LTC3875_15 Datasheet, PDF (34/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Applications Information
INTV_CC
60k
20k _INTVCC
THERMAL
SENSOR
L1
0.33µH
(0.32mΩ DCR)
R2
R1
931Ω 4.64k
V_OUT
+
C_OUT3
470µF
×2
C_OUT4
100µF
×2
4.7µF
D1
CMDSH-3
M1
BSC050NE2LS
C_B1 0.1µF
V_IN INTV_CC
PHASMD
CLKOUT
RUN1,2
PGOOD
ILIM
IFAST
ENTMPB MODE/PLLIN
TG1
TG2
LTC3875
BOOST1
BOOST2
10µF
×4
D2
CMDSH-3
M3
BSC050NE2LS
C_B2 0.1µF
M2
BSC010NE2LSI
R_TRSET1 10k
220nF
220nF
R_TAVG
5k
SW1
EXTV_CC
BG1
TAVG
TRSET1
SNSA1⁺
SW2
BG2
PGND
TRSET2
SNSA2⁺
SNS1^–
SNS2^–
SNSD1⁺
SNSD2⁺
TCOMP1
VV_{OOSSNNSS11+–}
I_TH1
TCOMP2
FREQ
VV_{OOSSNNSS22+–}
TK/SS1 TK/SS2 ^ITH2
0.1µF
M4
BSC010NE2LSI
R_TRSET2 10k
220nF
220nF
1.5nF
100k 10k
V_IN
270µF ^{4.5V TO 20V}
50V
THERMAL
SENSOR
L2
0.33µH
(0.32mΩ DCR)
R1
R2
4.64k 931Ω
R_B 30.1k
R_A
20k
C_{OUT1 +}
100µF
×2
V_OUT
1.5V
60A
C_OUT2
470µF
×2
3875 F16
Figure 16. High Efficiency Dual Phase 400kHz, 1.5V/60A Step-Down Converter with Optional Thermal Balancing
The frequency is set by biasing the FREQ pin to 1V (see
Figure 12).
The inductance values are based on a 35% maximum
ripple current assumption (10.5A for each channel). The
highest value of ripple current occurs at the maximum
input voltage:
L
=
f
•
V_OUT
∆I_{L(MAX )}



1–
V_OUT
V_{IN(MAX )}



This design will require 0.33µH. The Würth 744301033,
0.32µH inductor is chosen. At the nominal input voltage
(12V), the ripple current will be:
∆I_L(NOM)
=
V_OUT
f •L

 ^1–
V_OUT
V_IN(NOM)


It will have 10A (33%) ripple. The peak inductor current
will be the maximum DC value plus one-half the ripple
current, or 35A.
The minimum on-time occurs at the maximum V_IN, and
should not be less than 90ns:
t_ON(MIN)
=
V_OUT
V_{IN(MAX )}
(f)
=
20V
1.5V
(400kHz)
=
187ns
DCR sensing is used in this circuit. If C1 and C2 are chosen
to be 220nF, based on the chosen 0.33µH inductor with
0.32mΩ DCR, R1 and R2 can be calculated as:
R1=
L
DCR •
C1
=
4.69k
R2
=
DCR
L
• C2
•
5
=
937Ω
3875fa
34
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