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PAM2841 Datasheet(PDF) 10 Page - Power Analog Micoelectronics |
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PAM2841 Datasheet(HTML) 10 Page - Power Analog Micoelectronics |
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10 / 14 page  Figure. 1 2). Using an External PWM Signal to NMOS Gate. Figure. 2 3).UsingaDCVoltage 4). Using a Filtered PWM Signal When PWM signal is at high level, N MOSFET turned on, then pull comp pin down, then the LED current should be zero. When PWM signal is at low level, N MOSFET turned off. The circuit uses resistor R1 to set the on state current. The average LED current changes proportionally with the duty cycle of the PWM signal. A 100% duty cycle turns off the PAM2841 and leads to zero LED current. A 0% duty cycle generates full current. Also the recommend frequency is between 100 and 500Hz. Frequency<100Hz can naturally causes LEDs to blink visibly. For some applications, a simple and direct way to control brightness is use an external variable DC voltage to vary the voltage drop on feedback resistor. This will make the PAM2841 adjust the output current to follow the change of feedback voltage. The circuit is shown in Figure 3. As the DC voltage increases, the voltage drop on R4 increases and the v oltage drop on R3 decreases. Thus, the LED current decreases. The selection of R4 and R5 will make the current from the variable DC source much smaller than the LED current and much larger than the FB pin current. For VCC range from 0V to 2V, the selection of resistors in Figure 3 gives dimming control of LED current from 0mA to 20mA. The filtered PWM signal can be considered as an adjustable DC voltage. Such regulated signal is often with some grade of ripple because of some simple configuration of circuit. With appropriated arrangement of PWM frequency and level, and filter parameters, it can be used to replace the variable DC voltage source in dimming control. The circuit is shown in Figure 4. I =I * (T –T )/(T +T ) where T : on time of a period T : 0.08ms T : offtimeofaperiod I : on state current (full current) AVE STATE ON STARTUP ON OFF ON STARTUP OFF STATE Figure. 3 PAM2841 1.5A SW Current, 40V Precision WLED Driver 10 , Power Analog Microelectronics Inc www.poweranalog.com 08/2008 Rev 1.3 U12 PAM2841 PGND 1 VIN 2 ENA 3 COMP 4 GND 5 FB 6 OV 7 SW 8 L1 22uH 1 2 D1 1 2 C3 1uF C2 1uF VIN C1 10uF 0 0 PWM C4 10n R1 910k R2 27k OVP Q1 PWM U12 PAM2841 PGND 1 VIN 2 ENA 3 COMP 4 GND 5 FB 6 OV 7 SW 8 L1 22uH 1 2 D1 1 2 R3 10 C3 1uF C2 1uF VIN C1 10uF 0 0 ENA C4 10n R1 910k R2 27k OVP U33 PAM2841 PGND 1 VIN 2 ENA 3 COMP 4 GND 5 FB 6 OV 7 SW 8 L1 22uH 1 2 D1 1 2 R3 10 C3 1uF C2 1uF VIN C1 10uF 0 0 ENA C4 10n R1 910k R2 27k DC R4 5k R5 90k OVP U33 PAM2841 PGND 1 VIN 2 ENA 3 COMP 4 GND 5 FB 6 OV 7 SW 8 L1 22uH 1 2 D1 1 2 R3 10 C3 1uF C2 1uF VIN C1 10uF 0 0 0V 5V ENA C4 10n R1 910k R2 27k R6 10k C5 100nF 0 PWM R4 5k R5 90k OVP Figure. 4 |
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