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LTC3865EUH#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36P37-P38
LTC3865/LTC3865-1
31
3865fb
APPLICATIONS INFORMATION
D3
D4
M1
0.1µF
L1
3.3µH
3.65k
1%
1800pF
100pF
0.1µF
0.1µF
22µF
50V
C
OUT1
100µF
s2
L1, L2: COILTRONICS HCP0703
M1, M2: VISHAY SILICONIX Si4816BDY
C
OUT1
, C
OUT2
: TAIYO YUDEN JMK325BJ107MM
4.75k
1%
V
OUT1
3.3V
5A
M2
0.1µF
L2
2.2µH
1.58k
1%
2200pF
100pF
C
OUT2
100µF
s2
5.49k
1%
162k
1%
V
OUT2
1.8V
5A
TG1 TG2
BOOST1 BOOST2
SW1 SW2
BG1
BG2
SGND
PGND
FREQ
SENSE1
+
SENSE2
+
VID21
SENSE1
–
SENSE2
–
V
OSENSE1
V
OSENSE2
I
TH1
I
TH2
V
IN
PGOOD INTV
CC
TK/SS1 TK/SS2
V
IN
7V TO 20V
3865 F14
EXTV
CC
0.1µF
0.1µF
LTC3865
MODE/PLLIN
I
LIM
RUN2RUN1
VID22
VID12
VID11
5.49k
1%
1.37k
1%
4.7µF
1µF
2.2Ω
+
Figure 14. High Effi ciency Dual 500kHz 3.3V/1.8V Step-Down Converter
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 fl oating 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
V
I
V
V
OUT
LMAX
OUT
IN MAX
=−
⎛
⎝
⎜
⎞
⎠
⎟
f •
() ()
Δ
1
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
V
L
V
V
L NOM
OUT OUT
IN NOM
()
()
•
=−
⎛
⎝
⎜
⎞
⎠
⎟
f
1
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
V
V
V
VkHz
n
ON MIN
OUT
IN MAX
()
()
.
()
== =
f
1 8
20 500
180ss
LTC3865/LTC3865-1
32
3865fb
APPLICATIONS INFORMATION
With I
LIM
fl oating, the equivalent R
SENSE
resistor value
can be calculated by using the minimum value for the
maximum current sense threshold (44mV).
R
V
I
I
SENSE EQUIV
SENSE MIN
LOAD MAX
L NOM
()
()
()
(
=
+
Δ
))
.
.
2
44
5
15
2
77=
+
≅
mV
A
A
mΩ
The equivalent R
SENSE
is the same for Channel 2.
The Coiltronics (Cooper) HCP0703-2R2 (20m DCR
MAX
at 20°C) and HCP0703-3R3 (30mΩ DCR
MAX
at 20°C) are
chosen. At 100°C, the estimated maximum DCR values are
26.4mΩ and 39.6mΩ. The divider ratios are:
R
R
DCR at T
m
m
D
SENSE EQUIV
MAX L MAX
==
()
()
.
.
77
26 4
Ω
Ω
==
≅
03
77
39 6
02
.;
.
.
.and
m
m
Ω
Ω
For each channel, 0.1µF is selected for C1.
RR
L
DCR at C C
H
mF
MAX
12
20 1
22
20 0 1
1
||
()•
.
•.
=
°
=
=
μ
μΩ
.. ;
.
•.
.1
33
30 0 1
11kand
H
mF
k
μ
μΩ
=
For channel 1, the DCR
SENSE
fi lter/divider values are:
R
RR
R
k
k
R
RR
R
k
D
D
D
1
12
11
02
55
2
1
1
55
==≅
=
−
=
||
.
.
.;
•.•
002
102
137
.
.
.
−
≅ k
The power loss in R1 at the maximum input voltage is:
PR
VVV
R
VV
LOSS
IN MAX OUT OUT
1
1
20 3 3 3
=
−
=
−
()•
(.)•
()
..
.
3
55
10
V
k
mW=
The respective values for Channel 2 are R1 = 3.66k,
R2 = 1.57k; and P
LOSS
R1 = 8mW.
