LTC3853_15 Datasheet, PDF (29/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
APPLICATIONS INFORMATION
M1
3.3µH
4.75k
V_OUT1
5V
5A
+
1430Ω
105k
1%
10pF
C_P1,2,3
330µF 20k
10V
1%
4.7µF
V_IN DRV_CC12 INTV_CC
TG1
TG2
PGOOD12
PGOOD3
BOOST1,2,3
SW1
SW2
BG1 ^LTC3853 BG2
0.1µF
R_ITH1,2,3
C_ITH1,2,3
MODE/PLLIN
PGND
ILIM
FREQ/PLLFLTR
SENSE1⁺
SENSE2⁺
SENSE2^–
SENSE1^–
V_FB2
V_FB1
I_TH1,2,3
RUN1,2,3
V_FB3
TG3
SW3
SGND
BG3
SENSE3⁺
EXTV_CC
SENSE3^–
TK/SS1,2,3
C_SS1,2,3
0.1µF
D_B1,2,3
C_B1,2,3 0.1µF
SW1,2,3
10k
INTV_CC
3.16k
1nF
M2
3.3µH
4.75k
0.1µF
1430Ω
63.4k 10pF
1%
20k
1%
V_OUT2
3.3V
5A
330µF
6V
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
V_IN
7V TO 20V
22µF
50V
M3
1.5µH
2.43k
0.068µF
1.82k
10k
1%
V_OUT3
1.2V
5A
20k
+ _330µF
1%
6V
3853 F13
Figure 13. High Efficiency Triple 5V/3.3V/1.2V Step-Down Converter
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 )}
+
∆I_L(NOM)
2
=
65mV
1.2
•


5A
+
∆I_{L(MAX )}
2


≅
9mΩ
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
= ^3.3µH
(DCR_MAX at 20°C) • C1 30mΩ • 0.1µF
=
1100 Ω
and
1.5µH
15mΩ • 0.1µF
=
1000 Ω
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.
For more information www.linear.com/LTC3853
3853fc
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