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LTM4648

4648f

For more information www.linear.com/LTM4648

applicaTions inForMaTion

where KD = 8.62−5, and knowing ln(ID/IS) is always posi-

tive because ID is always greater than IS, leaves us with

the equation that:

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 approximately –2mV/°C temperature rela-

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

fact, the IS term increases with temperature, reducing the

ln(ID/IS)absolutevalueyieldinganapproximately–2mV/°C

composite diode voltage slope.

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 JESD51-9 and are intended for use with

finite element analysis (FEA) software modeling tools that

leveragetheoutcomeofthermalmodeling,simulation,and

correlationtohardwareevaluationperformedonaµModule

package mounted to a hardware test board—also defined

by JESD51-9 (“Test Boards for Area Array Surface Mount

Package Thermal Measurements”). The motivation for

providingthesethermalcoefficientsinfoundinJESD51-12

(“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 µModule regulator’s thermal performance in their ap-

plication at various electrical and environmental operating

conditions to compliment any FEA activities. Without FEA

software, the thermal resistances reported in the Pin Con-

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

providing guidance of thermal performance; instead, the

derating curves provided in the data sheet can be used in

a manner that yields insight and guidance pertaining to

one’s application usage, and can be adapted to correlate

thermal performance to one’s own application.

The Pin Configuration section typically gives four thermal

coefficients explicitly defined in JESD51-12; these coef-

ficients are quoted or paraphrased below:

1. θJA: the thermal resistance from junction to ambient, is

the natural convection junction-to-ambient air thermal

resistance measured in a one cubic foot sealed enclo-

sure. This environment is sometimes referred to as

“still air” although natural convection causes the air to

move. This value is determined with the part mounted

toaJESD51-9definedtestboard, whichdoesnotreflect

an actual application or viable operating condition.

4648 F07

TEMPERATURE (°C)

–173

–73

127

0.4

0.6

0.8

1.0

∆VD

ID = 100µA

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

for Different Bias Currents

An external diode connected PNP transistor can be pulled

up to VIN with a resistor to set the current to 100µA for

using this diode connected transistor as a general tem-

perature monitor by monitoring the diode voltage drop

with temperature, or a specific temperature monitor can

be used that injects two currents that are at a 10:1 ratio

for very accurate temperature monitoring. See Figure 22

for an example.

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