Datasheet LT1528 (Analog Devices) - 8

LT1528
APPLICATIONS INFORMATION
The LT1528 is specifi ed with the SENSE pin tied to Table 1a lists thermal resistance for the DD package. the OUTPUT pin. This sets the output voltage to 3.3V. For the TO-220 package (Table 1b) thermal resistance is Specifi cations for output voltage greater than 3.3V will given for junction-to-case only since this package is usu- be proportional to the ratio of the desired output voltage ally mounted to a heat sink. Measured values of thermal to 3.3V (VOUT/3.3V). For example, load regulation for an resistance for several different copper areas are listed for output current change of 1mA to 1.5A is – 5mV (typical) at the DD package. All measurements were taken in still air VOUT = 3.3V. At VOUT = 12V, load regulation would be: on 3/32" FR-4 board with one ounce copper. This data can be used as a rough guideline in estimating thermal (12V/3.3V) • (–5mV) = (–18mV) resistance. The thermal resistance for each application will
Thermal Considerations
be affected by thermal interactions with other components as well as board size and shape. Some experimentation The power handling capability of the device will be limited will be necessary to determine the actual value. by the maximum rated junction temperature (125°C). The power dissipated by the device will be made up of two
Table 1a. Q-Package, 5-Lead DD
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 2500 sq mm 2500 sq mm 2500 sq mm 23°C/W 1000 sq mm 2500 sq mm 2500 sq mm 25°C/W 2. GND pin current multiplied by the input voltage, 125 sq mm 2500 sq mm 2500 sq mm 33°C/W IGND • VIN. *Device is mounted on topside. The GND pin current can be found by examining the GND
Table 1b. T Package, 5-Lead TO-220
Pin Current curves in the Typical Performance Character- istics. Power dissipation will be equal to the sum of the Thermal Resistance (Junction-to-Case) 2.5°C/W two components listed above.
Calculating Junction Temperature
The LT1528 has internal thermal limiting designed to pro- Example: Given an output voltage of 3.3V, an input voltage tect the device during overload conditions. For continuous range of 4.5V to 5.5V, an output current range of 0mA to normal load conditions the maximum junction temperature 500mA and a maximum ambient temperature of 50°C, rating of 125°C must not be exceeded. It is important to what will the maximum junction temperature be? give careful consideration to all sources of thermal resis- tance from junction-to-ambient. Additional heat sources The power dissipated by the device will be equal to: mounted nearby must also be considered. IOUT(MAX) • (VIN(MAX) – VOUT) + [IGND • VIN(MAX)] For surface mount devices heat sinking is accomplished where, by using the heat spreading capabilities of the PC board IOUT(MAX) = 500mA and its copper traces. Experiments have shown that the VIN(MAX) = 5.5V heat spreading copper layer does not have to be electri- IGND at (IOUT = 500mA, VIN = 5.5V) = 4mA cally connected to the tab of the device. The PC material so, can be very effective at transmitting heat between the P = 500mA • (5.5V – 3.3V) + (4mA • 5.5V) = 1.12W pad area, attached to the tab of the device, and a ground or power plane either inside or on the opposite side of If we use a DD package, the thermal resistance will be in the board. Although the actual thermal resistance of the the range of 23°C/W to 33°C/W depending on the copper PC material is high, the length/area ratio of the thermal area. So the junction temperature rise above ambient will resistor between layers is small. Copper board stiffeners be approximately equal to: and plated through holes can also be used to spread the 1.12W • 28°C/W = 31.4°C heat generated by power devices. 1528fb 8