Datasheet

LTM8052/LTM8052A
19
8052fc
For more information www.linear.com/LTM8052
applicaTions inForMaTion
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 LTM8052/
LTM8052A when the input voltage exceeds some thresh
-
old. 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 LTM8052/LTM8052A. When
this happens, the input capacitance needs to absorb the
energy stored within the LTM8052/LTM8052A’s internal
inductor, resulting in an additional voltage rise. As shown
in the Block Diagram, the internal inductor value is 2.2uH.
If the LTM8052/LTM8052A negative current limit is set
to 5A, for example, the energy that the input capacitance
V
IN
ZENER
DIODE
SCR
8052 F05a
LTM8052
LOAD
CURRENT
GND
V
OUT
FUSE
SOURCING
LOAD
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 LTM8052/LTM8052A
if the Input Rises Above a Predetermined Threshold. When the
LTM8052/LTM8052A Turns Off, the Energy Stored in the Internal
Inductor Will Raise V
IN
a Small Amount Above the Threshold.
V
IN
RUN
8052 F05d
10µF
LTM8052
LOAD
CURRENT
GND
V
OUT
SOURCING
LOAD
EXTERNAL
REFERENCE
VOLTAGE
+
–
must absorb is 1/2 LI
2
= 27.5μJ. Suppose the comparator
circuit in Figure 5d is set to pull the RUN pin down when
V
TRIP
= 15V. The input voltage will rise according to the
capacitor energy equation:
1
2
C (V
IN
2
− V
TRIP
2
) = 27.5µJ
If the total input capacitance is 10μF, the input voltage
will rise to:
27.5µJ =
1
2
10µF(V
IN
2
− 15V
2
)
V
IN
= 15.2V
For applications where only sourcing current (one quadrant
operation) is desired, use LTM8026.