LTC3865-1_15 Datasheet, PDF (31/38 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
APPLICATIONS INFORMATION
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 floating 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=
f
V_OUT
• ΔI_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:
ΔI_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 _{= 180ns}
20V(500kHz)
2.2Ω
1μF
+ _22μF
50V
V_IN
7V TO 20V
V_OUT1
3.3V
5A
L1
3.3μH
5.49k
1%
C_OUT1
100μF
s2
M1
1.37k
1%
4.7μF
D3
0.1μF
V_IN PGOOD EXTV_CC INTV_CC
TG1
TG2
BOOST1
SW1
BG1
LTC3865
BOOST2
SW2
BG2
1800pF
4.75k
1%
0.1μF
100pF
MODE/PLLIN
I_LIM
SENSE1⁺
PGND
FREQ
SENSE2⁺
RUN1
SENSE1^–
VID11
VID12
V_OSENSE1
I_TH1
TK/SS1
SGND
RUN2
SENSE2^–
VID21
VID22
V_OSENSE2
I_TH2
TK/SS2
0.1μF
0.1μF
D4
M2
0.1μF
0.1μF
1.58k
1%
2200pF
162k 5.49k
1% 1%
100pF
L1, L2: COILTRONICS HCP0703
M1, M2: VISHAY SILICONIX Si4816BDY
C_OUT1, C_OUT2: TAIYO YUDEN JMK325BJ107MM
Figure 14. High Efficiency Dual 500kHz 3.3V/1.8V Step-Down Converter
L2
2.2μH
3.65k
1%
V_OUT2
1.8V
5A
C_OUT2
100μF
s2
3865 F14
3865fb
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