Datasheet

LTM8026
17
8026fb
For more information www.linear.com/LTM8026
applicaTions inForMaTion
LTM8026s equally sharing output current is shown in the
Typical Applications section. The modulation of the CTL_I
inputs is performed at a high bandwidth, so use an op
amp with a gain bandwidth product greater than 1MHz.
The example circuit in the Typical Applications section
uses the LTC6255, which has a minimum gain bandwidth
product of 2MHz.
The LTM8026’s CVCC operation provides the ability to
power share the load among several input voltage sources.
An example of this is shown in the Typical Applications
section; please refer to the schematic while reading this
discussion. Suppose the application powers 2.5V at 8A
and the system under consideration has regulated 24V
and 12V input rails available. The power budget for the
power rails says that each can allocate only 750mA to
produce 2.5V. From the Input Current vs Output Current
graph in the Typical Performance Characteristics section
for 2.5V
OUT
, 750mA from the 24V rail can support more
than 5A output current, so apply a 66.5k/140k from V
REF
to the CTL_I pin of the LTM8026 powered from 24V
IN
to
set the output current to 5A. These resistor values were
derived as follows:
1. The typical output current limit is 5.6A for CTL_I = 1.5V
and above.
2. To get 5A, make the voltage on CTL_I = 1.5V 5A/5.6A
= 1.34V.
3. The V
REF
node is a regulated 2V, so applying the
66.5k/140k network yields 2V 140k/(66.5k + 140k) =
1.35V
The LTM8026 powered from 12V
IN
needs to supply the
rest of the load current, or 3A. Again referring to the Input
Current vs Output Current graph in the Typical Performance
Characteristics section for 2.5V
OUT
, 750mA will support
more than 3A when operated from 12V
IN
. Using a method
similar to the above, apply a resistor network of 132k/78.7k
to the CTL_I pin:
1. To get 2.5A, make the voltage on CTL_I = 1.5V 3 A/5.6A
= 0.8V
2. Applying a 132k/88.7k network to V
REF
and CTL_I yields
2V • 88.7k/(88.7k + 132k) = 0.8V
As seen in the graph accompanying the schematic in the
Typical Applications section, the input currents to each
LTM8026 stays below 750mA for all loads below 8A.
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8026. The LTM8026 is neverthe
-
less
a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specified
operation with a haphazard or poor layout. See Figure 5
for a suggested layout. Ensure that the grounding and
heat sinking are acceptable.
A few rules to keep in mind are:
1. Place the R
ADJ
and R
T
resistors as close as possible to
their respective pins.
2. Place the C
IN
capacitor as close as possible to the V
IN
and GND connection of the LTM8026.
Figure 5. Layout Showing Suggested External Components, GND
Plane and Thermal Vias.
CTL_T
CTL_I
V
REF
COMP
SS
ADJ
SYNC
V
IN
V
IN
8026 F05
GND
GND
C
IN
V
OUT
V
OUT
C
OUT
GND
THERMAL AND INTERCONNECT VIAS
RUN
RT