Datasheet LTM8064 (Analog Devices) - 24

LTM8064
applicaTions inForMaTion
LOAD LOAD CURRENT CURRENT VIN VOUT VIN VOUT ZENER LTM8064 LTM8064 DIODE GND SOURCING RUN GND SOURCING Q LOAD LOAD 10µF R – + EXTERNAL 8064 F06a REFERENCE VOLTAGE
Figure 6a. The MOSFET Q Dissipates Momentary Energy to GND. The Zener Diode and Resistor Are Chosen to Ensure That the MOSFET
8064 F06d
Turns On Above the Maximum VIN Voltage Under Normal Operation Figure 6d. This Comparator Circuit Turns Off the LTM8064 if
LOAD
the Input Rises Above a Predetermined Threshold. When the
CURRENT V V
LTM8064 Turns Off, the Energy Stored in the Internal Inductor
IN OUT OPTIONAL
Will Raise VIN a Small Amount Above the Threshold
LTM8064 HYSTERESIS RESISTOR GND SOURCING LOAD + circuit must be designed to be able to dissipate 7.5W and Q – accept 7.5W/50V = 150mA. VREF Figures 6a through 6c are crowbar circuits, which attempt to prevent the input voltage from rising above some level 8064 F06b by clamping the input to GND through a power device. In
Figure 6b. The Comparator in This Circuit Activates the Q
some cases, it is possible to simply turn off the LTM8064
MOSFET at a More Precise Voltage Than the One Shown
when the input voltage exceeds some threshold. This
in Figure 5a
is possible when the voltage power source that drives current into V LOAD OUT never exceeds VIN. An example of this CURRENT circuit is shown in Figure 6d. When the power source on VIN VOUT the output drives VIN above a predetermined threshold, ZENER FUSE LTM8064 DIODE the comparator pulls down on the RUN pin and disables SCR GND SOURCING LOAD the LTM8064. When this happens, the input capacitance needs to absorb the energy stored within the LTM8064’s internal inductor, resulting in an additional voltage rise. As 8064 F06a shown in the Block Diagram, the internal inductor value is
Figure 6c. The SCR Latches On When the Activation Threshold Is
4.7µH. If the LTM8064 sinking current limit is set to 6A,
Reached, So a Fuse or Some Other Method of Disconnecting the
for example, the energy that the input capacitance must
Load Should be Used
absorb is 1/2 LI2 = 58.75μJ. Suppose the comparator circuit in Figure 6d is set to pull the RUN pin down when As mentioned, the LTM8064 sinks current by energy con- VTRIP = 15V. The input voltage will rise according to the version and not dissipation. Thus, no matter the protection capacitor energy equation: circuit used, the amount of power that the protection circuit 1/2 • C(V 2 – V 2) = 58.75µJ must absorb depends upon the amount of power at the IN TRIP input. For example, if the output voltage is 2.5V and can If the total input capacitance is 10μF, the input voltage sink 5A, the input protection circuit should be designed will rise to: to absorb at least 7.5W. In Figures 6a and 6b, let us say 58.75µJ = 1/2 • 10µF(V 2 − 15V2) that the protection activation threshold is 50V. Then the IN VIN = 15.39V 8064fa 24 For more information www.linear.com/LTM8064 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts