Datasheet LTC1844 (Analog Devices) - 10

LTC1844 Series
U U W U APPLICATIONS INFORMATION Thermal Considerations
The power dissipated by the device will be equal to: The power handling capability of the device will be limited IOUT(MAX)(VIN(MAX) – VOUT) by the maximum rated junction temperature (125°C). The where: power dissipated by the device will be the output current multiplied by the input/output voltage differential: IOUT(MAX) = 50mA (IOUT)(VIN – VOUT). VIN(MAX) = 6V The LTC1844 series regulators have internal thermal lim- So: iting designed to protect the device during momentary P = 50mA(6V – 3.3V) = 0.135W overload conditions. For continuous normal conditions, the maximum junction temperature rating of 125°C must The power dissipated by the LTC1844’s quiescent current not be exceeded. It is important to give careful consider- (240µW) is insignificant. The thermal resistance will be in ation to all sources of thermal resistance from junction to the range of 125°C/W to 150°C/W depending on the ambient. Additional heat sources mounted nearby must copper area. The junction temperature rise above ambient also be considered. will be approximately equal to: For surface mount devices, heat sinking is accomplished 0.135W(150°C/W) = 20.3°C by using the heat-spreading capabilities of the PC board The maximum junction temperature will then be equal to and its copper traces. Copper board stiffeners and plated the maximum junction temperature rise above ambient through holes can also be used to spread the heat gener- plus the maximum ambient temperature or: ated by power devices. T = 50°C + 20.3°C = 70.3°C Table 1 lists thermal resistance for several different board sizes and copper areas. All measurements were taken in
Protection Features
still air on 3/32" FR-4 board with one ounce copper. The LTC1844 regulators incorporate several protection
Table 1. Measured Thermal Resistance
features which make them ideal for use in battery-powered
COPPER AREA THERMAL RESISTANCE
circuits. In addition to the usual protection features asso-
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
ciated with monolithic regulators, such as current limiting 2500mm2 2500mm2 2500mm2 125°C/W and thermal limiting, the devices are protected against 1000mm2 2500mm2 2500mm2 125°C/W reverse input voltages and reverse voltages from output to 225mm2 2500mm2 2500mm2 130°C/W input. 100mm2 2500mm2 2500mm2 135°C/W Current limit protection and thermal overload protection 50mm2 2500mm2 2500mm2 150°C/W are intended to protect the device against current overload *Device is mounted on topside. conditions at the output of the device. For normal opera- tion, the junction temperature should not exceed 125°C.
Calculating Junction Temperature
The input of the device will withstand input voltages of Example: Given an output voltage of 3.3V, an input voltage – 7V. Current flow into the device will be limited to less of 4V to 6V, an output current range of 0mA to 50mA and than 500µA (typically less than 200µA) and only a small a maximum ambient temperature of 50°C, what will the negative voltage will appear at the output (~ –300mV with maximum junction temperature be? no load). The LTC1844 will protect both itself and the load against batteries plugged in backward. The shutdown pin will require current limiting if used (see Pin Functions). 1844fa 10