LT3071EUFD LINER [Linear Technology], LT3071EUFD Datasheet - Page 22

• Manufacturer Part Number: LT3071EUFD
• Description: 5A, Low Noise, Programmable Output, 85mV Dropout Linear Regulator with Analog Margining
• Manufacturer: LINER [Linear Technology]

LT3071
ApplicAtions inFormAtion
Calculating Junction Temperature
Example: Given an output voltage of 0.9V, an input voltage
range of 1.2V ± 5%, a BIAS voltage of 2.5V, a maximum out-
put current of 4A and a maximum ambient temperature of
50°C, what will the maximum junction temperature be?
The power dissipated by the device equals:
where:
I
V
I
I
thus:
With the QFN package soldered to maximum copper
area, the thermal resistance is 30°C/W. So the junction
temperature rise above ambient equals:
1.448W at 30°C/W = 43.44°C
The maximum junction temperature equals the maximum
ambient temperature plus the maximum junction tempera-
ture rise above ambient or:
T
Applications that cannot support extensive PCB space
for heat sinking the LT3071 require a derating of output
current or increased airflow.

I
+ I
P = 4A • (1.26V – 0.9V) + (6.91mA – 0.87mA) • 0.9V
+ 0.87mA • 2.5V = 1.448W
OUT(MAX)
OUT(MAX)
BIAS
GND
JMAX
IN(MAX)
GND
at (I
at (I
= 50°C + 43.44°C = 93.44°C
• V
= 1.26V
OUT
OUT
• (V
= 4A
BIAS
= 4A, V
= 4A, V
IN(MAX)
BIAS
BIAS
– V
= 2.5V) = 0.87mA
= 2.5V) = 6.91mA
OUT
) + (I
BIAS
– I
GND
) • V
OUT
Paralleling Devices for Higher I
Multiple LT3071s may be paralleled to obtain higher output
current. This paralleling concept borrows from the scheme
employed by the LT3080.
To accomplish this paralleling, tie the REF/BYP pins of
the paralleled regulators together. This effectively gives
an averaged value of multiple 600mV reference voltage
sources. Tie the OUT pins of the paralleled regulators to
the common load plane through a small piece of PC trace
ballast or an actual surface mount sense resistor beyond
the primary output capacitors of each regulator. The re-
quired ballast is dependent upon the application output
voltage and peak load current. The recommended ballast
is that value which contributes 1% to load regulation. For
example, two LT3071 regulators configured to output 1V,
sharing a 10A load require 2mΩ of ballast at each output.
The Kelvin SENSE pins connect to the regulator side of
the ballast resistors to keep the individual control loops
from conflicting with each other (see Figures 8 and 9).
Keep this ballast trace area free of solder to maintain a
controlled resistance.
Table 4 shows a simple guideline for PCB trace resistance
as a function of weight and trace width.
Table 4. PC Board Trace Resistance
*Trace resistance is measured in milliohms/in
WEIGHT (Oz)
1
2
100 MIL WIDTH*
5.43
2.71
OUT
200 MIL WIDTH*
2.71
1.36
3071f