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LT1121 Datasheet(PDF) 8 Page - Linear Technology |
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LT1121 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1121/LT1121-3.3/LT1121-5 S APPLICATI I FOR ATIO The LT1121 is a micropower low dropout regulator with shutdown, capable of supplying up to 150mA of output current at a dropout voltage of 0.4V. The device operates with very low quiescent current (30 µA). In shutdown the quiescent current drops to only 16 µA. In addition to the low quiescent current the LT1121 incorporates several protection features which make it ideal for use in battery- powered systems. The device is protected against both reverse input voltages and reverse output voltages. In battery backup applications where the output can be held up by a backup battery when the input is pulled to ground, the LT1121 acts like it has a diode in series with its output and prevents reverse current flow. Adjustable Operation The adjustable version of the LT1121 has an output voltage range of 3.75V to 20V. The output voltage is set by the ratio of two external resistors as shown in Figure 1. 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 1. 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 slight positive temperature coefficient of approximately 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 voltage 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 150mA is –12mV typical at VOUT = 3.75V. At VOUT = 12V, load regulation would be: 12 375 12 38 V V mV mV . –– × () = () Thermal Considerations Power handling capability will be limited by maximum rated junction temperature (125 °C). Power dissipated by the device will be made up of two components: 1. Output current multiplied by the input/output voltage differential: IOUT × (VIN – VOUT), and 2. Ground pin current multiplied by the input voltage: IGND × VIN. The ground pin current can be found by examining the Ground Pin Current curves in the Typical Performance Characteristics. Power dissipation will be equal to the sum of the two components listed above. The LT1121 series regulators have internal thermal limit- ing designed to protect the device during overload condi- tions. For continuous normal load conditions the maxi- mum junction temperature rating of 125 °C must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. Additional heat sources mounted nearby must also be considered. Heat sinking, for surface mount devices, is accomplished by using the heat spreading capabilities of the PC board and its copper traces. Copper board stiffeners and plated through holes can also be used to spread the heat gener- ated by power devices. Tables 1 through 5 list thermal resistances for each package. Measured values of thermal resistance for several different board sizes and copper areas are listed for each package. All measurements were Figure 1. Adjustable Operation IN LT1121 GND SHDN 1121 • F01 + 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 |
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