LTM4644 datasheet(20/32 Pages) LINER

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LTM4634

4634f

For more information www.linear.com/LTM4634

applicaTions inForMaTion

If we take this equation and differentiate it with respect to

temperature T, then:

dVD

= –

VG0 – VD

This dVD/dT term is the temperature coefficient equal to

about –2mV/K or –2mV/°C. The equation is simplified for

the first order derivation.

Solving for T, T = –(VG0 – VD)/(dVD/dT) provides the

temperature.

1st Example: Figure 5 for 27°C, or 300K the diode

voltage is 0.598V, thus, 300K = –(1200mV – 598mV)/

–2.0 mV/K)

2nd Example: Figure 5 for 75°C, or 350K the diode

voltage is 0.50V, thus, 350K = –(1200mV – 500mV)/

–2.0mV/K)

Converting the Kelvin scale to Celsius is simply taking the

Kelvin temp and subtracting 273 from it.

A typical forward voltage is given in the electrical charac-

teristics section of the data sheet, and Figure 5 is the plot

of this forward voltage. Measure this forward voltage at

27°C to establish a reference point. Then using the above

expression while measuring the forward voltage over

temperature will provide a general temperature monitor.

Connect resistors between TEMP1, TEMP2 and VIN to set

the currents to 100µA each. See Figure 25 for an example.

Safety Considerations

The LTM4634 module does not provide galvanic isolation

from VIN to any of the three VOUTs. There is no internal

fuse. If required, a slow blow fuse with a rating higher

than the maximum input current can be used to protect

the unit in case of a catastrophic failure. An inline circuit

breaker function can also be used instead of a fuse.

The fuse or circuit breaker should be selected to limit the

current to the regulator during overvoltage in case of an

internal top MOSFET fault. If the internal top MOSFET fails,

then turning it off will not resolve the overvoltage, thus

the internal bottom MOSFET will turn on indefinitely trying

to protect the load. Under this fault condition, the input

voltage will source very large currents to ground through

the failed internal top MOSFET and enabled internal bot-

tom MOSFET. This can cause excessive heat and board

damage depending on how much power the input voltage

can deliver to this system. A fuse or circuit breaker can

be used as a secondary fault protector in this situation.

Figure 5. Diode Voltage VD vs Temperature T(°C)

Figure 6. Thermal Plot 24V to 3.3V at 5A, 5V at 5A, and 12V

at 4A, Airflow = 200LFM, Ambient = 25°C

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4634 F05

125

ID = 100µA

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