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CAT32 датащи(PDF) 9 Page - Catalyst Semiconductor |
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CAT32 датащи(HTML) 9 Page - Catalyst Semiconductor |
9 / 14 page CAT32 9 Doc No. 5001, Rev. AC LED DIMMING WITH PWM SIGNAL PWM brightness control provides the widest dimming range (greater than 20:1). By turning the LEDs ON and OFF using the control signal the LEDs operate at either zero or full current, but their average current changes with the PWM signal duty cycle. Typically, a 5kHz to 40kHz PWM signal is used. PWM dimming with the CAT32 can be accomplished two different ways. The SHDN pin can be driven directly or a resistor can be added to drive the RSET pin. If the SHDN pin is used, increasing the duty cycle will increase the LED bright- ness. Using this method, the LEDs can be dimmed and turned off completely using the same control signal. A 0% duty cycle signal will turn off the CAT32, reducing the total quiescent current to near zero. If the RSET pin is used, increasing the duty cycle will decrease the brightness. Using this method, the LEDs are dimmed using RSET and turned off completely using SHDN. If the RSET pin is used to provide PWM dimming, the approximate value of RPWM should be calculated (where VMAX is the “HIGH” value of the PWM signal): RR V V PWM SET MAX =× − 015 1 . In addition to providing the widest dimming range, PWM brightness control also ensures the “purest” white LED color over the entire dimming range. The true color of a white LED changes with operating current, and is the “purest” white at a specific forward current, usually 15mA or 20mA. If the LED current is less than or more than this value, the emitted light becomes more blue. Applications involving color LCDs can find the blue tint objectionable. When a PWM control signal is used to drive the SHDN pin of the CAT32, the LEDs are turned off and on at the PWM frequency. The current through them alternates between full current and zero current, so the average current changes with duty cycle. This ensures that when the LEDs are on, they can be driven at the appropriate current to give the purest white light. LED brightness varies linearly with the PWM duty cycle. LED DIMMING WITH A LOGIC SIGNAL For applications that need to adjust the LED brightness in discrete steps, a logic signal can be used. RMIN sets the minimum LED current value (when the NMOS is OFF): R V I MIN LED MIN =× 225 01 . () R INCR determines how much LED current increases when the external NMOS switch is turned ON. R V I INCR LED Increase =× 225 01 . () LED DIMMING WITH A DC VOLTAGE R VV I I ADJ MAX LED MAX LEAD MIN =× − − 225 01 . () ( ) CAT32 SHDN 5 PWM CAT32 RSET 4 PWM RPWM RSET CAT32 RSET PWM 4 10k Ω RSET RPWM 0.1µF CAT32 RSET VDC 4 RSET RADJ CAT32 RSET 4 RMIN RINCR LOGIC SIGNAL Figure 3: LED Dimming Circuits |
Аналогичный номер детали - CAT32 |
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Аналогичное описание - CAT32 |
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