Datasheet LTC1844 (Analog Devices) - 7

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