LTM8052A_15 Datasheet, PDF (19/32 Pages) Linear Technology – 36VIN, 5A, 2-Quadrant CVCC Step-Down Module Regulator

LTM8052/LTM8052A
Applications Information
Input Precautions
When the LTM8052/LTM8052A is sinking current, it main-
tains its output voltage regulation by power conversion, not
power dissipation. This means that the energy provided to
the LTM8052/LTM8052A 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/LTM8052A’s
input voltage will rise. Left unchecked, the energy can raise
the input voltage above the absolute maximum voltage
rating and damage the LTM8052/LTM8052A.
In many cases, the system load on the LTM8052/LTM8052A
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/LTM8052A delivers. In cases where
the LTM8052/LTM8052A is the largest or only power
converter, this may not be true and some means may need
to be devised to prevent the LTM8052/LTM8052A’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 is typically
set well above the maximum V_IN voltage under normal
operating conditions. This circuit does not have a preci-
sion 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_REF output of the LTM8052/LTM8052A to establish
a reference 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.
As mentioned, the LTM8052/LTM8052A 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.
LOAD
CURRENT
V_IN
V_OUT
ZENER
DIODE
LTM8052
Q
GND
SOURCING
LOAD
R
8052 F05a
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
OPTIONAL
HYSTERESIS
RESISTOR
Q
V_IN
V_OUT
LTM8052
V_{REF GND}
LOAD
CURRENT
SOURCING
LOAD
8052 F05b
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 LTM8052/LTM8052A
For more information www.linear.com/LTM8052
8052ff
19