LM3405AXMYEVAL National Semiconductor, LM3405AXMYEVAL Datasheet - Page 12

• Manufacturer Part Number: LM3405AXMYEVAL
• Description: BOARD EVAL FOR LM3405 8MSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3405AXMYEVAL

• Current - Output / Channel: 750mA
• Outputs And Type: 1, Non-Isolated
• Features: Dimmable
• Voltage - Input: 5 ~ 18V
• Utilized Ic / Part: LM3405
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Cycle-by-Cycle Current Limit,
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

www.national.com
The gate charge loss is associated with the gate charge Q
required to drive the switch:
The power loss required for operation of the internal circuitry:
I
1.8mA for the LM3405A.
The total power loss in the IC is:
An example of power losses for a typical application is shown
in Table 2:
Q
D is calculated to be 0.36
is the quiescent operating current, and is typically around
R
T
T
V
DS(ON)
I
V
f
V
Q
OUT
RISE
FALL
OUT
SW
I
D1
Q
IN
G
Σ ( P
Conditions
P
TABLE 2. Power Loss Tabulation
INTERNAL
COND
1.6MHz
300mΩ
1.8mA
0.45V
1.4nC
+ P
P
P
3.9V
1.0A
18ns
12ns
12V
INTERNAL
G
= P
SW
= f
P
Q
COND
SW
+ P
= I
x V
Q
Q
= 445mW
+ P
x V
+ P
IN
SW
P
IN
x Q
G
P
COND
P
P
SW
) = P
+ P
Q
G
G
Power loss
G
INTERNAL
+ P
Q
108mW
288mW
22mW
27mW
G
12
PCB Layout Considerations
When planning the layout there are a few things to consider
when trying to achieve a clean, regulated output. The most
important consideration when completing the layout is the
close coupling of the GND connections of the input capacitor
C1 and the catch diode D1. These ground ends should be
close to one another and be connected to the GND plane with
at least two through-holes. Place these components as close
to the IC as possible. The next consideration is the location
of the GND connection of the output capacitor C2, which
should be near the GND connections of C1 and D1.
There should be a continuous ground plane on the bottom
layer of a two-layer board except under the switching node
island.
The FB pin is a high impedance node and care should be
taken to make the FB trace short to avoid noise pickup that
causes inaccurate regulation. The LED current setting resis-
tor R1 should be placed as close as possible to the IC, with
the GND of R1 placed as close as possible to the GND of the
IC. The V
the inductor and any other traces that are switching.
High AC currents flow through the V
so they should be as short and wide as possible. Radiated
noise can be decreased by choosing a shielded inductor.
The remaining components should also be placed as close
as possible to the IC. Please see Application Note AN-1229
for further considerations and the LM3405A demo board as
an example of a four-layer layout.
OUT
trace to LED anode should be routed away from
IN
, SW and V
OUT
traces,