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LD24W Datasheet(PDF) 3 Page - MicroPower Direct, LLC |
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LD24W Datasheet(HTML) 3 Page - MicroPower Direct, LLC |
3 / 4 page MicroPower Direct • 292 Page Street Ste D Stoughton, MA 02072 • TEL: (781) 344-8226 • FAX: (781) 344-8481 • E-Mail: sales@micropowerdirect.com www.micropowerdirect.com PWM Output Current Control An LED operates at its maximum efficiency when operated at the rated drive current specified by the manufacturer. Operating an LED at lower than its rated forward current not only decreases the system efficiency; but may cause color (or wave- length) shifting. In illumination applications, this could cause visible changes to lighting. A preferred method is using pulse width modula- tion (PWM). As shown at left, the output current is adjusted by applying a PWM signal to the VADJ input. By varying the signal duty cycle the average output current is adjusted up or down. To avoid visible flicker, the PWM signal should be greater than 100 Hz. For duty cycles (DPWM) between 0 and 1, the output current is derived by the formula: II X D NOM MAX PWM = The VADJ input may be driven via an open collec- tor transistor (as shown). The diode and resistor suppress high amplitude negative spikes that may be caused by the drain-source capacitance of the transistor. Negative spikes on the control input of the unit could cause errors in output current or erratic operation. The VADJ input can also be driven by the open drain output of a microcontroller. Again, any high amplitude negative spikes that may be caused by the drain- source capacitance of the FET must be supressed. PWM Control Signals PWM Dimming Application A simple method of achieving digital (or PWM) dimming is by using a 555 timer to apply a series of pulses to the VADJ input, as illustrated above. The 555 oper- ates over a supply voltage range of 4.5 VDC to 15VDC. Here it is connected to the 12 VDC output of the SR7805 switching regulator (this is also the VIN of the LED driver). Care should be taken to minimize ripple at the VCC input. Excess ripple could cause timing errors. The timer is connected for astable (free run) operation. The frequency is set by R1, R2 and C4. The timing capacitor (C4) charges through R1 and D2. When it reaches the level of 2/3 VCC, the discharge pin (pin 7) goes low and C4 will discharge through D1 and R2 to the internal discharge transistor. When the C4 voltage drops to 1/3 VCC, the discharge pin goes high and C4 begins to charge again. The formulas for calculating the frequency and duty cycle are included in the MPD application note “Driving LEDs”. The diodes (D1 and D2) allow duty cycles below 50% to be set. Diode D1 bypasses R2 while C4 is charging. Diode D2 is optional (but recommended), essentially blocking R2 during the charge period. Theoretically, this circuit will allow for duty cycles over a range of approximately 5% to 95%. If manual adjustment is desired, a potentiometer may be substituted for R2 (with some adjustment of the circuit). The size of C4 is generally not critical, but it should be as low leakage as pos- sible. In order to avoid excessive current flow through the internal discharge transistor, it is recommended that R1 be at least 5 k . Get a full explanation of this circuit in “Driving LEDs”. Available free at the MPD website |
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Descripción similar - LD24W |
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