LT1129/LT1129-3.3/LT1129-5
APPLICATIONS INFORMATION
Adjustable Operation
Thermal Considerations
TheadjustableversionoftheLT1129hasanoutputvoltage
range of 3.75V to 30V. The output voltage is set by the
ratio of two external resistors as shown in Figure 2. The
device servos the output voltage to maintain the voltage
at the adjust pin at 3.75V. The current in R1 is then equal
to 3.75V/R1. The current in R2 is equal to the sum of the
current in R1 and the adjust pin bias current. The adjust
pin bias current, 150nA at 25°C, flows through R2 into the
adjust pin. The output voltage can be calculated according
to the formula in Figure 2. The value of R1 should be less
than 400k to minimize errors in the output voltage caused
by the adjust pin bias current. Note that in shutdown the
output is turned off and the divider current will be zero.
Curves of Adjust Pin Voltage vs Temperature and Adjust
Pin Bias Current vs Temperature appear in the Typical
Performance Characteristics. The reference voltage at
the adjust pin has a positive temperature coefficient of
approximately 15ppm/°C. The adjust pin bias current has
anegativetemperaturecoefficient. Theseeffectsaresmall
and will tend to cancel each other.
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
• (V – V ), and
OUT
IN OUT
2. Ground pin current multiplied by the input voltage:
• V .
I
GND
IN
The ground pin current can be found by examining the
Ground Pin Current curves in the Typical Performance
Characteristics. Power dissipation will be equal to the
sum of the two components listed above.
The LT1129 series regulators have internal thermal
limiting designed to protect the device during overload
conditions. For continuous normal load conditions the
maximum 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 ambi-
ent. Additionalheatsourcesmountednearbymustalsobe
considered.
The adjustable device is specified with the adjust pin tied
to the output pin. This sets the output voltage to 3.75V.
Specifications for output voltages greater than 3.75V will
be proportional to the ratio of the desired output voltage
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.
to3.75V(V /3.75V).Forexample:loadregulationforan
OUT
outputcurrentchangeof1mAto700mAis–6mVtypicalat
V
= 3.75V. At V
= 12V, load regulation would be:
OUT
OUT
12V
3.75V
ꢀ
ꢂ
ꢃ
ꢄ
• –6mV = –19mV
(
) (
)
ꢅ
ꢁ
V
OUT
IN
OUT
ADJ
LT1129
R2
R1
+
SHDN
GND
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 board sizes and
copper areas are listed for each package. All measure-
ments were taken in still air on 3/32" FR-4 board with 1-oz
112935 F02
R2
V
= 3.75V 1 +
(
+ I
• R2
ADJ
(
)
OUT
)
R1
V
= 3.75V
ADJ
I
= 150nA at 25°C
ADJ
OUTPUT RANGE = 3.75V to 30V
Figure 2. Adjustable Operation
112935ff
10
Linear [ Linear ]
