ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 30

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3853
For channel 1, the DCR
The power loss in R1 at the maximum input voltage is:
The respective values for Channel 2 are R1 = 4.75k, R2 =
1430Ω; and P
R1 = 2.21k, R2 = 1.82k; and P
Burst Mode operation is chosen for high light load effi ciency
(Figure 14) 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 (9mΩ). At heavier loads, DCR sensing provides
higher effi ciency.
30
R
R
P
LOSS
1
2
=
=
R
R
Figure 14. Design Example Effi ciency vs Load
1
R
1
1
R
−
1
100
||
60
50
40
•
90
80
70
D
=
=
R
0.01
R
R
D
(
2
LOSS
(
D
20
V
=
IN MAX
=
V
1100
(
R1 = 11.6mW. And for Channel 3 are
4
4
−
0 23
.
. .
7 7 5
.
75
1 0 23
LOAD CURRENT (mA)
5
0.1
SENSE
−
V
)
k • .
k
Ω
−
) •
R
.
V
≅
5
0 23
1
OUT
V
fi lter/divider values are:
4 75
LOSS
.
=
) •
15 8
1
≅
EFFICIENCY
POWER LOSS
9mΩ
k
R1 = 10.2mW.
V
1430
;
.
OUT
mW
DCR
DCR
3853 F14
Ω
10
10
1
0.1
0.01
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Siliconix Si4816BDY dual MOSFET
results in: R
At maximum input voltage with T(estimated) = 50°C:
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
which is less than under full-load conditions.
C
temperature assuming only one channel is on. C
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:
IN
V
P
I
P
(
⎡ ⎡
⎢
⎣
SC
is chosen for an RMS current rating of at least 2A at
0 023
MAIN
ORIPPLE
SYNC
5 2 3
.
– .
=
1
(
=
1 3 75
=
=
Ω Ω
0 009
/
20
84
= R
+
20
DS(ON)
)
.
5
+
V
)
2 3
mW
V
20
V
ESR
(
1
.
20
( )
–
Ω
mV
5
V
⎤
⎥
⎦
5
= 0.023Ω/0.016Ω, C
(
(ΔI
V
2
500
V
)
[
–
2
(
1 0 005 50
L
2 5
) = 0.02Ω(1.5A) = 30mV
⎛
⎝ ⎜
+
2
1
kHz
.
DS(ON)
5
( .
⎛
⎝ ⎜
2
A
A
90
⎞
⎠ ⎟
) (
)
2
3 3
( )(
=
ns
2
.
1 125 0 016
Ω
243
and δ = (0.005/°C)(25)
(
)(
μ
.
20
H
100
mW
V
°
)
)
C
⎞
⎠ ⎟
(
MILLER
pF •
–
=
.
25
2 2 5 . A
)
°
C
Ω
≅ 100pF .
) •
P-P
]
)
OUT
3853f
is