Analog Devices Linear Technology LTM8026 Manual page 17

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 , 750mA from the 24V rail can support more
OUT
than 5A output current, so apply a 66.5k/140k from V
to the CTL_I pin of the LTM8026 powered from 24V
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 node is a regulated 2V, so applying the
REF
66.5k/140k network yields 2V • 140k/(66.5k + 140k) =
1.35V
The LTM8026 powered from 12V
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
more than 3A when operated from 12V
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 • 3A/5.6A
= 0.8V
2. Applying a 132k/88.7k network to V
2V • 88.7k/(88.7k + 132k) = 0.8V
REF
to
IN
needs to supply the
IN
, 750mA will support
OUT
. Using a method
IN
and CTL_I yields
REF
For more information
•
•
C
OUT
•
•
V
GND V
OUT
•
THERMAL AND INTERCONNECT VIAS
Figure 5. Layout Showing Suggested External Components, GND
Plane and Thermal Vias.
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 and R
ADJ
their respective pins.
2. Place the C capacitor as close as possible to the V
IN
and GND connection of the LTM8026.
www.linear.com/LTM8026
LTM8026
• • • •
• • • •
GND
• • • • •
• • •
• • •
• • •
• • • • • •
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• • •
V
IN
OUT
C
IN
GND V
IN
8026 F05
resistors as close as possible to
T
SYNC
RUN
IN
8026fd
17