Burst Mode operation is chosen for high light load effi ciency
(Figure 15) by fl oating the MODE/PLLIN pin. Power loss
due to the DCR sensing network is slightly higher at light
loads than would have been the case with a suitable sense
resistor (8mΩ). At heavier loads, DCR sensing provides
higher effi ciency.
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Siliconix Si4816BDY dual MOSFET
results in: R
DS(ON)
= 0.023Ω/0.016Ω, C
MILLER
≅ 100pF.
At maximum input voltage with T(estimated) = 50°C:
P
V
V
CC
MAIN
=
()
+°°
[]
33
20
5 1 0 005 50 25
00
2
.
(. )( – )•
.
223 20
5
2
2 100
1
523
1
2
2
Ω
()
+
()
⎛
⎝
⎜
⎞
⎠
⎟
Ω
()( )
+
V
A
pF •
–. .. 3
500 186
⎡
⎣
⎢
⎤
⎦
⎥
()
=kHz mW
LOAD CURRENT (mA)
0.01
70
EFFICIENCY (%)
POWER LOSS (mW)
80
90
0.1 1 10
60
50
40
100
0.1
1
0.01
10
3865 F16
DCR
8m
POWER LOSS
EFFICIENCY
Figure 15. Design Example Effi ciency vs Load
LTC3865/LTC3865-1
33
3865fb
APPLICATIONS INFORMATION
A short-circuit to ground will result in a folded back cur-
rent of:
I
mV
ns V
H
SC
=
()
Ωμ
⎛
⎝
⎜
⎞
⎠
⎟
=
1350
0008
1
2
90 20
33
/
.
–
()
.
11 8.A
with a typical value of R
DS(ON)
and δ = (0.005/°C)(20)
= 0.1. The resulting power dissipated in the bottom
MOSFET is:
P
VV
V
A
m
SYNC
=
()( )
Ω
()
=
20 3 3
20
1 8 1 125 0 016
48
2
–.
...
WW
which is less than under full-load conditions.
C
IN
is chosen for an RMS current rating of at least 2A at
temperature assuming only channel 1 or 2 is on. C
OUT
is
chosen with an ESR of 0.02Ω for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
V
ORIPPLE
= R
ESR
(ΔI
L
) = 0.02Ω(1.5A) = 30mV
P-P
0.1µF
L2
0.47µH
Q1
RJK0305DPB
Q3
RJK0305DPB
Q4
RJK0330DPB
D2
Q2
RJK0330DPB
1000pF
1000pF 1000pF
22µF
50V
10k
V
OUT1
1.5V
15A
0.1µF
L2
0.47µH
1000pF
C
OUT4
100µF
15k
162k
1%
V
OUT2
1.2V
15A
TG1 TG2
BOOST1 BOOST2
SW1 SW2
BG1
BG2
SGND
PGND
FREQ
SENSE1
+
SENSE2
+
RUN2
SENSE1
–
SENSE2
–
V
OSENSE1
V
OSENSE2
I
TH1
I
TH2
V
IN
PGOOD INTV
CC
TK/SS1 TK/SS2
V
IN
4.5V TO 24V
3865 F16
0.1µF
100pF
0.1µF
LTC3865
10µF
35V
10µF
35V
MODE/PLLIN
I
LIM
RUN1
VID11
VID12
VID21
VID22
EXTV
CC
4.7µF
C
OUT1
100µF
C
OUT2,3
100pF
100Ω
100Ω
100Ω
100Ω
2.2Ω
1µF
D4D3
D1, D2: VISHAY B340A
L1, L2: VISHAY IHLP4040DZERR47M11
C
OUT1
, C
OUT4
: TDK C322JX5R0J107MT
C
OUT2
, C
OUT3
, C
OUT5
, C
OUT6
: SANYO 4TPE 220µF
2mΩ
2mΩ
+
C
OUT5,6
+
+
D1
Figure 16. 1.5V/15A, 1.2V/15A Converter Using Sense Resistors
TYPICAL APPLICATIONS
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LTC3865EUH#TRPBF

Mfr. #:
Buy LTC3865EUH#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs
Lifecycle:
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