LTC3563
13
3563f
With this information we can calculate L using:
L
fI
V
V
V
L
OUT
OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
1
1
•
••–
∆
Substituting V
OUT
= 1.87V, V
IN
= 4.2V, ΔI
L
= 200mA and
f = 2.25MHz gives:
L
V
MHz mA
V
V
=
⎛
⎝
⎜
⎞
⎠
⎟
=
187
2 25 200
1
187
42
231
.
.•
•–
.
.
. µµH
With V V
L
V
MHz mA
OUT
=
=
128
128
2 25 200
1
12
.
.
.•
•–
.
88
42
198
V
V
µH
.
.
⎛
⎝
⎜
⎞
⎠
⎟
=
Choosing a vendor’s closest inductor value of 2.2µH results
in a maximum ripple current of:
For V V
I
V
MHz µH
V
OUT
L
=
=
187
187
225 22
1
187
.
.
.•.
•–
.
∆
442
209 6
128
128
2
.
.
.
.
.
V
mA
For V V
I
V
OUT
L
⎛
⎝
⎜
⎞
⎠
⎟
=
=
=∆
225 2 2
1
128
42
179 8
MHz µH
V
V
mA
•.
•–
.
.
.
⎛
⎝
⎜
⎞
⎠
⎟
=
APPLICATIO S I FOR ATIO
WUUU
Hot Swap is a trademark of Linear Technology Corporation.
C
IN
will require an RMS current rating of at least
0.25A ≅ I
LOAD(MAX)
/2 at temperature and C
OUT
will require
ESR of less than 0.2Ω. In most cases, ceramic capacitors
will satisfy these requirements. Select C
OUT
= 10µF and
C
IN
= 10µF.
Figure 3 shows the complete circuit along with its effi ciency
curve, load step response and recommended layout.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3563. These items are also illustrated graphically
in Figure 3b. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the V
IN
trace should be kept short, direct and
wide.
2. Does the (+) plate of C
IN
connect to V
IN
as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
3. Keep the (–) plates of C
IN
and C
OUT
as close as possible.
Figure 3a. Typical Application
V
IN
LTC3563
RUN
3563 F03
L
C
IN
C
OUT
SW
V
IN
2.7V TO 5.5V
V
OUT
V
OUT
V
SEL
GND
1.28V 1.87V
