ltc3405a Linear Technology Corporation, ltc3405a Datasheet - Page 12

• Manufacturer Part Number: ltc3405a
• Description: 1.5mhz, 300ma Synchronous Step-down Regulator In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3405A
1. The power traces, consisting of the GND trace, the SW
2. Does the V
3. Does the (+) plate of C
4. Keep the (–) plates of C
5. Keep the switching node, SW, away from the sensitive
Design Example
As a design example, assume the LTC3405A is used in a
single lithium-ion battery-powered cellular phone
application. The V
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.25A but most of the time it will be in
standby mode, requiring only 2mA. Efficiency at both low
and high load currents is important. Output voltage is
2.5V. With this information we can calculate L using
equation (1),
12
trace and the V
wide.
resistors? The resistive divider R1/R2 must be con-
nected between the (+) plate of C
possible? This capacitor provides the AC current to the
internal power MOSFETs.
V
FB
node.
FB
IN
IN
pin connect directly to the feedback
U
trace should be kept short, direct and
will be operating from a maximum of
IN
U
IN
and C
connect to V
OUT
W
V
OUT
OUT
as close as possible.
SW
L1
and ground.
IN
Figure 6. LTC3405A Suggested Layout
as closely as
PIN 1
U
C
OUT
LTC3405A
VIA TO V
GND
Substituting V
f = 1.5MHz in equation (3) gives:
For best efficiency choose a 300mA or greater inductor
with less than 0.3Ω series resistance.
C
I
of less than 0.5Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
For the feedback resistors, choose R1 = 412k. R2 can
then be calculated from equation (2) to be:
Figure 7 shows the complete circuit along with its effi-
ciency curve.
LOAD(MAX)
IN
IN
C
R
L
L
IN
will require an RMS current rating of at least 0.125A ≅
2
=
=
=
1 5
( )
f
.
⎛
⎜
⎝
C
( )
V
R1
R2
FWD
MHz
/2 at temperature and C
1
∆
0 8
OUT
.
I
L
2 5
OUT
.
VIA TO GND
(
V
100
−
V
OUT
1 1 875 5
= 2.5V, V
⎞
⎟
⎠
mA
R
⎛
⎜
⎝
1
V
=
)
−
3405A F06
IN
⎛
⎜
⎝
1
V
V
−
OUT
IN
IN
VIA TO V
2 5
4 2
. ;
.
.
= 4.2V, ∆I
⎞
⎟
⎠
k use
V
V
OUT
⎞
⎟ ≅
⎠
OUT
will require an ESR
6 8
8
.
87k
L
µ
= 100mA and
H
3405afa
(3)