Datasheet

LTM4615
15
4615fb
For more information www.linear.com/LTM4615
applicaTions inForMaTion
Reverse input current will spike up as LDO_IN gets to
within about 30mV of LDO_OUT as reverse current protec-
tion circuitry is disabled and normal operation resumes.
As LDO_IN transitions above LDO_OUT the reverse current
transitions into short cir
cuit current as long as LDO_OUT
is held below the regulation voltage.
Thermal Considerations and Output Current Derating
The power loss curves in Figures 5 and 6 can be used
in coordination with the load current derating curves in
Figures 7 to 10 for calculating an approximate θ
JA
thermal
resistance for the LTM4615 with various heat sinking and
airflow conditions. Both of the LTM4615 outputs are at full
4A load current, and the power loss curves in Figures 5
and 6 are combined power losses plotted for both output
voltages up to 4A each. The VLDO regulator is set to have
a power dissipation of 0.5W since it is generally used with
dropout voltages of 0.5V or less. For example: 1.2V to 1V,
1.5V to 1V, 1.5V to 1.2V and 1.8V to 1.5V. Other dropout
voltages can be supported at VLDO maximum load, but
further thermal analysis will be required for the VLDO.
The 4A output voltages are 1.2V and 3.3V. These voltages
are chosen to include the lower and higher output voltage
ranges for correlating the thermal resistance. Thermal
models are derived from several temperature measure
-
ments in a controlled temperature chamber along with
thermal modeling analysis. The junction temperatures are
monitored while ambient temperature is increased with and
without airflow. The junctions are maintained at ~120°C
Figure 5. 1.2V Power Loss Figure 6. 3.3V Power Loss
while lowering output current or power with increasing
ambient temperature. The 120°C value is chosen to allow
for a 5°C margin window relative to the maximum 125°C
limit. The decreased output current will decrease the inter
-
nal module loss as ambient temperature is increased. The
power loss curves in Figures 5 and 6 show this amount of
power loss as a function of load current that is specified
for
both channels. The monitored junction temperature of
120°C minus the ambient operating temperature specifies
how much module temperature rise can be allowed. As
an example, in Figure 7 the load current is derated to 3A
for each channel with 0LFM at ~90°C and the power loss
for both channels at 5V to 1.2V at 3A output is ~1.4W.
Add the VDLO power loss of 0.5W to equal 1.9W. If the
90°C ambient temperature is subtracted from the 120°C
maximum junction temperature, then the difference of 30°C
divided by 1.9W equals a 15.7°C/W thermal resistance.
Table 2 specifies a 15°C/W value which is very close. Table
2 and Table 3 provide equivalent thermal resistances for
1.2V and 3.3V outputs with and without air flow and heat
sinking. The combined power loss for the two 4A outputs
plus the VLDO power loss can be summed together and
multiplied by the thermal resistance values in Tables 2 and
3 for module temperature rise under the specified condi
-
tions. The printed circuit board is a 1.6mm thick four layer
board with two ounce copper for the two outer layers and 1
ounce copper for the two inner layers. The PCB dimensions
are 95mm
× 76mm. The BGA heat sinks are listed below
Table 3. The data sheet lists the θ
JC
(Junction to Case)
thermal resistances under the Pin Configuration diagram.
LOAD CURRENT (A)
0
POWER LOSS (W)
1.0
1.5
4
4615 F05
0.5
0
1
2
3
2.5
2.0
V
IN
= 5V
LOAD CURRENT (A)
0
0
POWER LOSS (W)
0.5
1.0
1.5
2.0
2.5
3.0
1 2 3 4
4615 F06
V
IN
= 5V