LTC3853 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
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
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
3853 F13
Figure 13. High Efficiency Triple 5V/3.3V/1.2V Step-Down Converter
standard values are 3.3μH, 3.3μH and 1.5μH. At the
maximum input voltage (20V), the ripple will be:
ΔI_L(MAX)
=
V_OUT
f •L
⎛
⎜^1−
⎝
V_OUT
V_IN(MAX)
⎞
⎟
⎠
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)
+
ΔI_L(NOM)
2
=
65mV
≅ 9mΩ
1.2
•
⎛
⎝⎜
5A
+
ΔI_L(MAX)
2
⎞
⎠⎟
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.
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 ≅ 0.45
19.8mΩ
For each channel, 0.1μF is selected for C1.
R1|| R2 =
L
= ^3.3μH
(DCR_MAX at 20°C) • C1 30mΩ • 0.1μF
= 1100Ω and ^1.5μH = 1000Ω
15mΩ • 0.1μF
3853f
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