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LTC3220-1 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3220-1 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  LTC3220/LTC3220-1 8 32201fc OPERATION Power Management The LTC3220/LTC3220-1 use a switched capacitor charge pump to boost CPO as much as 2 times the input voltage up to 5.1V. The part starts up in 1x mode. In this mode, VIN is connected directly to CPO. This mode provides maximum efficiency and minimum noise. The LTC3220/ LTC3220-1 will remain in 1x mode until an 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 LTC3220/ LTC3220-1 will switch into 1.5x mode. The CPO voltage will then start to increase and will attempt to reach 1.5 × VIN up to 4.6V. Any subsequent dropout will cause the part to enter the 2x mode. The CPO voltage will attempt to reach 2 × VIN up to 5.1V. A 2-phase non-overlapping clock activates the charge pump switches. In the 2x mode the flying capacitors are charged on alternate clock phases from VIN to minimize CPO voltage ripple. In 1.5x mode the flying capacitors are charged in series during the first clock phase and stacked in parallel on VIN during the second phase. This sequence of charging and discharging the flying capacitors continues at a constant frequency of 850kHz. The current delivered by each LED current source is con- trolled by an associated DAC. Each DAC is programmed via the I2C port. + – ROL CPO 3220 F01 1.5VIN OR 2VIN + – Figure 1. Charge Pump Open-Loop Thevenin Equivalent Circuit Soft-Start Initially, when the part is in shutdown, a weak switch connects VIN to CPO. This allows VIN to slowly charge the CPO output capacitor and prevent large charging currents from occurring. The LTC3220/LTC3220-1 also employ a soft-start feature on the 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 125μs. Soft-start occurs at the start of both 1.5x and 2x mode changes. Charge Pump Strength When the LTC3220/LTC3220-1 operate in either 1.5x mode or 2x mode, the charge pump can be modeled as a Theve- nin-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 non-overlap period of the switching circuit. However, for a given ROL, the amount of current available will be directly proportional to the advantage voltage of 1.5VIN – CPO for 1.5x mode and 2VIN – CPO for 2x mode. Consider the example of driving LEDs from a 3.1V supply. |
Similar Part No. - LTC3220-1_15 |
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Similar Description - LTC3220-1_15 |
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