LT3514
19
3514fa
For more information www.linear.com/LT3514
Figure 7. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It
Also Protects the Circuit from a Reversed Input. The LT3514 Runs Only When the Input Is Present
3514 F07
D4
V
IN
V
OUT
BACKUP
GND
LT3514
SW1
DA1
FB1
EN/UVLO
SKY
SW5
V
IN
V
IN
RUN/SS1
RT/SYNC
tied to V
IN
), then the LT3514’s internal circuitry will pull
its quiescent current through its SW pin. This is fine if
your system can tolerate a few mA in this state. If you
ground the EN/UVLO pin, the SW pin current will drop to
essentially zero. However, if the V
IN
pin is grounded while
the output is held high, then parasitic diodes inside the
LT3514 can pull large currents from the output through
the SW pin and the V
IN
pin. Figure 7 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
High Temperature Considerations
While the LT3514 is capable of delivering total output
current up to 4A, total power dissipation for an applica
-
tion circuit and the resulting temperature rise must be
considered, especially if all three channels are operating
at high duty cycle.
The
die
temperature of the LT3514 must be lower than the
maximum rating of 125°C. This is generally not a concern
unless the ambient temperature is above 85°C. For higher
temperatures, extra care should be taken in the layout of
the circuit to ensure good heat sinking of the LT3514. The
maximum load current should be derated as the ambient
temperature approaches 125°C. Programming the LT3514
to a lower switching frequency will improve efficiency and
reduce the dependence of efficiency on input voltage. The
die temperature is calculated by multiplying the LT3514
power dissipation by the thermal resistance from junc
-
tion to ambient. Power dissipation within the LT3514 can
be estimated by calculating the total power loss from an
efficiency measurement and subtracting the catch diode
losses. Thermal resistance depends on the layout of the
cir
cuit board, but 43°C/W is typical for the QFN pack
-
age and 33°C/W is typical for the FE package. Thermal
shutdown will turn off the buck regulators and the boost
regulator when the die temperature exceeds 175°C, but
this is not a warrant to allow operation at die temperatures
exceeding 125°C.
Outputs Greater Than 9V
For
outputs greater than
9V, add a 1k resistor in series
with a 1nF capacitor across the inductor to damp the
discontinuous ringing of the SW node, preventing unin
-
tended SW current.
Other Linear Technology Publications
Application Notes 19, 35, 44 contain more detailed descrip
-
tions and design information for step-down regulators and
other switching regulators. Design Note 318 shows how to
generate a bipolar output supply using a step-down regulator.
APPLICATIONS INFORMATION
