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LT1521/LT1521-3
LT1521-3.3/LT1521-5
1521335fb
APPLICATIONS INFORMATION
WUU
U
is required to prevent oscillations. The LT1521 is a
micropower device and output transient response will be
a function of output capacitance. See the Transient
Response curves in the Typical Performance Characteris-
tics. Larger values of output capacitance will decrease the
peak deviations and provide improved output transient
response for larger load current deltas. Bypass capaci-
tors, used to decouple individual components powered by
the LT1521, will increase the effective value of the output
capacitor.
Protection Features
The LT1521 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 ther-
mal limiting, the device is protected against reverse input
voltages, reverse output 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 temperatures should not exceed 125°C.
The input of the device will withstand reverse voltages of
20V. Current flow into the device will be limited to less
than 1mA (typically less than 100µA) and no negative
voltage will appear at the output. The device will protect
both itself and the load. This provides protection against
batteries that can be plugged in backward.
For fixed voltage versions of the device, the output can be
pulled below ground without damaging the device. If the
input is left open circuit or grounded, the output can be
pulled below ground by 20V. The output will act like an
open circuit, no current will flow out of the pin. If the input
is powered by voltage source, the output will source the
short-circuit current of the device and will protect itself by
thermal limiting. For the adjustable version of the device,
the output pin is internally clamped at one diode drop
below ground. Reverse current for the adjustable device
must be limited to 5mA.
Table 3. SOT-223 Package
(Thermal Resistance Junction-to-Tab 20°C/W)
COPPER AREA
THERMAL RESISTANCE
TOPSIDE* BACKSIDE BOARD AREA
(JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
50°C/W
1000mm
2
2500mm
2
2500mm
2
50°C/W
225mm
2
2500mm
2
2500mm
2
58°C/W
100mm
2
2500mm
2
2500mm
2
64°C/W
1000mm
2
1000mm
2
1000mm
2
57°C/W
1000mm
2
0 1000mm
2
60°C/W
* Tab of device attached to topside copper.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4.5V to 7V, an output current range of 0mA to
150mA 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)
= 150mA
V
IN(MAX)
= 7V
I
GND
at (I
OUT
= 150mA, V
IN
= 7V) = 2.1mA
So,
P = 150mA(7V – 3.3V) + (2.1mA)(7V) = 0.57W
If we use a SOT-223 package, then the thermal resistance
will be in the range of 50°C/W to 65°C/W depending on the
copper area. So the junction temperature rise above
ambient will be approximately equal to:
0.57W(60°C/W) = 34.2°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 + 34.2°C = 84.2°C
Output Capacitance and Transient Performance
The LT1521 is designed to be stable with a wide range of
output capacitors. A minimum output capacitor of 1.5µF
