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LD24-24-1000 Datasheet(PDF) 3 Page - MicroPower Direct, LLC |
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LD24-24-1000 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 Get a full explanation of this circuit in “Driving LEDs”. Available free at the MPD website 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. Again, we are powering the LED driver with the MPM-30D-15IS AC power supply. The 555 operates over a supply voltage range of 4.5 VDC to 18VDC. Here it is connected to the 15 VDC output of the SR7805 switching regulator (also driven by the MPM-20S-24EPB). 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 C6. The timing capacitor (C6) charges through R1 and D2. When it reaches the level of 2/3 VCC, the discharge pin (pin 7) goes low and C6 will discharge through D1 and R2 to the internal discharge transistor. When the C6 voltage drops to 1/3 VCC, the discharge pin goes high and C6 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 C6 is charging. Diode D2 is optional (but recommended), essentially block- ing R2 during the charge period. Theoretically, this circuit will allow for duty cycles over a range of about 5% to 95%. If manual adjustment is desired, a potentiometer may be substituted for R2 (with some adjustment of the circuit). The size of C6 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 . |
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