LT1129/LT1129-3.3/LT1129-5
OPERATION
copper. This data can be used as a rough guideline in
estimating thermal resistance. 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.
The power dissipated by the device will be equal to:
I
• (V
– V ) + (I
• V
)
IN MAX
OUT MAX
IN MAX
OUT
GND
where, I
= 500mA
= 5.5V
OUT IN
OUT MAX
V
IN MAX
I
at (I
= 500mA, V = 5.5V) = 25mA
GND
so,
P = 500mA • (5.5V – 3.3V) + (25mA • 5.5V)
= 1.24W
Table 1. Q Package, 5-Lead DD
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE*
BACKSIDE
BOARD AREA
If we use a DD package, then the thermal resistance will be
in the range of 25°C/W to 35°C/W depending on copper
area. So the junction temperature rise above ambient will
be approximately equal to:
2500 sq. mm 2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm 2500 sq. mm
125 sq. mm 2500 sq. mm 2500 sq. mm
* Tab of device attached to topside copper
25°C/W
27°C/W
35°C/W
1.24W • 30°C/W = 37.2°C
Table 2. ST Package, 3-Lead SOT-223
COPPER AREA
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
THERMAL RESISTANCE
TOPSIDE*
BACKSIDE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq. mm 2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm 2500 sq. mm
225 sq. mm 2500 sq. mm 2500 sq. mm
100 sq. mm 2500 sq. mm 2500 sq. mm
* Tab of device attached to topside copper
45°C/W
45°C/W
53°C/W
59°C/W
T
= 50°C + 37.2°C = 87.2°C
JMAX
Output Capacitance and Transient Performance
The LT1129 is designed to be stable with a wide range
of output capacitors. The minimum recommended value
is 3.3μF with an ESR of 2Ω or less. The LT1129 is a
micropower device and output transient response will
be a function of output capacitance. See the Transient
Response curves in the Typical Performance Character-
istics. Larger values of output capacitance will decrease
thepeakdeviationsandprovideimprovedoutputtransient
response. Bypass capacitors, used to decouple individual
components powered by the LT1129, will increase the
effective value of the output capacitor.
Table 3. S8 Package, 8-Lead Plastic SOIC
COPPER AREA
THERMAL RESISTANCE
TOPSIDE*
BACKSIDE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq. mm 2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm 2500 sq. mm
225 sq. mm 2500 sq. mm 2500 sq. mm
100 sq. mm 2500 sq. mm 2500 sq. mm
* Device attached to topside copper
55°C/W
55°C/W
63°C/W
69°C/W
T Package, 5-Lead TO-220
Thermal Resistance (Junction-to-Case) = 5°C/W
Protection Features
Calculating Junction Temperature
The LT1129 incorporates several protection features
which make it ideal for use in battery-powered circuits.
In addition to the normal protection features associated
with monolithic regulators, such as current limiting and
thermal limiting, the device is protected against reverse
input voltages, and reverse voltages from output to input.
For fixed voltage devices the output and sense pins are
tied together at the output.
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?
112935ff
11
Linear [ Linear ]
