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LTC3209EUF-1 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3209EUF-1 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC3209-1/LTC3209-2 9 320912fa The LTC3209-1 has 6 MAIN outputs, 1 CAM output and 1 AUX output. The LTC3209-2 has 5 MAIN outputs, 2 CAM outputs and 1 AUX output. Power Management The LTC3209-1/LTC3209-2 use a switched capacitor charge pump to boost CPO to as much as 2 times the input voltage up to 5.1V. The part starts up in 1x mode. In this mode, VBAT is connected directly to CPO. This mode provides maximum efficiency and minimum noise. The LTC3209-1/LTC3209-2 will remain in 1x mode until a MAIN or CAM LED current source drops out. Dropout occurs when a current source voltage becomes too low for the programmed current to be supplied. When dropout is detected, the LTC3209-1/LTC3209-2 will switch into 1.5x mode. The CPO voltage will then start to increase and will attempt to reach 1.5x VBAT up to 4.6V. Any subsequent dropout will cause the part to enter the 2x mode. The CPO voltage will attempt to reach 2x VBAT up to 5.1V. The part will be reset to 1x mode whenever a DAC data bit is updated via the I2C port or on the falling edge of the CAMHL signal. A 2-phase nonoverlapping clock activates the charge pump switches. In the 2x mode the flying capacitors are charged on alternate clock phases from VBAT to minimize input current ripple and CPO voltage ripple. In 1.5x mode the flying capacitors are charged in series during the first clock phase and stacked in parallel on VBAT during the second phase. This sequence of charging and discharging the flying capacitors continues at a constant frequency of 850kHz. The currents delivered by the LED current sources are controlled by an associated DAC. Each DAC is programmed via the I2C port. Soft-Start Initially, when the part is in shutdown, a weak switch connects VBAT1 to CPO. This allows VBAT1 to slowly charge the CPO output capacitor and prevent large charging currents to occur. The LTC3209-1/LTC3209-2 also employs a soft-start feature on its charge pump to prevent excessive inrush current and supply droop when switching into the step-up modes. The current available to the CPO pin is increased linearly over a typical period of 125 µs. Soft-start occurs at the start of both 1.5x and 2x mode changes. Charge Pump Strength When the LTC3209-1/LTC3209-2 operate in either 1.5x mode or 2x mode, the charge pump can be modeled as a Thevenin-equivalent circuit to determine the amount of current available from the effective input voltage and effective open-loop output resistance, ROL (Figure 1). ROL is dependent on a number of factors including the switching term, 1/(2fOSC • CFLY), internal switch resis- tances and the nonoverlap period of the switching circuit. However, for a given ROL, the amount of current available will be directly proportional to the advantage voltage of 1.5VBAT - CPO for 1.5x mode and 2VBAT- CPO for 2x mode. OPERATIO 1.5VBAT OR 2VBAT ROL CPO 320912 F01 – + + – Figure 1. Charge Pump Thevenin Equivalent Open-Loop Circuit |
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