Datasheet

LTM8001
17
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event. The SCR latches when the input voltage threshold
is exceeded, so this circuit should be used with a fuse, as
shown, or employ some other method to interrupt current
from the load.
As mentioned, the LTM8001 sinks current by energy
conversion and not dissipation. Thus, no matter what
protection circuit that is used, the amount of power that the
protection circuit must absorb depends upon the amount
of power at the input. For example, if the output voltage is
2.5V and can sink 5A, the input protection circuit should
be designed to absorb at least 7.5W. In Figures 5a and 5b,
let us say that the protection activation threshold is 30V.
Then the circuit must be designed to be able to dissipate
7.5W and accept 7.5W/30V = 250mA.
Figures 5a through 5c are crowbar circuits, which attempt
to prevent the input voltage from rising above some level
by clamping the input to GND through a power device. In
some cases, it is possible to simply turn off the LTM8001
when the input voltage exceeds some threshold. This
is possible when the voltage power source that drives
current into V
OUT
never exceeds V
IN
. An example of this
circuit is shown in Figure 5d. When the power source on
the output drives V
IN
above a predetermined threshold,
the comparator pulls down on the RUN pin and stops
switching in the LTM8001. When this happens, the input
capacitance needs to absorb the energy stored within the
LTM8001’s internal inductor, resulting in an additional
voltage rise. As shown in the Block Diagram, the internal
APPLICATIONS INFORMATION
V
IN
ZENER
DIODE
R
Q
8001 F05a
LTM8001
LOAD
CURRENT
GND
V
OUT0
SOURCING
LOAD
V
IN
V
REF
8001 F05b
Q
LTM8001
LOAD
CURRENT
GND
V
OUT0
SOURCING
LOAD
OPTIONAL
HYSTERESIS
RESISTOR
+
–
V
IN
ZENER
DIODE
SCR
8001 F05a
LTM8001
LOAD
CURRENT
GND
V
OUT0
FUSE
SOURCING
LOAD
V
IN
RUN
8001 F05d
10µF
LTM8001
LOAD
CURRENT
GND
V
OUT0
SOURCING
LOAD
EXTERNAL
REFERENCE
VOLTAGE
+
–
Figure 5a. The MOSFET Q Dissipates Momentary Energy to
GND. The Zener Diode and Resistor Are Chosen to Ensure That
the MOSFET Turns On Above the Maximum V
IN
Voltage Under
Normal Operation
Figure 5b. The Comparator in This Circuit Activates the Q
MOSFET at a More Precise Voltage Than the One Shown in
Figure 5a. The Reference for the Comparator is Derived from
the V
REF
Pin of the LTM8001
Figure 5c. The SCR Latches On When the Activation Threshold is
Reached, So a Fuse or Some Other Method of Disconnecting the
Load Should be Used
Figure 5d. This Comparator Circuit Turns Off the LTM8001 if
the Input Rises Above a Predetermined Threshold. When the
LTM8001 Turns Off, the Energy Stored in the Internal Inductor
Will Raise V
IN
a Small Amount Above the Threshold.