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LTC3230EUD-PBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3230EUD-PBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3230 10 3230fa Charge Pump Soft-Start In shutdown, CPO is disconnected from VIN and is pulled downthrougha14.3kresistor.Whenenabled,aweakswitch connects VIN to CPO. This allows VIN to slowly charge the CPO output to prevent large charging currents. The LTC3230 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 50μs. Soft-start occurs at the start of both 1.5x and 2x modes. Charge Pump Strength and Regulation Regulation is achieved by sensing the voltage at the CPO pin and modulating the charge pump strength based on the error signal. The CPO regulation voltages are set internally, and are dependent on the charge pump modes as shown in Table 1. Table 1. Charge Pump Output Regulation Voltages CHARGE PUMP MODE REGULATED VCPO 1.5x 4.5V 2x 5V When the LTC3230 operates 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 2). 1.5 • VIN – CPO for 1.5x mode and 2 • VIN – CPO for 2x mode. Consider the example of driving white LEDs from a 3.1V supply. If the LED forward voltage is 3.8V and the current sources require 100mV, the advantage voltage for 1.5x mode is 3.1V • 1.5 – 3.8V – 0.1V or 750mV. Notice that if the input voltage is raised to 3.2V, the advantage voltage jumps to 900mV – a 20% improvement in avail- able strength. From Figure 2, for 1.5x mode the available current is given by: I VV R OUT IN CPO OL = 15 .• – For 2x mode, the available current is given by: I VV R OUT IN CPO OL = 2• – Notice that the advantage voltage in this case is 3.1V • 2 – 3.8V – 0.1V = 2.3V. ROL is higher in 2x mode but a significant increase in available current is achieved. Typical values of ROL as a function of temperature are shown in Figures 3 and 4. Mode Switching The LTC3230 will automatically switch from 1x mode to 1.5x mode and subsequently to 2x mode whenever a dropout condition is detected at any LED pin. Dropout occurs when a current source voltage becomes too low for the programmed current to be supplied. The time from dropout detection to mode switching is typically 0.5ms. The charge pump mode is reset back to 1x when the LED drivers are shut down (ENM = ENS = Low) or on the falling edge of either ENM or ENS. An internal comparator will not allow the main switches to connect VIN and CPO in 1x mode until the voltage at the CPO pin has decayed to less than or equal to the voltage at the VIN pin. LDO Operation Two independent low drop-out linear regulators are in the LTC3230. Each regulator may be independently enabled (ENLDO1 and ENLDO2) from each other and from the OPERATION + – ROL CPO 1.5VIN OR 2VIN + – 3230 F02 Figure 2. Charge Pump Thevenin Equivalent Open-Loop Circuit ROL is dependent on a number of factors including the switching term, 1/(2 • fOSC • 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 |
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