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LTC3203B1 Datasheet(PDF) 10 Page  Linear Technology 

LTC3203B1 Datasheet(HTML) 10 Page  Linear Technology 
10 / 16 page LTC3203/LTC32031 LTC3203B/LTC3203B1 10 32031fa APPLICATIO S I FOR ATIO Power Efficiency The power efficiency ( η) of the LTC3203/LTC32031/ LTC3203B/LTC3203B1 in 1.5x mode is similar to that of a linear regulator with an effective input voltage of 1.5 times the actual input voltage. This occurs because the input current for a 1.5x fractional charge pump is approxi mately 1.5 times the load current. In an ideal regulating 1.5x charge pump the power efficiency would be given by: η 15 15 15 . .. XIdeal OUT IN OUT OUT IN OUT OUT IN P P VI VI V V == • • = Similarly, in 2x mode, the efficiency is similar to that of a linear regulator with an effective input voltage of twice the actual input voltage. In an ideal regulating voltage doubler the power efficiency would be given by: η 2 22 XIdeal OUT IN OUT OUT IN OUT OUT IN P P VI VI V V == • • = At moderate to high output power the switching losses and quiescent current of the LTC3203/LTC32031/ LTC3203B/LTC3203B1 are negligible and the expression above is valid. As evident from the above two equations, with the same VIN, the 1.5x mode will give higher efficiency than the 2x mode. Programming the LTC3203/LTC3203B Output Voltage (FB Pin) While the LTC32031/LTC3203B1 have internal resistive dividers to program the output voltage, the programmable LTC3203/LTC3203B may be set to an arbitrary voltage via an external resistive divider. Since it operates as a voltage doubling charge pump when MODE is less than VMODEL, it is not possible to achieve output voltages greater than twice the available input voltage in this case. Similarly, when MODE is greater than VMODEH, the achievable output voltage is less than 1.5 times the available input voltage. Figure 1 shows the required voltage divider connection. Figure 1. Programming the LTC3203/LTC3203B Output Voltage The voltage divider ratio is given by the expression: R R V V or V R R V OUT OUT 1 20 91 1 1 2 10 91 = − =+ ⎛ ⎝⎜ ⎞ ⎠⎟ . •. Typical values for total voltage divider resistance can range from several k Ωs up to 1MΩ. The compensation capacitor (CFB) is necessary to counteract the pole caused by the large valued resistors R1 and R2, and the input capacitance of the FB pin. For best results, CFB should be 5pF for all R1 or R2 greater than 10k and can be omitted if both R1 and R2 are less than 10k. The LTC3203/LTC3203B can also be configured to control a current. In white LED applications the LED current is programmed by the ratio of the feedback set point voltage and a sense resistor as shown in Figure 2. The current of the remaining LEDs is controlled by virtue of their similar ity to the reference LED and the ballast voltage across the sense resistor. 3203 F02 LTC3203/ LTC3203B VOUT FB GND COUT • • • ILED = VFB RX 9, 11 2 5 RX RX Figure 2. Programming the LTC3203/LTC3203B Output Current 3203 F01 LTC3203/ LTC3203B COUT VOUT FB GND R1 R2 2 5 9, 11 CFB In this configuration the feedback factor ( ∆VOUT/∆IOUT) will be very near unity since the small signal LED imped ance will be considerably less than the current setting 
