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LTC3252EDE Datasheet(PDF) 6 Page - Linear Technology |
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LTC3252EDE Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page LTC3252 6 3252f OPERATIO The LTC3252 has two switched capacitor charge pumps to step down VIN to two regulated output voltages. The two charge pumps operate 180 ° out of phase to reduce input ripple. Regulation is achieved by sensing each output voltage through an external resistor divider and modulat- ing the charge pump output current based on the error signal. A 2-phase nonoverlapping clock activates the two charge pumps running them out of phase from each other. On the first phase of the clock current is transferred from VIN, through the external flying capacitor 1, to OUT1 via the switches of charge pump 1. Not only is current being delivered to OUT1 on the first phase, but the flying capaci- tor is also being charged up. On the second phase of the clock, flying capacitor 1 is connected from OUT1 to ground, transferring the charge stored during the first phase of the clock to OUT1 via the switches of charge pump 1. Charge pump 2 operates in the same manner to supply current to OUT2, but with the phases of the clock reversed relative to charge pump 1. Using this method of switching, only half of the output current for each output is delivered from VIN, thus achieving a 50% increase in efficiency over a conventional LDO. A spread spectrum oscillator, which utilizes random switching frequencies between 1MHz and 1.6MHz, sets the rate of charging and discharging of the flying capacitors. This architecture achieves extremely low output noise. Input noise is signifi- cantly reduced compared to conventional charge pumps. The outputs also have a low current burst mode to improve efficiency even at light loads. In shutdown mode all circuitry is turned off and the LTC3252 draws only leakage current from the VIN supply. Furthermore, OUT1 and OUT2 are disconnected from VIN. The EN1 and EN2 pins are CMOS inputs with threshold voltages of approximately 0.8V to allow regulator control with low voltage logic levels. The LTC3252 is in shutdown when a logic low is applied to both enable pins. Since the mode pins are high impedance CMOS inputs, they should never be allowed to float. Always drive the enable pins with valid logic levels. (Refer to Simplified Block Diagram) Short-Circuit/Thermal Protection The LTC3252 has built-in short-circuit current limiting as well as over temperature protection. During short-circuit conditions, internal circuitry automatically limits each output to approximately 500mA of current. If fault condi- tions (such as shorted outputs) cause excessive self heating on chip such that the junction temperature ex- ceeds approximately 160 °C, the thermal shutdown cir- cuitry will disable the charge pumps. The IC resumes operation once the junction temperature drops back to approximately 155 °C. The LTC3252 will cycle in and out of thermal shutdown without latchup or damage until the overstress condition is removed. Long term overstress (IOUT1 or IOUT2 > 400mA, and/or TJ > 125°C) should be avoided as it can degrade the performance or shorten the life of the part. Soft-Start To prevent excessive current flow at VIN during start-up, the LTC3252 has built-in soft-start circuitry on each out- put. When an output is enabled, the soft-start circuitry increases the amount of current available from the output linearly over a period of approximately 500 µs. The soft- start circuitry is disabled shortly after the output achieves regulation. Spread Spectrum Operation Switching regulators can be particularly troublesome where electromagnetic interference (EMI) is concerned. Switch- ing regulators operate on a cycle-by-cycle basis to transfer power to an output. In most cases, the frequency of operation is either fixed or is a constant based on the output load. This method of conversion creates large components of noise at the frequency of operation (funda- mental) and multiples of the operating frequency (har- monics). |
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