Datasheet

LTC3850/LTC3850-1
28
38501fc
voltage will decrease as V
IN
approaches 5V, lowering the
switching frequency. If a separate 5V supply is connected to
EXTV
CC
, INTV
CC
will remain at 5V even if V
IN
decreases.
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
OUT
f • ∆I
L(MAX)
1−
V
OUT
V
IN(MAX)






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:
∆I
L(NOM)
=
V
OUT
f • L
1−
V
OUT
V
IN(NOM)






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
ON(MIN)
=
V
OUT
V
IN(MAX)
f
=
1.8V
20V(500kHz)
=180ns
With I
LIM
floating, the equivalent R
SENSE
resistor value
can be calculated by using the minimum value for the
maximum current sense threshold (40mV).
R
SENSE(EQUIV)
=
V
SENSE(MIN)
I
LOAD(MAX)
+
∆I
L(NOM)
2
=
40mV
5A +
1.5A
2
≅ 7mΩ
The equivalent R
SENSE
is the same for Channel 2.
APPLICATIONS INFORMATION
Figure 14. High Efficiency Dual 500kHz 3.3V/1.8V Step-Down Converter
D3
D4
M1
0.1µF
63.4k
1%
L1
3.3µH
4.12k
1%
1800pF
100pF
0.1µF
0.1µF
22µF
50V
C
OUT1
100µF
X2
L1, L2: COILTRONICS HCP0703
M1, M2: VISHAY SILICONIX Si4816BDY
C
OUT1
, C
OUT2
: TAIYO YUDEN JMK325BJ107MM
20k
1%
4.75k
1%
V
OUT1
3.3V
5A
M2
0.1µF
25.5k
1%
L2
2.2µH
1.5k
1%
2200pF
100pF
C
OUT2
100µF
X2
20k
1%
5.49k
1%
3.16k
1%
V
OUT2
1.8V
5A
TG1 TG2
BOOST1 BOOST2
SW1 SW2
BG1
BG2
SGND
PGND
FREQ/PLLFLTR
SENSE1
+
SENSE2
+
RUN2
SENSE1
–
SENSE2
–
V
FB1
V
FB2
I
TH1
I
TH2
V
IN
PGOOD INTV
CC
TK/SS1 TK/SS2
V
IN
7V TO
20V
38501 F14
EXTV
CC
0.1µF
0.1µF
LTC3850
MODE/PLLIN
I
LIM
RUN1
6.19k
1%
1.33k
1%
10k, 1%
4.7µF
33pF33pF
1µF
2.2Ω