lt3070 Linear Technology Corporation, lt3070 Datasheet - Page 22

• Manufacturer Part Number: lt3070
• Description: 5a, Low Noise, Programmable Output, 85mv Dropout Linear Regulator
• Manufacturer: Linear Technology Corporation

ApplicAtions inFormAtion
LT3070
heat sink resistance or circuit board to ambient as the
application dictates. Also, consider additional heat sources
mounted in proximity to the LT3070. The LT3070 is a surface
mount device and as such, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Surface mount heat sinks and
plated through-holes can also be used to spread the heat
generated by power devices. Junction-to-case thermal
resistance is specified from the IC junction to the bottom
of the case directly below the die. This is the lowest resis-
tance path for heat flow. Proper mounting is required to
ensure the best possible thermal flow from this area of the
package to the heat sinking material. Note that the exposed
pad is electrically connected to GND.
Table 3 lists thermal resistance as a function of copper
area in a fixed board size. All measurements were taken
in still air on a 4-layer FR-4 board with 1 oz solid internal
planes and 2 oz top/bottom external trace planes with a
total board thickness of 1.6mm. PCB layers, copper weight,
board layout and thermal vias affect the resultant thermal
resistance. For further information on thermal resistance
and high thermal conductivity test boards, refer to JEDEC
standard JESD51, notably JESD51-12 and JESD51-7.
Achieving low thermal resistance necessitates attention
to detail and careful PCB layout.
Table 3, UFD Plastic Package, 28-Lead QFN
*Device is mounted on topside
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:

TOPSIDE*
2500mm
1000mm
225mm
100mm
I
+ I
OUT(MAX)
COPPER AREA
GND
2
2
2
2
• V
BACK SIDE
2500mm
2500mm
2500mm
2500mm
• (V
BIAS
IN(MAX)
2
2
2
2
BOARD AREA
2500mm
2500mm
2500mm
2500mm
– V
OUT
2
2
2
2
) + (I
(JUNCTION-TO-AMBIENT)
THERMAL RESISTANCE
BIAS
– I
30°C/W
32°C/W
33°C/W
35°C/W
GND
) • V
OUT
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 LT3070 require a derating of output
current or increased airflow.
Paralleling Devices for Higher I
Multiple LT3070s 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 IN pins and the OUT
pins of the multiple devices together. Also, tie the REF/BYP
pins of the multiple outputs together. This effectively gives
an averaged value of multiple 600mV reference voltage
sources. The OUT of each LT3070 is connected to the
common load using a small piece of PC trace as a ballast
resistor (≅2mΩ) or an actual sense resistor, beyond the
primary output capacitors of each regulator. The ballast
resistor ensures output current sharing (see Figures 8
and 9). The amount of ballast resistance used may need
to increase at high V
this ballast trace area free of solder to maintain a controlled
resistance.
P = 4A(1.26V – 0.9V) + (6.91mA – 0.87mA)0.9V +
0.87mA(2.5V) = 1.448W
OUT(MAX)
BIAS
GND
JMAX
IN(MAX)
at (I
at (I
= 50°C + 43.44°C = 93.44°C
= 1.26V
OUT
= 4A
OUT
= 4A, V
= 4A, V
OUT
BIAS
BIAS
s to ensure current sharing. Keep
= 2.5V) = 0.87mA
= 2.5V) = 6.91mA
OUT
3070f