LTC3245_15 Datasheet, PDF (10/18 Pages) Linear Technology – Wide VIN Range, Low Noise, 250mA Buck-Boost Charge Pump

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
protection 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.
LTC3245
V_OUT
OUTS
GND
V_OUT
FIXED 3.3V OR
FIXED 5V
C_OUT
3245 F01
Figure 1: Fixed 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.
LTC3245
V_OUT
ADJ
V_OUT
R_A
( ) ^1.2V
1+
R_A
R_B
R_B
C_OUT
GND
3245 F02
Figure 2: Adjustable Output Operation
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 =} V_{OUT – 1}
R_B 1.2V
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.
3245fa
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