DC1245A Linear Technology, DC1245A Datasheet - Page 17

• Manufacturer Part Number: DC1245A
• Description: BOARD DEMO LTM4616
• Manufacturer: Linear Technology
• Series: µModuler

Specifications of DC1245A

• Design Resources: LTM4616 Spice Model LTM4616 Gerber Files DC1245 Design Files
• Main Purpose: DC/DC, Step Down
• Outputs And Type: 2, Non-Isolated
• Power - Output: -
• Voltage - Output: 1.2V, 1.8V
• Current - Output: 8A, 8A
• Voltage - Input: 2.7 ~ 5.5 V
• Regulator Topology: Buck
• Frequency - Switching: 1.5MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LTM4616
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Thermal Considerations and Output Current Derating
The power loss curves in Figures 7 and 8 can be used
in coordination with the load current derating curves in
Figures 9 to16 for calculating an approximate θ
resistance for the LTM4616 with various heat sinking and
airflow conditions. Both LTM4616 outputs are placed in
parallel for a total output current of 16A, and the power
loss curves are plotted for specific output voltages up to
16A. The derating curves are plotted with each output at
8A combined for a total of 16A. The output voltages are
1.2V, 2.5V and 3.3V. These are chosen to include the lower
and higher output voltage ranges for correlating the ther-
mal resistance. Thermal models are derived from several
temperature measurements in a controlled temperature
chamber along with thermal modeling analysis. The junc-
tion temperatures are monitored while ambient temperature
increases with and without airflow. The junctions are main-
tained at ~115°C while lowering output current or power
with increasing ambient temperature. The 115°C value is
chosen to allow for 10°C of margin relative to the maxi-
mum temperature of 125°C. The decreased output current
will decrease the internal module loss as ambient tem-
perature is increased. The power loss curves in Figures 7
and 8 show this amount of power loss as a function of
load current that is specified with both channels in paral-
lel. The monitored junction temperature of 115°C minus
the ambient operating temperature specifies how much
applications inForMation
8
7
6
5
4
3
2
1
0
0
Figure 7. 1.2V, 2.5V Power Loss
4
LOAD CURRENT (A)
8
3.3V
3.3V
12
IN
IN
1.2V
2.5V
4616 F07
OUT
OUT
16
JA
thermal
module temperature rise can be allowed. As an example, in
Figure 10 the load current is derated to 10A at ~ 80°C and
the power loss for the 5V to 1.2V at 10A output is ~3.2W.
If the 80°C ambient temperature is subtracted from the
115°C maximum junction temperature, then difference of
35°C divided by 3.2W equals a 10.9°C/W. Table 4 specifies
a 10.5°C/W value which is very close. Table 4 and Table 5
provide equivalent thermal resistances for 1.2V and 3.3V
outputs, with and without airflow and heat sinking. The
printed circuit board is a 1.6mm thick four layer board
with two ounce copper for the two outer layers and one
ounce copper for the two inner layers. The PCB dimensions
are 95mm × 76mm. The BGA heat sinks are listed below
Table 5. At load currents on each channel from 3A to 8A
(6A to16A in parallel on the derating curves), the thermal
resistance values in Tables 4 and 5 are fairly accurate. As
the load currents go below the 3A level on each channel
the thermal resistance starts to increase due to the reduced
power loss on the board. The approximate thermal resis-
tance values for these lower currents is 15°C/W.
Safety Considerations
The LTM4616 modules do not provide isolation from V
to V
fuse with a rating twice the maximum input current needs
to be provided to protect each unit from catastrophic
failure. The device does support thermal shutdown and
overcurrent protection.
OUT
. There is no internal fuse. If required, a slow blow
8
7
6
5
4
3
2
1
0
0
Figure 8. 1.2V, 3.3V Power Loss
4
LOAD CURRENT (A)
8
5V
5V
12
IN
IN
1.2V
3.3V
LTM4616
4616 F08
OUT
OUT
16
17
4616fc
IN