LTC3865/LTC3865-1
31
3865fb
APPLICATIONS INFORMATION
D3
D4
M1
0.1µF
L1
3.3µH
3.65k
1%
1800pF
100pF
0.1µF
0.1µF
22µF
50V
C
OUT1
100µF
s2
L1, L2: COILTRONICS HCP0703
M1, M2: VISHAY SILICONIX Si4816BDY
C
OUT1
, C
OUT2
: TAIYO YUDEN JMK325BJ107MM
4.75k
1%
V
OUT1
3.3V
5A
M2
0.1µF
L2
2.2µH
1.58k
1%
2200pF
100pF
C
OUT2
100µF
s2
5.49k
1%
162k
1%
V
OUT2
1.8V
5A
TG1 TG2
BOOST1 BOOST2
SW1 SW2
BG1
BG2
SGND
PGND
FREQ
SENSE1
+
SENSE2
+
VID21
SENSE1
–
SENSE2
–
V
OSENSE1
V
OSENSE2
I
TH1
I
TH2
V
IN
PGOOD INTV
CC
TK/SS1 TK/SS2
V
IN
7V TO 20V
3865 F14
EXTV
CC
0.1µF
0.1µF
LTC3865
MODE/PLLIN
I
LIM
RUN2RUN1
VID22
VID12
VID11
5.49k
1%
1.37k
1%
4.7µF
1µF
2.2Ω
+
Figure 14. High Effi ciency Dual 500kHz 3.3V/1.8V Step-Down Converter
Design Example
As a design example for a 2-channel medium current
regulator, assume V
IN
= 12V (nominal), V
IN
= 20V
(maximum), V
OUT1
= 3.3V, V
OUT2
= 1.8V, I
MAX1,2
= 5A, and
f = 500kHz (see Figure 14).
The regulated output voltages are set by connecting VID11
and VID22 to INTV
CC
and fl oating VID12 and VID21.
The frequency is set by biasing the FREQ pin to 1.2V (see
Figure 9).
The inductance values are based on a 35% maximum
ripple current assumption (1.75A for each channel). The
highest value of ripple current occurs at the maximum
input voltage:
L
V
I
V
V
OUT
LMAX
OUT
IN MAX
=−
⎛
⎝
⎜
⎞
⎠
⎟
f •
() ()
Δ
1
Channel 1 will require 3.2µH, and Channel 2 will require
1.9µH. The next highest standard values are 3.3µH
and 2.2µH. At the nominal input voltage (12V), the ripple
will be:
ΔI
V
L
V
V
L NOM
OUT OUT
IN NOM
()
()
•
=−
⎛
⎝
⎜
⎞
⎠
⎟
f
1
Channel 1 will have 1.45A (29%) ripple, and Channel 2 will
have 1.4A (28%) ripple. The peak inductor current will be
the maximum DC value plus one-half the ripple current,
or 5.725A for Channel 1 and 5.7A for Channel 2.
The minimum on-time occurs on Channel 1 at the maximum
V
IN
, and should not be less than 90ns:
t
V
V
V
VkHz
n
ON MIN
OUT
IN MAX
()
()
.
()
== =
f
1 8
20 500
180ss