Datasheet

LTC3853
29
3853fa
standard values are 3.3µH, 3.3µH and 1.5µH. At the
maximum input voltage (20V), the ripple will be:
DI
L(MAX)
=
V
OUT
f • L
1−
V
OUT
V
IN(MAX)






Channel 1 will have ~2.3A (46%) ripple, and both channel
2 and channel 3 will have ~1.75A (35%) ripple. The peak
inductor current will be the maximum DC value plus one-
half the ripple current, or 6.15A for channel 1 and 5.88A
for channels 2 and 3.
With I
LIM
high, the equivalent R
SENSE
resistor value can be
calculated by using the minimum value for the maximum
current sense threshold (65mV).
R
SENSE(EQUIV)
=
V
SENSE(MIN)
I
LOAD(MAX)
+
DI
L(NOM)
2
=
65mV
1.2 • 5A +
DI
L(MAX)
2






@ 9mΩ
The 1.2 factor adds margin for component variation and
overcurrent headroom during full load transients.
The equivalent R
SENSE
is the same for channels 1, 2
and 3.
The Vishay IHLP2525CZER3R3M01 (30mΩ DCR
MAX
at
20°C) and IHLP2525CZER1R5M01 (15mΩ DCR
MAX
at
20°C,) are chosen. At 100°C, the estimated maximum
DCR values are 39.6mV and 19.8mV. The divider ratios are:
R
D
=
R
SENSE(EQUIV)
DCR
MAX
at T
L(MAX)
=
9mΩ
39.6mΩ
= 0.23;
and
9mΩ
19.8mΩ
@ 0.45
For each channel, 0.1µF is selected for C1.
R1|| R2 =
L
(DCR
MAX
at 20°C) • C1
=
3.3µH
30mΩ • 0.1µF
= 1100Ω and
1.5µH
15mΩ • 0.1µF
= 1000Ω
APPLICATIONS INFORMATION
Figure 13. High Efficiency Triple 5V/3.3V/1.2V Step-Down Converter
TG1 TG2
BOOST1,2,3
SW1
SW2
BG1 BG2
SENSE1
+
SENSE2
+
SENSE1
–
SENSE2
–
V
FB1
V
FB2
I
TH1,2,3
V
IN
INTV
CC
LTC3853
DRV
CC12
MODE/PLLIN
20k
1%
FREQ/PLLFLTR
ILIM
PGND
PGOOD12
SGND
EXTV
CC
TK/SS1,2,3
V
FB3
SENSE3
–
SENSE3
+
SW3
TG3
BG3
RUN1,2,3
PGOOD3
4.75k
1430Ω
20k
1%
3.16k 1nF
63.4k
1%
4.7µF
10k
INTV
CC
V
OUT1
5V
5A
V
OUT2
3.3V
5A
20k
1%
3853 F13
2.43k
4.75k
V
OUT3
1.2V
5A
V
IN
7V TO 20V
C
ITH1,2,3
R
ITH1,2,3
0.1µF
330µF
10V
M1, M2, M3: Si4816BDY
CHANNEL 1: R
ITH1
= 15k, C
ITH1
= 1.5nF, C
P1
= 220pF
CHANNEL 2: R
ITH2
= 18k, C
ITH2
= 1.5nF, C
P2
= 220pF
CHANNEL 3: R
ITH3
= 10k, C
ITH3
= 1.5nF, C
P3
= 330pF
FOR CHANNEL 3, 0.068µF WAS SUBSTITUTED FOR 0.1µF AND 2.43k WAS SUBSTITUTED
FOR 2.21k TO MEET MINIMUM 15mV RIPPLE AT SENSE INPUT REQUIREMENT
C
P1,2,3
22µF
50V
C
SS1,2,3
0.1µF
0.1µF
10pF
1430Ω
3.3µH3.3µH
1.5µH
C
B1,2,3
0.1µF
SW1,2,3
D
B1,2,3
M2M1 M3
0.068µF
1.82k
+
330µF
6V
+
330µF
6V
+
10k
1%
105k
1%
10pF