LT1521/LT1521-3
LT1521-3.3/LT1521-5
U
W U U
APPLICATIONS INFORMATION
The ground pin current can be found by examining the
Ground Pin Current curves in the Typical Performance
Characteristics. Powerdissipationwillbeequaltothesum
of the two components listed above.
V
IN
OUT
OUT
+
R2
V
LT1521
GND
IN
SHDN
ADJ
The LT1521 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
allsourcesofthermalresistancefromjunctiontoambient.
Additional heat sources mounted nearby must also be
considered.
R1
R2
V
V
= 3.75V 1 +
+ (I
ADJ
+ R2)
OUT
ADJ
)
)
R1
= 3.75V
I
= 50nA AT 25°C
ADJ
OUTPUT RANGE = 3.75V TO 20V
LT1521 • F01
Figure 2. Adjustable Operation
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 ap-
proximately 15ppm/°C. The adjust pin bias current has a
negativetemperaturecoefficient.Theseeffectswilltendto
cancel each other.
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.
The following tables list thermal resistance for each pack-
age. Measured values of thermal resistance for several
different board sizes and copper areas are listed for each
package. All measurements were taken in still air on
3/32"FR-4boardwithoneouncecopper.AllNCleadswere
connected to the ground plane.
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
to 3.75V; (VOUT/3.75V). For example: load regulation for
an output current change of 1mA to 300mA is –20mV
typical at VOUT = 3.75V. At VOUT = 12V, load regulation
would be:
Table 1. MS8 Package
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE** BACKSIDE
BOARD AREA
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
1000mm2
225mm2
100mm2
2500mm2
2500mm2
2500mm2
2500mm2
110°C/W
115°C/W
120°C/W
130°C/W
(12V/3.75V)(–20mV) = –64mV
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:
* Pin 4 is ground. ** Device is mounted on topside.
Table 2. S8 Package*
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
BOARD AREA
1. Output current multiplied by the input/output voltage
differential: IOUT(VIN – VOUT), and
2. Ground pin current multiplied by the input voltage:
(IGND)(VIN)
TOPSIDE** BACKSIDE
2500mm2
1000mm2
225mm2
100mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
60°C/W
60°C/W
68°C/W
74°C/W
* Pins 3, 6, 7 are ground. ** Device is mounted on topside.
10
Linear [ Linear ]
