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EL7900ILCZ Datasheet(PDF) 6 Page - Intersil Corporation |
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EL7900ILCZ Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 8 page 6 FN7377.8 September 18, 2009 brightness and 1.25V represents the maximum. The first step would be to determine the ratio of R1 and R2: Next, the value of R2 can be calculated based on the maximum output current coming from the EL7900 under the application's maximum light exposure. Suppose the current has been determined to be about 125µA. Thus, R2 can be calculated as shown in Equations 5 and 6: and In Figure 11, user adjustable bias control has been added to allow control over the minimum and maximum output voltage. This allows the user to adjust the output brightness to personal preference over a limit range via the 3V PWM control. Figure 12 shows how a fully manual override can be quickly added by using the EN pin. Short Circuit Current Limit The EL7900 does not limit the output short circuit current. If the output is directly shorted to the ground continuously, the output current could easily increase for a strong input light such that the device may be damaged. Maximum reliability is maintained if the output continuous current never exceeds 6mA by adding a load resistor at the output. This limit is set by the design of the internal metal interconnects. Suggested PCB Footprint Footprint pads should be a nominal 1-to-1 correspondence with package pads. The large, exposed central die-mounting paddle in the center of the package requires neither thermal nor electrical connections to PCB, and such connections should be avoided. Power Supply Bypassing and Printed Circuit Board Layout The EL7900 is relatively insensitive to the printed circuit board layout due to its low speed operation. Nevertheless, good printed circuit board layout is necessary for optimum performance. Ground plane construction is highly recommended, lead length should be as short as possible and the power supply pins must be well bypassed to reduce the risk of oscillation. For normal single supply operation, where the GND pin is connected to ground, a 0.1µF ceramic capacitor should be placed from VCC pin to GND pin. A 4.7µF tantalum capacitor should then be connected in parallel, placed close to the device. R 1 R 2 3.0V 0.25V ---------------- 1 – ⎝⎠ ⎛⎞ × 11 R 2 × == (EQ. 4) 3V TO 5V SUPPLY VOLTAGE 3V DC VOLTAGE EL7900 OUT EN GND VDO TO INVERTER BRIGHTNESS CONTROLLER C1 R2 R1 FIGURE 10. AUTOMATIC DIMMING SOLUTION 10k 25µF 110k INPUT OR LED DRIVER R 2 1.25V 125 μA ------------------ ⎝⎠ ⎛⎞ 10k Ω == (EQ. 5) R 1 11 R 2 × 110k Ω == (EQ. 6) 3V TO 5V SUPPLY VOLTAGE 3V PWM CONTROL EL7900 OUT EN GND VDO FIGURE 11. AUTOMATIC DIMMING SOLUTION WITH ADJUSTABLE BIAS CONTROL C1 R2 R1 10k 25µF 110k TO INVERTER BRIGHTNESS CONTROLLER INPUT OR LED DRIVER 3V TO 5V SUPPLY VOLTAGE 3V PWM CONTROL EL7900 OUT EN GND VDO FIGURE 12. AUTOMATIC DIMMING SOLUTION WITH ADJUSTABLE BIAS CONTROL AND MANUAL OVERRIDE ENABLE/ CONTROL DISABLE C1 R2 R1 10k 25µF 110k TO INVERTER CONTROLLER BRIGHTNESS LED DRIVER INPUT OR EL7900 |
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