LTM8001
19
8001fd
For more information www.linear.com/LTM8001
and damaging the part. If the input supply is poorly con-
trolled or the user will be plugging the LTM8001
into an
energized supply, the input network should be designed
to prevent this overshoot. This can be accomplished by
installing a small resistor in series to V
IN0
, but the most
popular method of controlling input voltage overshoot is
to add an electrolytic bulk capacitor to the V
IN0
net. This
capacitor’s relatively high equivalent series resistance
damps the circuit and eliminates the voltage overshoot.
The extra capacitor improves low frequency ripple filtering
and can slightly improve the performance of the circuit,
though it may be physically large.
Shorted Input Protection
Care needs to be taken in systems where the V
OUT0
out-
put will be held high when the input to the LTM8001 is
absent.
If the V
IN0
is allowed to float and the RUN pin is
held high (either by a logic signal or because it is tied to
V
IN0
), then the LTM8001’s internal circuitry will pull its
quiescent current through its internal power switch. This
is fine if your system can tolerate this state. If the RUN pin
is pulled low, the input current will drop to essentially zero.
However, if the V
IN0
is grounded while the V
OUT0
output is
held high, then parasitic diodes inside the LTM8001 can
pull large currents from the output through the V
IN0
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.
Charging Applications
The LTM8001’s internal switching step-down regula
-
tor’s CVCC operation makes it well suited for battery or
super
capacitor
charging applications. A schematic of the
LTM8001 charging a supercapacitor and then distribut
-
ing power to various loads through the onboard LDOs is
shown in the Typical Applications section. In this applica-
tion, the supercapacitor is charged through the step-down
switching regulator and not the LDOs
.
Each LDO is rated
for positive and differential voltages between its input
and output, but may experience a negative voltage during
start-up or turn-off transients if its output is connected to
a battery, supercapacitor or energized load. Avoid using
the LTM8001 in applications where the internal LDOs can
experience a negative voltage.
Thermal Considerations
The LTM8001 output current may need to be derated if it
is required to operate in a high ambient temperature. The
amount of current derating is dependent upon the input
voltage, output power and ambient temperature. The
temperature rise curves given in the Typical Performance
Characteristics section can be used as a guide. These curves
were generated by the LTM8001 mounted to a 59cm
2
4-layer FR4 printed circuit board. Boards of other sizes
and layer count can exhibit different thermal behavior, so
it is incumbent upon the user to verify proper operation
over the intended system’s line, load and environmental
operating conditions.
For increased accuracy and fidelity to the actual application,
many designers use finite element analysis (FEA) to predict
thermal performance. To that end, the Pin Configuration
of this data sheet typically gives four thermal coefficients:
θ
JA
: Thermal resistance from junction to ambient
θ
JCbottom
: Thermal resistance from junction to the bottom
of the product case
θ
JCtop
: Thermal resistance from junction to top of the
product case
θ
JB
: Thermal resistance from junction to the printed
circuit board
applicaTions inForMaTion
V
IN
RUN
RT
V
OUT0
GND
8001 F07
LTM8001
V
IN
V
OUT
Figure 7. The Input Diode Prevents a Shorted Input
from Discharging a Backup Battery Tied to the Output.
It Also Protects the Circuit from a Reversed Input. The
LTM8001 Runs Only When the Input is Present
