Datasheet LT1529, LT1529-3.3, LT1529-5 (Analog Devices) - 9

LT1529 LT1529-3.3/LT1529-5
O U U W U APPLICATI S I FOR ATIO Thermal Considerations
tance. The thermal resistance for each application will be affected by thermal interactions with other components as The power handling capability of the device will be limited well as board size and shape. Some experimentation will by the maximum rated junction temperature (125°C). The be necessary to determine the actual value. power dissipated by the device will be made up of two components:
Table 1. Q Package, 5-Lead DD
1. Output current multiplied by the input/output voltage
COPPER AREA THERMAL RESISTANCE
differential: I
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
OUT • (VIN – VOUT), and 2500 sq. mm 2500 sq. mm 2500 sq. mm 23°C/W 2. Ground pin current multiplied by the input voltage: 1000 sq. mm 2500 sq. mm 2500 sq. mm 25°C/W IGND • VIN . 125 sq. mm 2500 sq. mm 2500 sq. mm 33°C/W The GND pin current can be found by examining the GND * Device is mounted on topside. Pin Current curves in the Typical Performance Character-
T Package, 5-Lead TO-220
istics. Power dissipation will be equal to the sum of the two Thermal Resistance (Junction-to-Case) = 2.5°C/W components listed above. The LT1529 series regulators have internal thermal limit-
Calculating Junction Temperature
ing designed to protect the device during overload condi- Example: Given an output voltage of 3.3V, an input voltage tions. For continuous normal load conditions the maxi- range of 4.5V to 5.5V, an output current range of 0mA to mum junction temperature rating of 125°C must not be 500mA, and a maximum ambient temperature of 50°C, exceeded. It is important to give careful consideration to what will the maximum junction temperature be? all sources of thermal resistance from junction to ambient. The power dissipated by the device will be equal to: Additional heat sources mounted nearby must also be considered. IOUT(MAX) • (VIN(MAX) – VOUT) + (IGND • VIN(MAX)) For surface mount devices heat sinking is accomplished where, IOUT(MAX) = 500mA by using the heat spreading capabilities of the PC board VIN(MAX) = 5.5V and its copper traces. Experiments have shown that the IGND at (IOUT = 500mA, VIN = 5.5V) = 3.6mA heat spreading copper layer does not need to be electri- so, P = 500mA • (5.5V – 3.3V) + (3.6mA • 5.5V) cally connected to the tab of the device. The PC material = 1.12W can be very effective at transmitting heat between the pad area, attached to the tab of the device, and a ground or If we use a DD package, then the thermal resistance will be power plane layer either inside or on the opposite side of in the range of 23°C/W to 33°C/W depending on copper the board. Although the actual thermal resistance of the PC area. So the junction temperature rise above ambient will material is high, the length/area ratio of the thermal be approximately equal to: resistor between layers is small. Copper board stiffeners 1.12W • 28°C/W = 31.4°C and plated through-holes can also be used to spread the heat generated by power devices. The maximum junction temperature will then be equal to the maximum junction temperature rise above ambient The following tables list thermal resistances for each plus the maximum ambient temperature or: package. For the TO-220 package, thermal resistance is given for junction-to-case only since this package is TJMAX = 50°C + 31.4°C = 81.4°C usually mounted to a heat sink. Measured values of
Output Capacitance and Transient Performance
thermal resistance for several different copper areas are listed for the DD package. All measurements were taken in The LT1529 is designed to be stable with a wide range of still air on 3/32" FR-4 board with 1-oz copper. This data can output capacitors. The minimum recommended value is be used as a rough guideline in estimating thermal resis- 22µF with an ESR of 0.2Ω or less. The LT1529 is a 152935fb 9