes the color. The chromaticity of the LEDs changes with the change in Example: forward current. Many applications cannot tolerate any shift in the color of the LEDs. Controlling the intensity of ƒ = 100Hz, tSETTLE = 50μs the LEDs with a direct PWM signal allows dimming of the LEDs without changing the color. In addition, direct PWM tPERIOD = 1 = 1 = 0 0 . 1s dimming offers a wider dimming range to the user. ƒ 100 Dimming the LEDs via a PWM signal essentially involves t . 0 0 s PERIOD 1 turning the LEDs on and off at the PWM frequency. The Dim Range = = = 200:1 t μs 50 typical human eye has a limit of ~60 frames per second. SETTLE By increasing the PWM frequency to ~80Hz or higher, t μs SETTLE 50 the eye will interpret that the pulsed light source is con- Min Duty Cycle = • 100 = • 10 00 = 0 5 . % t . 0 0 s 1 tinuously on. Additionally, by modulating the duty cycle PERIOD (amount of “on-time”), the intensity of the LEDs can be Duty Cycle Range = 100%→ 0 5 . % at 100Hz controlled. The color of the LEDs remains unchanged in this scheme since the LED current value is either zero or The calculations show that for a 100Hz signal the dimming a constant value. range is 200 to 1. In addition, the minimum PWM duty cycle of 0.5% ensures that the LED current has enough The time it takes for the LED current to reach its pro- time to settle to its fi nal value. Figure 7 shows the dim- grammed value sets the achievable dimming range for a ming range achievable for three different frequencies with given PWM frequency. For example, the settling time of a settling time of 50μs. the LED current in Figure 6 is approximately 50μs for a PWM 5V/DIV 100Hz VSW 20V/DIV 1kHz 10kHz ILED 20mA/DIV VIN = 48V 2ms/DIV 3590 F06 4 LEDs 1 10 100 1000 PWM DIMMING RANGE 3590 F07
Figure 6. Direct PWM Dimming Waveforms Figure 7. Dimming Range Comparison of Three PWM Frequencies
3590f 10
APPLICATIONS INFORMATION Direct PWM Dimming
48V input voltage. The achievable dimming range for this application and 100Hz PWM frequency can be determined Changing the forward current fl owing in the LEDs not only using the following method. changes the intensity of the LEDs, it also changes the color. The chromaticity of the LEDs changes with the change in Example: forward current. Many applications cannot tolerate any shift in the color of the LEDs. Controlling the intensity of ƒ = 100Hz, tSETTLE = 50μs the LEDs with a direct PWM signal allows dimming of the LEDs without changing the color. In addition, direct PWM tPERIOD = 1 = 1 = 0 0 . 1s dimming offers a wider dimming range to the user. ƒ 100 Dimming the LEDs via a PWM signal essentially involves t . 0 0 s PERIOD 1 turning the LEDs on and off at the PWM frequency. The Dim Range = = = 200:1 t μs 50 typical human eye has a limit of ~60 frames per second. SETTLE By increasing the PWM frequency to ~80Hz or higher, t μs SETTLE 50 the eye will interpret that the pulsed light source is con- Min Duty Cycle = • 100 = • 10 00 = 0 5 . % t . 0 0 s 1 tinuously on. Additionally, by modulating the duty cycle PERIOD (amount of “on-time”), the intensity of the LEDs can be Duty Cycle Range = 100%→ 0 5 . % at 100Hz controlled. The color of the LEDs remains unchanged in this scheme since the LED current value is either zero or The calculations show that for a 100Hz signal the dimming a constant value. range is 200 to 1. In addition, the minimum PWM duty cycle of 0.5% ensures that the LED current has enough The time it takes for the LED current to reach its pro- time to settle to its fi nal value. Figure 7 shows the dim- grammed value sets the achievable dimming range for a ming range achievable for three different frequencies with given PWM frequency. For example, the settling time of a settling time of 50μs. the LED current in Figure 6 is approximately 50μs for a PWM 5V/DIV 100Hz VSW 20V/DIV 1kHz 10kHz ILED 20mA/DIV VIN = 48V 2ms/DIV 3590 F06 4 LEDs 1 10 100 1000 PWM DIMMING RANGE 3590 F07
Figure 6. Direct PWM Dimming Waveforms Figure 7. Dimming Range Comparison of Three PWM Frequencies
3590f 10
