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LT1129CQ-3.3 Datasheet(PDF) 8 Page - Linear Technology |
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LT1129CQ-3.3 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1129/LT1129-3.3/LT1129-5 PI FU CTIO S S APPLICATI I FOR ATIO The LT1129 is a micropower low dropout regulator with shutdown, capable of supplying 700mA of output current at a dropout voltage of 0.4V. The device operates with very low quiescent current (50 µA). In shutdown the quiescent current drops to only 16 µA. In addition to the low quies- cent current the LT1129 incorporates several protection features which make it ideal for use in battery-powered systems. The device is protected against reverse input voltages. In battery backup applications where the output can be held up by a backup battery when the input is pulled to ground, the LT1129 acts like it has a diode in series with its output and prevents reverse current flow. Adjustable Operation The adjustable version of the LT1129 has an output voltage range of 3.75V to 30V. The output voltage is set by the ratio of two external resistors as shown in Figure 2. The device servos the output voltage to maintain the voltage at the adjust pin at 3.75V. The current in R1 is then equal to 3.75V/R1. The current in R2 is equal to the sum of the current in R1 and the adjust pin bias current. The adjust pin bias current, 150nA at 25 °C, flows through R2 into the adjust pin. The output voltage can be calculated according to the formula in Figure 2. The value of R1 should be less than 400k to minimize errors in the output voltage caused by the adjust pin bias current. Note that in shutdown the output is turned off and the divider current will be zero. Curves of Adjust Pin Voltage vs Temperature and Adjust Pin Bias Current vs Temperature appear in the Typical Performance Characteristics. The reference voltage at the adjust pin has a positive temperature coefficient of ap- proximately 15ppm/ °C. The adjust pin bias current has a negative temperature coefficient. These effects are small and will tend to cancel each other. The adjustable device is specified with the adjust pin tied to the output pin. This sets the output voltage to 3.75V. Specifications for output voltages greater than 3.75V will be proportional to the ratio of the desired output voltage to 3.75V (VOUT/3.75V). For example: load regulation for an output current change of 1mA to 700mA is – 6mV typical at VOUT = 3.75V. At VOUT = 12V, load regulation would be: 12 375 619 V V mV mV . –– × () = () IN LT1129 GND SHDN LT1129 • F02 + ADJ OUT R2 R1 VOUT = 3.75V 1 + + IADJ • R2 VADJ = 3.75V IADJ = 150nA at 25°C OUTPUT RANGE = 3.75V to 30V R2 R1 () () VOUT Figure 2. Adjustable Operation Figure 1. Kelvin Sense Connection current is 15 µA at the nominal regulated output voltage. This pin is internally clamped to – 0.6V (one VBE). Adjust Pin: For the LT1129 (adjustable version) the adjust pin is the input to the error amplifier. This pin is internally clamped to 6V and – 0.6V (one VBE). This pin has a bias current of 150nA which flows into the pin. See Bias Current curve in the Typical Performance Characteristics. The adjust pin reference voltage is equal to 3.75V referenced to ground. IN LT1129 GND SHDN LT1129 • F01 SENSE OUT RP + 1 µF LOAD + 10 µF RP |
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Similar Description - LT1129CQ-3.3 |
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