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