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LTC1682CMS8 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1682CMS8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1682/LTC1682-3.3/LTC1682-5 Operation The LTC1682 uses a switched-capacitor charge pump to generate a CPO voltage of approximately 2(VIN). CPO powers an internal low dropout linear regulator that sup- plies a regulated output at VOUT. Internal comparators are used to sense CPO and VIN voltages for power-up condi- tioning. The output current is sensed to determine the charge pump operating mode. A trimmed internal bandgap is used as the voltage reference and a trimmed internal oscillator is used to control the charge pump switches. The charge pump is a doubler configuration that uses one external flying capacitor. When enabled, a 2-phase nonoverlapping clock controls the charge pump switches. At start-up, the LDO is disabled and the load is removed from CPO. When CPO reaches 1.75(VIN) the LDO is enabled. If CPO falls below 1.45(VIN) the LDO will be disabled. Generally, the charge pump runs open loop with continuous clocking for low noise. If CPO is greater than 1.95(VIN) and IOUT is less than 100µA, the charge pump will operate in Burst Mode operation for increased effi- ciency but slightly higher output noise. In Burst Mode operation, the clock is disabled when CPO reaches 1.95(VIN) and enabled when CPO droops by about 100mV. The switching frequency is precisely controlled to ensure that the frequency is above 455kHz and at the optimum rate to ensure maximum efficiency. The switch edge rates are also controlled to minimize noise. The effective output resistance at CPO is dependent on the voltage at VIN, CPO and the junction temperature. A low ESR capacitor of ≥2µF should be used at CPO for minimum noise. The LDO is used to filter the ripple on CPO and to set an output voltage independent of CPO. VOUT is set by an external or internal resistor divider. The LDO requires a capacitor on VOUT for stability and improved load transient response. A low ESR capacitor of ≥2µF should be used. Output Voltage Selection The LTC1682-3.3/LTC1682-5 versions have internal resistor networks to set the regulated output voltage. The LTC1682 output voltage is set using an external resistor divider (see Figure 3). The output voltage is determined using the following formula: VOUT = 1.235V(1 + R1/R2) The output voltage range is 2.5V to 5.5V. Maximum VOUT and IOUT Calculations The maximum available output voltage and current can be calculated based on the open circuit CPO voltage, the dropout voltage of the LDO and the effective output resistance of the charge pump. The open circuit CPO voltage is approximately 2(VIN) (see Figure 4). Figure 3. Powering an Auxiliary Regulator from CPO APPLICATIONS INFORMATION R1 56k R2 18k C1 0.22 µF C4 4.7 µF 1682 F03 VIN 3.6V C3 4.7 µF 5.1V VRIPPLE = 800µVP-P 1 2 3 4 8 7 6 5 VOUT SHDN FB GND CPO C+ VIN C– LTC1682 C2 4.7 µF IN 3.3V OUT GND C5 EXTERNAL LDO VOUT = 1.235V(1 + R1/R2) + – 2VIN RCPO CCPO VCPO VOUT ILOAD 1682 F04 RDROPOUT VDROPOUT + – Figure 4. Equivalent Circuit |
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