LTC3418 Datasheet, PDF (14/20 Pages) Linear Technology – 8A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3418
APPLICATIO S I FOR ATIO
Using a 0.2µH inductor results in a maximum ripple
current of:
∆I_L
=
⎛
⎝⎜
2.5V ^⎞
(^1MHz)(^0.2µH)⎠⎟
⎛⎝⎜_1–
2.5V ⎞
3.3V ⎠⎟
=
3.03A
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.
If we set the sum of R2 and R3 to 200k, then the following
equations can be solved.
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, five
100µF ceramic capacitors will be used.
C_IN should be sized for a maximum current rating of:
I_RMS
=
(^8A)^{⎛⎝⎜}
2.5V
3.3V
⎞⎠⎟
3.3V
2.5V
–
1
=
3.43A_RMS
Decoupling the PV_IN and SV_IN pins with four 100µF
capacitors 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
R2 + R3 = 200k
1+
R2
R3
=
0.8V
0.67V
The two equations shown above result in the following
values for R2 and R3: R2 = 33.2k, R3 = 169k. The value
of R1 can now be determined by solving the equation:
1+
R1
202.2k
=
2.5V
0.8V
R1= 430k
A value of 432k will be selected for R1. Figure 4 shows the
complete schematic for this design example.
V_IN
3.3V
C_SVIN
1µF
X7R
C_IN
100µF
×4
R_SVIN
100Ω
R_SS R_PG
2.2M 100k
3
PV_IN
4
PV_IN
9
PV_IN
10
PV_IN
22
PV_IN
23
PV_IN
28
PV_IN
29
PV_IN
24
SV_IN
1
SW
2
SW
11
SW
12
SW
20
SW
21
SW
30
SW
31
SW
25
V_FB
C_SS
1000pF
X7R
C_ITH
820pF
X7R
R_ITH
7.5k
C1
47pF
X7R
LTC3418
35
27
TRACK SYNC/MODE
5
PGOOD
38
PGND
7
RUN/SS
37
PGND
26
R_OSC 69.8k ₆
8
I_TH
R_T
SGND
36
PGND
34
PGND
33
PGND
13
PGND
32
PGND
14
PGND
19
PGND
15
PGND
18
PGND
17
PGND
16
V_REF
C_IN, C_OUT: AVX 18126D107MAT
L1: TOKO FDV0620-R20M
L1
0.2µH
C1
22pF
X7R
R1
432k
C_OUT
100µF
×5
V_OUT
2.5V
8A
R2
33.2k
R3
169k
C_REF
2.2µF
X7R
3418 F04
V_REF
Figure 4. 2.5V, 8A Regulator at 1MHz, Burst Mode Operation
3418f
14