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LTC1844-3.3 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1844-3.3 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LTC1844 Series 1844ia APPLICATIO S I FOR ATIO The LTC1844 family are a series of 150mA ultralow dropout regulators with micropower quiescent current and shutdown. The devices are capable of supplying 150mA at a dropout voltage of 90mV (LTC1844-3.3, see Electrical Characteristics for dropout voltage of other versions). Output voltage noise is as low as 30 µVRMS over a 10Hz to 100kHz bandwidth with the addition of a 0.1 µF bypass capacitor. The low operating quiescent current (35 µA) drops to 10nA in shutdown. In addition to the low quiescent current, the LTC1844 regulators incorporate several protection features which make them ideal for use in battery-powered systems. The devices are protected against both reverse input voltages and reverse voltages from output to input (reverse current protection). The devices also include current limit and thermal overload protection, and will survive an output short circuit indefinitely. The fast transient response over- comes the traditional tradeoff between dropout voltage, quiescent current and load transient response inherent in most regulators by using a proprietary new architecture (see Figure 1). Adjustable Operation The adjustable version of the LTC1844 has an output voltage range of 1.25V to 6V. The output voltage is set by the ratio of two external resistors as shown in Figure 2. The device servos the output to maintain the ADJ pin voltage at 1.25V (referenced to ground). The current in R1 is then equal to 1.25V/R1 and the current in R2 is the current in R1 plus the ADJ pin bias current. The ADJ pin bias current, 30nA at 25 °C, flows through R2 into the ADJ pin. The output voltage can be calculated using the formula in Figure 2. The value of R1 should be no greater than 1M to minimize errors in the output voltage caused by the ADJ pin bias current. Note that in shutdown the output is turned off and the divider current will be zero once COUT is discharged. Adjustable devices are tested and specified with the ADJ pin tied to the OUT pin for an output voltage of 1.25V. Specifications for output voltages greater than 1.25V will be proportional to the ratio of the desired output voltage to 1.25V: VOUT/1.25V. For example, load regulation for an Figure 2. Adjustable Operation TIME ( µs) 0 –0.02 0.02 80 1844 F01 50 –0.04 0 0.04 0 20 10 40 30 60 70 90 50 100 VIN = 3V CIN = 1µF COUT = 1µF Figure 1. LTC1844-2.5 Transient Response 1mA to 50mA to 1mA IN 1844 F02 R2 LTC1844 OUT VIN VOUT ADJ GND R1 CFF + VV R R IR VV InA OUT ADJ ADJ ADJ =+ + ()( ) = =° 125 1 2 1 2 125 30 . . AT 25 C OUTPUT RANGE = 1.25V TO 6V C OPTIONAL FF |
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