LM2622
Operation
(Continued)
gin can also be improved by adding C
C2
as discussed earlier
in the section. The equation for A
DC
is given below with
additional equations required for the calculation:
where R
L
is the minimum load resistance, V
IN
is the mini-
mum input voltage, g
m
is the error amplifier transconduc-
tance found in the
Electrical Characteristics
table, and R
D-
SON
is the value chosen from the graph "R
DSON
vs. V
IN
" in
the
Typical Performance Characteristics
section.
LAYOUT CONSIDERATIONS
The input bypass capacitor C
IN
, as shown in the typical
operating circuit, must be placed close to the IC. This will
reduce copper trace resistance which effects input voltage
ripple of the IC. For additional input voltage filtering, a 100nF
bypass capacitor can be placed in parallel with C
IN
, close to
the V
IN
pin, to shunt any high frequency noise to ground. The
output capacitor, C
OUT
, should also be placed close to the
IC. Any copper trace connections for the C
OUT
capacitor can
increase the series resistance, which directly effects output
voltage ripple. The feedback network, resistors R
FB1
and
R
FB2
, should be kept close to the FB pin, and away from the
inductor, to minimize copper trace connections that can in-
ject noise into the system. Trace connections made to the
inductor and schottky diode should be minimized to reduce
power dissipation and increase overall efficiency. For more
detail on switching power supply layout considerations see
Application Note AN-1149:
Layout Guidelines for Switching
Power Supplies.
mc
)
0.072fs (in V/s)
11
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