Electronic Components Datasheet Search |
|
VLCF4020T-220MR56 Datasheet(PDF) 9 Page - Linear Integrated Systems |
|
VLCF4020T-220MR56 Datasheet(HTML) 9 Page - Linear Integrated Systems |
9 / 16 page LT3591 9 3591f APPLICATIONS INFORMATION Table 4. RSENSE Value Selection for 200mV Sense ILED (mA) RSENSE (Ω) 540 10 20 15 13.3 20 10 DIMMING CONTROL There are three different types of dimming control circuits. The LED current can be set by modulating the CTRL pin with a DC voltage, a filtered PWM signal or directly with a PWM signal. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The CTRL pin voltage can be modulated to set the dim- ming of the LED string. As the voltage on the CTRL pin increases from 0V to 1.5V, the LED current increases from 0 to ILED. As the CTRL pin voltage increases beyond 1.5V, it has no effect on the LED current. The LED current can be set by: I mV R when V V I V LED SENSE CTRL LED CTRL ≈> ≈ 200 15 6 ,. .2 25 125 • ,. R when V V SENSE CTRL < Feedback voltage variation versus control voltage is given in the Typical Performance Characteristics. Using a Filtered PWM Signal A filtered PWM signal can be used to control the brightness of the LED string. The PWM signal is filtered (Figure 5) by a RC network and fed to the CTRL pin. The corner frequency of R1, C1 should be much lower than the frequency of the PWM signal. R1 needs to be much smaller than the internal impedance of the CTRL pin which is 10MΩ (typ). Direct PWM Dimming Changing the forward current flowing in the LEDs not only changes the intensity of the LEDs, it also changes the color. The chromaticity of the LEDs changes with the change in forward current. Many applications cannot tolerate any shift in the color of the LEDs. Controlling the intensity of the LEDs with a direct PWM signal allows dimming of the LEDs without changing the color. In addition, direct PWM dimming offers a wider dimming range to the user. Dimming the LEDs via a PWM signal essentially involves turning the LEDs on and off at the PWM frequency. The typical human eye has a limit of ~60 frames per second. By increasing the PWM frequency to ~80Hz or higher, the eye will interpret that the pulsed light source is con- tinuously on. Additionally, by modulating the duty cycle (amount of “on-time”), the intensity of the LEDs can be controlled. The color of the LEDs remains unchanged in this scheme since the LED current value is either zero or a constant value. Figure 6 shows a Li-Ion powered driver for ten white LEDs. Direct PWM dimming method requires an external NMOS tied between the cathode of the lowest LED in the string LT3591 CTRL C1 0.1 µF PWM 10kHz TYP 3591 F05 R1 100k Figure 5. Dimming Control Using a Filtered PWM Signal CTRL PWM FREQ VIN L1 22 µH VIN 3V TO 5V RSENSE 10 Ω LT3591 SW CAP LED 100k GND 0V 5V C2 2.2 µF Q1 Si2308 C1 1 µF Figure 6. Li-Ion to Ten White LEDs with Direct PWM Dimming |
Similar Part No. - VLCF4020T-220MR56 |
|
Similar Description - VLCF4020T-220MR56 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |