10
LTC1844 Series
1844fa
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
power dissipated by the device will be the output current
multiplied by the input/output voltage differential:
(I
OUT
)(V
IN
– V
OUT
).
The LTC1844 series regulators have internal thermal lim-
iting designed to protect the device during momentary
overload conditions. For continuous normal conditions,
the maximum junction temperature rating of 125°C must
not be exceeded. It is important to give careful consider-
ation to all sources of thermal resistance from junction to
ambient. Additional heat sources mounted nearby must
also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat-spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through holes can also be used to spread the heat gener-
ated by power devices.
Table 1 lists thermal resistance for several different board
sizes and copper areas. All measurements were taken in
still air on 3/32" FR-4 board with one ounce copper.
Table 1. Measured Thermal Resistance
COPPER AREA THERMAL RESISTANCE
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
125°C/W
1000mm
2
2500mm
2
2500mm
2
125°C/W
225mm
2
2500mm
2
2500mm
2
130°C/W
100mm
2
2500mm
2
2500mm
2
135°C/W
50mm
2
2500mm
2
2500mm
2
150°C/W
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
of 4V to 6V, an output current range of 0mA to 50mA and
a maximum ambient temperature of 50°C, what will the
maximum junction temperature be?
The power dissipated by the device will be equal to:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
)
where:
I
OUT(MAX)
= 50mA
V
IN(MAX)
= 6V
So:
P = 50mA(6V – 3.3V) = 0.135W
The power dissipated by the LTC1844’s quiescent current
(240µW) is insignificant. The thermal resistance will be in
the range of 125°C/W to 150°C/W depending on the
copper area. The junction temperature rise above ambient
will be approximately equal to:
0.135W(150°C/W) = 20.3°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
T = 50°C + 20.3°C = 70.3°C
Protection Features
The LTC1844 regulators incorporate several protection
features which make them ideal for use in battery-powered
circuits. In addition to the usual protection features asso-
ciated with monolithic regulators, such as current limiting
and thermal limiting, the devices are protected against
reverse input voltages and reverse voltages from output to
input.
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C.
The input of the device will withstand input voltages of
– 7V. Current flow into the device will be limited to less
than 500µA (typically less than 200µA) and only a small
negative voltage will appear at the output (~–300mV with
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).
APPLICATIONS INFORMATION
WUU
U
