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LTC3204B-5 Datasheet(PDF) 7 Page - Linear Technology |
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LTC3204B-5 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LTC3204-3.3/LTC3204-5/ LTC3204B-3.3/LTC3204B-5 7 3204fa TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5 use a switched capacitor charge pump to boost VIN to a regulatedoutputvoltage.Regulationisachievedbysensing the output voltage through an internal resistor divider and modulating the charge pump output current based on the error signal. A 2-phase nonoverlapping clock activates the chargepumpswitches.Theflyingcapacitorischargedfrom VIN on the first phase of the clock. On the second phase of the clock it is stacked in series with VIN and connected to VOUT. This sequence of charging and discharging the flying capacitor continues at a free running frequency of 1.2MHz (typ). Shutdown Mode In shutdown mode, all circuitry is turned off and the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5 draws only leakage current from the VIN supply. Further- more, VOUT is disconnected from VIN. The SHDN pin is a CMOSinputwithathresholdvoltageofapproximately0.7V. TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5 are in shutdown when a logic low is applied to the SHDN pin. Since the SHDN pin is a very high impedance CMOS input, it should never be allowed to float. To ensure that its state is defined, it must always be driven with a valid logic level. Since the output voltages of these devices can go above the input voltage, special circuitry is required to control theinternallogic.Detectionlogicwilldrawaninputcurrent of 5µA when the devices are in shutdown. However, this current will be eliminated if the output voltage (VOUT) is less than approximately 0.8V. Burst Mode Operation The LTC3204-3.3/LTC3204-5 provide automatic Burst Mode operation to reduce supply current at light loads. Burst Mode operation is initiated if the output load current falls below an internally programmed threshold. Once (Refer to the Block Diagram) Burst Mode operation is initiated, the part shuts down the internal oscillator to reduce the switching losses and goes into a low current state. This state is referred to as the sleep state in which the IC consumes only about 40µA from the input. When the output voltage droops enough to overcome the burst comparator hysteresis, the part wakes up and commences normal fixed frequency opera- tion. The output capacitor recharges and causes the part to reenter the sleep state if the output load still remains less than the Burst Mode threshold. This Burst Mode threshold varies with VIN, VOUT and the choice of output storage capacitor. Soft-Start TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5 havebuilt-insoft-startcircuitrytopreventexcessivecurrent flowduringstart-up.Thesoft-startisachievedbycharging an internal capacitor with a very weak current source. The voltage on this capacitor, in turn, slowly ramps the amount of current available to the output storage capacitor from zero to a value of 300mA over a period of approximately 0.75ms. The soft-start circuit is reset in the event of a commanded shutdown or thermal shutdown. Short-Circuit/Thermal Protection TheLTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5 havebuilt-inshort-circuitcurrentlimitaswellasover-tem- perature protection. During a short-circuit condition, they willautomaticallylimittheiroutputcurrenttoapproximately 300mA. At higher temperatures, or if the input voltage is high enough to cause excessive self-heating of the part, the thermal shutdown circuitry will shutdown the charge pumponcethejunctiontemperatureexceedsapproximately 160°C. It will enable the charge pump once the junction temperature drops back to approximately 150°C. The LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5will cycle in and out of thermal shutdown indefinitely without latchup or damage until the short-circuit condition on VOUT is removed. OPERATIO |
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Similar Description - LTC3204B-5 |
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