LTM4614_15 Datasheet, PDF (11/20 Pages) Linear Technology – Dual 4A per Channel Low VIN DC/DC Module Regulator

LTM4614
Applications Information
Parallel Switching Regulator Operation
The LTM4614 switching regulators are inherently current
mode control. Paralleling will have very good current shar-
ing. This will balance the thermals on the design. Figure
13 shows a schematic of a parallel design. The voltage
feedback equation changes with the variable N as chan-
nels are paralleled.
The equation:
V_OUT
= 0.8V
•
4.99k
N
+
R_FB
R_FB
N is the number of paralleled channels.
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 LTM4614 with various heat sinking
and airflow conditions. Both of the LTM4614 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 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 measurements in a controlled tem-
perature 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 while lowering output
current or power while increasing ambient temperature.
The 120°C is chosen to allow for a 5°C margin window
relative to the maximum 125°C. The decreased output
current will decrease the internal module loss as ambi-
ent 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 chan-
nels. The monitored junction temperature of 120°C minus
the ambient operating temperature specifies how much
2.5
2.0
1.5
1.0
0.5
0
V_IN = 5V
0
1
2
3
4
LOAD CURRENT (A)
4614 F05
Figure 5. 1.2V Power Loss
3.0
2.5
2.0
1.5
1.0
0.5
0
V_IN = 5V
0
1
2
3
4
LOAD CURRENT (A)
4614 F06
Figure 6. 3.3V Power Loss
4614fb
11