LTM4645 datasheet(21/34 Pages) LINER

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LTM4646

4646f

For more information www.linear.com/LTM4646

Figure 7. Diode Voltage VD vs Temperature T(°C)

for Different Bias Currents

VD T(KELVIN) • KD • ln

where VD appears to increase with temperature. It is com-

mon knowledge that a silicon diode biased with a current

source has an approximate –2mV/°C temperature rela-

tionship (Figure 7), which is at odds with the equation

APPLICATIONS INFORMATION

K’D = KD • In(10) = 198µV/k

yields

∆VD = K’D • T(KELVIN)

Solving for temperature:

T(KELVIN)

∆VD

K'D

T(KELVIN) [ C] 273.15,

[ C] T(KELVIN) 273.15

means that if we take the difference in voltage across the

diode measured at two currents with a ratio of 10, the result-

ing voltage is 198V per Kelvin of the junction with a zero

intercept at 0 Kelvin. The diode connected PNP transistor at

the TEMP+, TEMP– pins can be used to monitor the internal

temperature of the LTM4646. A general temperature moni-

tor can be implemented by connecting a resistor between

TEMP+ and VIN to set the current to 100µA, grounding the

TEMP– pin, and then monitoring the diode voltage drop

with temperature. A more accurate temperature monitor

can be achieved with a circuit injecting two currents that

are at a 10:1 ratio. See LTC2997 data sheet.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters defined by JESD51-12 and are intended for use with

finite element analysis (FEA) software modeling tools that

leverage the outcome of thermal modeling, simulation,

and correlation to hardware evaluation performed on a

Module® package mounted to a hardware test board.

The motivation for providing these thermal coefficients

is found in JESD51-12 (“Guidelines for Reporting and

Using Electronic Package Thermal Information”). Many

designers may opt to use laboratory equipment and a test

vehicle such as the demo board to anticipate the Module

regulator’s thermal performance in their application at

various electrical and environmental operating conditions

to compliment any FEA activities. Without FEA software,

the thermal resistances reported in the Pin Configuration

section are in-and-of themselves not relevant to provid-

ing guidance of thermal performance; instead, the derat-

ing curves provided in the data sheet can be used in a

4646 F07

TEMPERATURE (°C)

–173

–73

127

0.4

0.6

0.8

1.0

∆VD

ID = 100µA

ID = 10µA

term, increases with temperature, reducing the ln(ID/IS)

absolute value yielding an approximate –2mV/°C com-

posite diode voltage slope.

To obtain a linear voltage proportional to temperature,

we cancel the IS variable in the natural logarithm term to

remove the IS dependency from the following equation.

This is accomplished by measuring the diode voltage at

two currents I1, and I2, where I1 = 10 • I2.

Subtracting we get:

∆VD T(KELVIN) • KD •In

T(KELVIN) • KD •In

Combining like terms, then simplifying the natural log

terms yields:

∆VD = T(KELVIN) • KD • In(10)

and redefining constant

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