Datasheet LT1521, LT1521-3, LT1521-3.3, LT1521-5 (Analog Devices) - 10

LT1521/LT1521-3 LT1521-3.3/LT1521-5
U U W U APPLICATIONS INFORMATION
The ground pin current can be found by examining the V IN OUT OUT + Ground Pin Current curves in the Typical Performance V R2 IN LT1521 Characteristics. Power dissipation will be equal to the sum of the two components listed above. SHDN ADJ GND The LT1521 series regulators have internal thermal limit- R1 V R2 ) OUT = 3.75V 1 + + (IADJ + R2) ing designed to protect the device during overload condi- R1 VADJ = 3.75V tions. For continuous normal load conditions the maxi- IADJ = 50nA AT 25°C mum junction temperature rating of 125°C must not be OUTPUT RANGE = 3.75V TO 20V LT1521 • F01 exceeded. It is important to give careful consideration to
Figure 2. Adjustable Operation
all sources of thermal resistance from junction to ambient. Additional heat sources mounted nearby must also be output is turned off and the divider current will be zero. considered. Curves of Adjust Pin Voltage vs Temperature and Adjust For surface mount devices, heat sinking is accomplished Pin Bias Current vs Temperature appear in the Typical by using the heat spreading capabilities of the PC board Performance Characteristics. The reference voltage at the and its copper traces. Copper board stiffeners and plated adjust pin has a positive temperature coefficient of ap- through-holes can also be used to spread the heat gener- proximately 15ppm/°C. The adjust pin bias current has a ated by power devices. negative temperature coefficient. These effects will tend to cancel each other. The following tables list thermal resistance for each pack- age. Measured values of thermal resistance for several The adjustable device is specified with the adjust pin tied different board sizes and copper areas are listed for each to the output pin. This sets the output voltage to 3.75V. package. All measurements were taken in still air on Specifications for output voltages greater than 3.75V will 3/32" FR-4 board with one ounce copper. All NC leads were be proportional to the ratio of the desired output voltage connected to the ground plane. to 3.75V; (VOUT/3.75V). For example: load regulation for an output current change of 1mA to 300mA is –20mV
Table 1. MS8 Package
typical at VOUT = 3.75V. At VOUT = 12V, load regulation
COPPER AREA THERMAL RESISTANCE
would be:
TOPSIDE** BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
(12V/3.75V)(– 20mV) = – 64mV 2500mm2 2500mm2 2500mm2 110°C/W 1000mm2 2500mm2 2500mm2 115°C/W
Thermal Considerations
225mm2 2500mm2 2500mm2 120°C/W The power handling capability of the device will be limited 100mm2 2500mm2 2500mm2 130°C/W by the maximum rated junction temperature (125°C). The * Pin 4 is ground. ** Device is mounted on topside. power dissipated by the device will be made up of two
Table 2. S8 Package*
components:
COPPER AREA THERMAL RESISTANCE
1. Output current multiplied by the input/output voltage
TOPSIDE** BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
differential: IOUT(VIN – VOUT), and 2500mm2 2500mm2 2500mm2 60°C/W 2. Ground pin current multiplied by the input voltage: 1000mm2 2500mm2 2500mm2 60°C/W (IGND)(VIN) 225mm2 2500mm2 2500mm2 68°C/W 100mm2 2500mm2 2500mm2 74°C/W * Pins 3, 6, 7 are ground. ** Device is mounted on topside. 1521335fb 10