LTM8026 LINER [Linear Technology], LTM8026 Datasheet - Page 17

• Manufacturer Part Number: LTM8026
• Description: 36VIN, 5A, 2-Quadrant CVCC Step-Down
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
Input Precautions
When the LTM8052 is sinking current, it maintains its
output voltage regulation by power conversion, not power
dissipation. This means that the energy provided to the
LTM8052 is in turn delivered to its input power bus.
There must be something on this power bus to accept or
use the energy, or the LTM8052’s input voltage will rise.
Left unchecked, the energy can raise the input voltage
above the absolute maximum voltage rating and damage
the LTM8052.
In many cases, the system load on the LTM8052 input
bus will be sufficient to absorb the energy delivered by the
μModule regulator. The power required by other devices
will consume more than enough to make up for what the
LTM8052 delivers. In cases where the LTM8052 is the
largest or only power converter, this may not be true and
some means may be need to be devised to prevent the
is typically set well above the maximum V
normal operating conditions. This circuit does not have a
precision threshold, and is subject to both part-to-part and
temperature variations, so it is not suitable for applications
where high accuracy is required or large voltage margins
are not available.
The circuit in Figure 5b also dissipates energy during mo-
mentary overvoltage conditions, but is more precise than
that in Figure 5a. It uses an inexpensive comparator and
the V
voltage. The optional hysteresis resistor in the comparator
circuit avoids MOSFET chatter. Figure 5c shows a circuit
that latches on and crowbars the input in an overvoltage
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.
LTM8052’s input from rising too high. Figure 5a shows a
passive crowbar circuit that will dissipate energy during
momentary input overvoltage conditions. The breakdown
voltage of the zener diode is chosen in conjunction with
the resistor R to set the circuit’s trip point. The trip point
REF
output of the LTM8052 to establish a reference
IN
voltage under
As mentioned, the LTM8052 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.
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
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
Q
Q
REF
Pin of the LTM8052
HYSTERESIS
OPTIONAL
RESISTOR
R
ZENER
DIODE
+
–
V
IN
LTM8052
GND
V
V
IN
REF
V
LTM8052
OUT
GND
CURRENT
V
LOAD
OUT
SOURCING
LTM8052
LOAD
IN
CURRENT
Voltage Under
8052 F05a
LOAD
SOURCING
LOAD
8052 F05b
17
8052fa