Datasheet LT1129, LT1129-3.3, LT1129-5 (Analog Devices) - 10

LT1129/LT1129-3.3/LT1129-5
APPLICATIONS INFORMATION Adjustable Operation Thermal Considerations
The adjustable version of the LT1129 has an output voltage The power handling capability of the device will be limited range of 3.75V to 30V. The output voltage is set by the by the maximum rated junction temperature (125°C). The ratio of two external resistors as shown in Figure 2. The power dissipated by the device will be made up of two device servos the output voltage to maintain the voltage components: at the adjust pin at 3.75V. The current in R1 is then equal 1. Output current multiplied by the input/output voltage to 3.75V/R1. The current in R2 is equal to the sum of the differential: IOUT • (VIN – VOUT), and current in R1 and the adjust pin bias current. The adjust pin bias current, 150nA at 25°C, fl ows through R2 into the 2. Ground pin current multiplied by the input voltage: adjust pin. The output voltage can be calculated according IGND • VIN. to the formula in Figure 2. The value of R1 should be less The ground pin current can be found by examining the than 400k to minimize errors in the output voltage caused Ground Pin Current curves in the Typical Performance by the adjust pin bias current. Note that in shutdown the Characteristics. Power dissipation will be equal to the output is turned off and the divider current will be zero. sum of the two components listed above. Curves of Adjust Pin Voltage vs Temperature and Adjust The LT1129 series regulators have internal thermal Pin Bias Current vs Temperature appear in the Typical limiting designed to protect the device during overload Performance Characteristics. The reference voltage at conditions. For continuous normal load conditions the the adjust pin has a positive temperature coeffi cient of maximum junction temperature rating of 125°C must not approximately 15ppm/°C. The adjust pin bias current has be exceeded. It is important to give careful consideration a negative temperature coeffi cient. These effects are small to all sources of thermal resistance from junction to ambi- and will tend to cancel each other. ent. Additional heat sources mounted nearby must also be The adjustable device is specifi ed with the adjust pin tied considered. to the output pin. This sets the output voltage to 3.75V. For surface mount devices heat sinking is accomplished Specifi cations for output voltages greater than 3.75V will by using the heat spreading capabilities of the PC board be proportional to the ratio of the desired output voltage and its copper traces. Experiments have shown that the to 3.75V (VOUT/3.75V). For example: load regulation for an heat spreading copper layer does not need to be electri- output current change of 1mA to 700mA is – 6mV typical at cally connected to the tab of the device. The PC material VOUT = 3.75V. At VOUT = 12V, load regulation would be: can be very effective at transmitting heat between the pad 12V area, attached to the tab of the device, and a ground or ( )=( ) –19mV 3.75V • –6mV power plane layer either inside or on the opposite side of the board. Although the actual thermal resistance of the PC material is high, the length/area ratio of the thermal IN OUT VOUT resistor between layers is small. Copper board stiffeners LT1129 and plated through holes can also be used to spread the R2 + heat generated by power devices. SHDN ADJ GND R1 The following tables list thermal resistances for each package. For the TO-220 package, thermal resistance 112935 F02 is given for junction-to-case only since this package R2 VOUT = 3.75V 1 + + IADJ • R2 is usually mounted to a heat sink. Measured values of ( ) ( ) R1 VADJ = 3.75V thermal resistance for several different board sizes and IADJ = 150nA at 25°C OUTPUT RANGE = 3.75V to 30V copper areas are listed for each package. All measure- ments were taken in still air on 3/32" FR-4 board with 1-oz
Figure 2. Adjustable Operation
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