LTM4613 datasheet(17/28 Pages) LINER

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LTM8001

8001f

For more information www.linear.com/8001

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 LTM8001 sinks current by energy

conversion and not dissipation. Thus, no matter what

protectioncircuitthatisused,theamountofpowerthatthe

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.

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 LTM8001

when the input voltage exceeds some threshold. This

is possible when the voltage power source that drives

current into VOUT never exceeds VIN. An example of this

circuit is shown in Figure 5d. When the power source on

the output drives VIN above a predetermined threshold,

the comparator pulls down on the RUN pin and stops

switching in the LTM8001. When this happens, the input

capacitance needs to absorb the energy stored within the

LTM8001’s internal inductor, resulting in an additional

voltage rise. As shown in the Block Diagram, the internal

APPLICATIONS INFORMATION

VIN

ZENER

DIODE

8001 F05a

LTM8001

LOAD

CURRENT

GND

VOUT0

SOURCING

LOAD

VIN

VREF

8001 F05b

LTM8001

LOAD

CURRENT

GND

VOUT0

SOURCING

LOAD

OPTIONAL

HYSTERESIS

RESISTOR

–

VIN

ZENER

DIODE

SCR

8001 F05a

LTM8001

LOAD

CURRENT

GND

VOUT0

FUSE

SOURCING

LOAD

VIN

RUN

8001 F05d

10µF

LTM8001

LOAD

CURRENT

GND

VOUT0

SOURCING

LOAD

EXTERNAL

REFERENCE

VOLTAGE

–

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 VIN Voltage Under

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 VREF Pin of the LTM8001

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 LTM8001 if

the Input Rises Above a Predetermined Threshold. When the

LTM8001 Turns Off, the Energy Stored in the Internal Inductor

Will Raise VIN a Small Amount Above the Threshold.

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