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CAT32TDI-T3 Datasheet(PDF) 8 Page - ON Semiconductor |
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CAT32TDI-T3 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 11 page CAT32 http://onsemi.com 8 Table 8. RSET RESISTOR VALUES ILED (mA) RSET 40 562 W 30 750 W 25 909 W 20 1.13 kW 15 1.50 kW 10 2.26 kW 5 4.53 kW For other LED current values, use the following equation to choose RSET. RSET + 255 0.1 V ILED Most white LEDs are driven at maximum currents of 15 mA to 20 mA. Some higher power designs will use two parallel strings of LEDs for greater light output, resulting in 30 mA to 40 mA (two strings of 15 mA to 20 mA) flowing into the LED pin. 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 5 kHz to 40 kHz 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 brightness. 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): RPWM + RSET VMAX 0.15 V * 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 15 mA or 20 mA. 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): RMIN + 255 0.1 V ILED(MIN) RINCR determines how much LED current increases when the external NMOS switch is turned ON. RINCR + 255 0.1 V ILED(Increase) LED Dimming with a DC Voltage RADJ + 225 VMAX * 0.1 V ILED(MAX) * ILEAD(MIN) PCB Layout Guidelines The CAT32 is a high−frequency switching regulator and therefore proper PCB board layout and component placement can minimize noise and radiation and increase efficiency. To maximize efficiency, the CAT32 design has fast switch rise and fall times. To prevent radiation and high frequency resonance problems minimize the length and area of all traces connected to the SW pin and use a ground plane under the switching regulator. The switch, schottky output diode and output capacitor signal path should be kept as short as possible. The ground connection for the RSET resistor should be tied directly to the GND pin and not be shared with other components. Figure 15. LED Dimming Circuits CAT32 5 PWM CAT32 RSET 4 PWM CAT32 RSET 4 CAT32 RSET PWM 10 kW 0.1 mF 4 CAT32 RSET Logic Signal 4 SHDN RSET RSET RPWM RPWM RSET RADJ VDC RMIN RINCR |
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