Datasheet

LTC3245
10
3245fa
For more information www.linear.com/LTC3245
applicaTions inForMaTion
Short-Circuit/Thermal Protection
The LTC3245 has built-in short-circuit current limiting as
well as overtemperature protection. During short-circuit
conditions the device will automatically limit the output
current.
The LTC3245 has thermal protection that will shut
down the device if the junction temperature exceeds the
overtemperature threshold (typically 175°C). Thermal
shutdown is included to protect the IC in cases of exces
-
sively high ambient temperatures, or in cases of excessive
power dissipation inside the IC. The charge transfer will
reactivate once the junction temperature drops back to
approximately 165°C.
When the thermal protection is active, the junction tem
-
perature is beyond the specified operating range. Thermal
p
r
otection is intended for momentary overload conditions
outside normal operation. Continuous operation above the
specified maximum operating junction temperature may
impair device reliability.
Soft-Start Operation
To prevent excessive current flow at V
IN
during start-up,
the LTC3245 has built-in soft-start circuitry. Soft-start is
achieved by increasing the amount of current available to
the output charge storage capacitor linearly over a period
of approximately 500μs. Soft-start is enabled whenever
the device is brought out of shutdown, and is disabled
shortly after regulation is achieved.
Programming the Output Voltage (OUTS/ADJ Pin)
The LTC3245 output voltage programming is very flexible
offering a fixed 3.3V output, fixed 5V output as well as
adjustable output that is programmed through an external
resistor divider. The desired output regulation method is
selected through the SET pins.
For a fixed output simply short OUTS (OUTS/ADJ pin) to
V
OUT
as shown in Figure 1. Fixed 3.3V operation is enabled
by driving both SEL1 and SEL2 pins high, while fixed 5V
operating is selected by driving SEL2 high with SEL1 low.
Driving both SEL1 and SEL2 low shuts down the device
causing V
OUT
to go high impedance.
Figure 1: Fixed Output Operation
Figure 2: Adjustable Output Operation
Adjustable output programming is accomplished by con-
necting ADJ (OUTS/ADJ pin) to a resistor divider between
V
OUT
and GND as shown in Figure 2. Adjustable operation
is enabled by driving SEL1 high and SEL2 low. Driving
both SEL1 and SEL2 low shuts down the device causing
V
OUT
to go high impedance.
V
OUT
OUTS
GND
LTC3245
V
OUT
FIXED 3.3V OR
FIXED 5V
3245 F01
C
OUT
V
OUT
ADJ
GND
LTC3245
V
OUT
3245 F02
C
OUT
R
A
R
B
1.2V
( )
1+
R
A
R
B
Using adjustable operation the output (V
OUT
) can be
programmed to regulate from 2.5V to 5V. The limited
programming range provides the required V
OUT
operating
voltage without overstressing the V
OUT
pin.
The desired adjustable output voltage is programmed by
solving the following equation for R
A
and R
B
:
R
A
R
B
=
V
OUT
1.2V
–
1
Select a value for R
B
in the range of 1k to 1M and solve
for R
A
. Note that the resistor divider current adds to the
total no load operating current. Thus a larger value for R
B
will result in lower operating current.