Datasheet

LTC3418
14
3418fb
Because ef ciency is important at both high and low load
current, Burst Mode operation will be utilized.
First, calculate the timing resistor:
R
OSC
=
7.3 10
10
1•10
6
2.5k = 70.5k
Use a standard value of 69.8k. Next, calculate the inductor
value for about 40% ripple current:
L =
2.5V
1M H z
()
3.2A
( )
1–
2.5V
3.3
V
= 0.19μH
Using a 0.2μH inductor results in a maximum ripple cur-
rent of:
I
L
=
2.5V
1M H z
()
0.2μH
( )
1–
2.5V
3.3
V
= 3.03A
C
OUT
will be selected based on the ESR that is required to
satisfy the output voltage ripple requirement and the bulk
capacitance needed for loop stability. For this design, fi ve
100μF ceramic capacitors will be used.
C
IN
should be sized for a maximum current rating of:
I
RMS
= 8A
()
2.5V
3.3
V
3.3V
2.5V
1=3.43A
RMS
Decoupling the PV
IN
and SV
IN
pins with four 100μF capaci-
tors is adequate for this application.
The burst clamp and output voltage can now be pro-
grammed by choosing the values of R1, R2 and R3. The
voltage on the MODE pin will be set to 0.67V by the resistor
divider consisting of R2 and R3. A burst clamp voltage of
0.67V will set the minimum inductor current, I
BURST
, to
approximately 1.2A.
PV
IN
PV
IN
PV
IN
PV
IN
PV
IN
PV
IN
PV
IN
PV
IN
SV
IN
TRACK
PGOOD
RUN/SS
I
TH
R
T
SGND
PGND
PGND
PGND
PGND
1
2
11
12
20
21
30
31
25
27
38
37
36
34
33
32
19
18
16
3
4
9
10
22
23
28
29
24
35
5
7
26
6
8
13
14
15
17
SW
SW
SW
SW
SW
SW
SW
SW
V
FB
SYNC/MODE
PGND
PGND
PGND
PGND
PGND
PGND
PGND
PGND
V
REF
LTC3418
L1
0.2μH
C1
22pF
X7R
C
OUT
100μF
×5
C
REF
2.2μF
X7R
C
IN
100μF
×4
V
IN
3.3V
V
OUT
2.5V
8A
R1
432k
R
PG
100k
R
SS
2.2M
C
SS
1000pF
X7R
C
ITH
820pF
X7R
R
ITH
7.5k
R
SVIN
100Ω
R
OSC
69.8k
R2
33.2k
R3
169k
V
REF
C1
47pF
X7R
3418 F04
C
IN
, C
OUT
: AVX 18126D107MAT
L1: TOKO FDV0620-R20M
C
SVIN
F
X7R
Figure 4. 2.5V, 8A Regulator at 1MHz, Burst Mode Operation
APPLICATIONS INFORMATION