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LTC3212 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3212 Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page LTC3212 0 3212fb Power Efficiency To calculate the power efficiency ( η) of an LED driver chip, the LED power should be compared to the input power. The difference between these two numbers represents lost power whether it is in the charge pump or the cur- rent sources. Stated mathematically, the power efficiency is given by: η = P P LED IN The efficiency of the LTC3212 depends upon the mode in which it is operating. Recall that the LTC3212 operates as a pass switch, connecting VIN to CPO, until dropout is detected at an ILED pin. This feature provides the op- timum efficiency available for a given input voltage and LED forward voltage. When it is operating as a switch, the efficiency is approximated by: η = = = P P V I V I V V LED IN LED LED BAT BAT LED BAT • • since the input current will be very close to the sum of the LED currents. At moderate to high output power, the quiescent current of the LTC3212 is negligible and the expression above is valid. Once dropout is detected at any LED pin, the LTC3212 enables the charge pump in 2x mode. In 2x boost mode, the efficiency is similar to that of a linear regulator with an effective input voltage of 2 times the actual input voltage. In an ideal 2x charge pump, the power efficiency would be given by: ηIDEAL LED IN LED LED BAT LED LED P P V I V I V V = = = • • • • 2 2 BBAT In some applications it may be possible to increase the efficiency of the LTC3212. If any of the LED’s maximum forward voltage is less than the minimum VIN supply volt- age minus ILED dropout voltage then the charge pump is not needed to drive that LED. This is often the case with the red LED due to its lower forward voltage. Its anode may be connected directly to VIN, bypassing the charge pump’s losses in 2x mode. Thermal Management Ifthejunctiontemperatureincreasesaboveapproximately 140°C the thermal shutdown circuitry will automatically deactivate the output current sources and charge pump. To reduce maximum junction temperature, a good thermal connection to the PC board is recommended. Connecting the Exposed Pad to a ground plane and maintaining a solid ground plane under the device will reduce the thermal resistance of the package and PC board considerably. applicaTions inForMaTion |
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