Datasheet

LTC3611
18
3611fd
Design Example
As a design example, take a supply with the following
specifications: V
IN
= 5V to 36V (12V nominal), V
OUT
=
2.5V ±5%, I
OUT(MAX)
= 10A, f = 550kHz. First, calculate
the timing resistor with V
ON
= V
OUT
:
R
ON
=
2.5V
(2.4) 550kHz
( )
10pF
( )
=187k
and choose the inductor for about 40% ripple current at
the maximum V
IN
:
L =
2.5V
550kHz
( )
0.4
( )
10A
( )
1−
2.5V
36V
⎛
⎝
⎜
⎞
⎠
⎟
= 1µH
Selecting a standard value of 1μH results in a maximum
ripple current of:
ΔI
L
=
2.5V
550kHz
( )
1µH
( )
1 –
2.5V
12V
⎛
⎝
⎜
⎞
⎠
⎟
= 3.6A
ApplicAtions inForMAtion
Next, set up V
RNG
voltage and check the I
LIMIT
. Tying
V
RNG
to 1V will set the typical current limit to 15A, and
tying V
RNG
to GND will result in a typical current around
10A. C
IN
is chosen for an RMS current rating of about 5A
at 85°C. The output capacitors are chosen for a low ESR
of 0.013Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
Δ
V
OUT(RIPPLE)
= ΔI
L(MAX)
(ESR)
= (3.6A) (0.013Ω) = 47mV
However, a 0A to 10A load step will cause an output
change of up to:
ΔV
OUT(STEP)
= ΔI
LOAD
(ESR) = (10A) (0.013Ω) =130mV
An optional 22μF ceramic output capacitor is included
to minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 6.
Figure 6. Design Example: 5V to 32V Input to 2.5V/10A at 550kHz
V
OUT
2.5V AT
10A
GND
GND
V
IN
V
IN
5V TO 32V
C
OUT1
100µF
×2
C5
22µF
6.3V
L1
1µH
C
IN
4.7µF
50V
×2
C6
100µF
50V
+
(OPTIONAL)
(OPTIONAL)
C
IN
= MURATA GRM32ER71H475K
C
OUT
= MURATA GRM43SR60J107M
L
1
= COOPER HCP0703-IRO
C5: MURATA GRM31CR60J226KE19
KEEP POWER AND SIGNAL GROUNDS SEPARATE.
CONNECT TO ONE POINT.
LTC3611
SGND
48
SGND
47
SGND
46
SGND
45
EXTV
CC
EXTV
CC
44
V
FB
43
SGND
42
I
ON
41
SGND
40
FCB
39
I
TH
38
V
RNG
37
PGOOD
36
V
ON
35
SGND
34
SGND
V
OUT
V
OUT
33
PV
IN
17
PV
IN
18
PV
IN
19
PV
IN
20
PV
IN
21
PV
IN
22
PV
IN
23
PV
IN
24
PV
IN
25
SW
SW
SW
SW
26
NC
27
SGND
28
BOOST
29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTV
CC
INTV
CC
INTV
CC
54
INTV
CC
INTV
CC
53
SV
IN
V
IN
52
SV
IN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PV
IN
V
IN
2Ω
12
PV
IN
13
PV
IN
14
PV
IN
15
PV
IN
16
3611 F06
C
F
0.1µF
50V
C
VCC
4.7µF
6.3V
C
ON
0.01µF
C4
0.01µF
C2C1
0.01µF
C
B1
0.22µF
D
B
CMDSH-3
C
SS
0.1µF
V
IN
R
SS1
510k
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
R1
9.5k
1%
R2
30.1k
1%
R
VON
0Ω
R
PG1
100k
39.2k
R
ON
182k
1%
R5
12.5k
R3
11k
C
C1
680pF
C
C2
100pF
(OPTIONAL)
SGNDPGND