LTM8052 Datasheet, PDF (18/28 Pages) Linear Technology – 36VIN, 5A, 2-Quadrant CVCC Step-Down

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
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 LTM8052. When this happens, the input
capacitance needs to absorb the energy stored within the
LTM8052’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 negative current
limit is set to 5A, for example, the energy that the input
capacitance must absorb is 1/2 LI² = 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
10
µF
(V_IN
2_−
15V
2₎
V_IN = 15.2V
For applications where only sourcing current (one quadrant
operation) is desired, use LTM8026.
LOAD
CURRENT
V_IN
V_OUT
ZENER
SCR
DIODE
LTM8052
GND
FUSE
SOURCING
LOAD
8052 F05a
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
10µF
LOAD
CURRENT
V_IN
V_OUT
LTM8052
RUN
GND
SOURCING
LOAD
–
+
EXTERNAL
REFERENCE
VOLTAGE
8052 F05d
Figure 5d. This Comparator Circuit Turns Off the LTM8052 if
the Input Rises Above a Predetermined Threshold. When the
LTM8052 Turns Off, the Energy Stored in the Internal Inductor
Will Raise V_IN a Small Amount Above the Threshold.
8052fa
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