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SP7121EK-LTR Datasheet(PDF) 5 Page - Sipex Corporation |
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SP7121EK-LTR Datasheet(HTML) 5 Page - Sipex Corporation |
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5 / 11 page  Sept 11 - 07 Rev E SP7121 Low Dropout LED Driver © 2007 Sipex Corporation Page 5 Example For a 2kHz signal, the minimum duty cycle for the PWM signal is %Minimum Duty cycle =3% Fault Operation If one of the LEDs is shorted, the LED voltage at this pin will be VCC but that channel will provide the nominal current value, thereby increasing power dissipation. If all channels are shorted, excessive power dissipation may damage the device. If an LED is open, the LED pin voltage will be pulled up to Vcc, and the LED current will be reduced to 3mA for the other channels. The SP7121 doesn’t have protection from the ISET pin being shorted to ground or the RSET value being too low. With an operational maximum current of 30mA per channel, the device may be damaged if the ISET pin is shorted to the ground or the RSET value is below 8k . APPLICATION INFORMATION Thermal Considerations The SP7121 does not have overtemperature protection. Special care needs to be taken to ensure that the driver’s junction temperature stays below 125˚C. The thermal resistance of the package is 160˚C per Watt. Under normal operation conditions with about 30mA of current per LED and a voltage drop across the part of 1V, the part will still only dissipate .06 Watts -- giving a temperature rise of only 11.4˚C Example of thermal calculations. Where P is total power dissipation Vd is the voltage drop across the SP7121 IOUT is the total output current Where P is the total power dissipation ΘJA is the thermal resistance of the package per Watt. Above example Temperature Rise = 11.4˚C The part will be able will operate up to 85˚C under these conditions. BOARD LAYOUT AND GROUNDING Although the SP7121 does not require an input capacitor for normal operation, a 1nF capacitor can be used to obtain better overall circuit performance. To obtain the best performance from the SP7121, a printed circuit board with ground plane is required. High quality, low series resistance ceramic 1nF bypass capacitors should be used at the Vcc and GND pins (pins 1 and 2). This capacitor must be located as close to the pins as possible. The traces connecting the pins and the capacitor must be kept short and should be made as wide as possible. |
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