Datasheet

LTC3876
37
3876f
The minimum on-time for channel 1 VDDQ is:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
•f
=
1.5V
14V•400kHz
= 268ns
The minimum on-time for channel 2 VTT is:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
•f
=
0.75V
14V•400kHz
= 134ns
Set the channel 1 VDDQ inductor value L1 to give 35%
ripple current at the maximum load to 20A for the maxi-
mum V
IN
of 14V using the adjusted operating frequency.
L1=
V
OUT
f
(
)
I
RIPPLE
(
)
1−
V
OUT
V
IN(MAX)
=
1.5V
400kHz
(
)
35% t 20A
(
)
1−
1.5
14
⎛
⎝
⎜
⎛
⎝
⎜
⎞
⎠
⎟
⎞
⎠
⎟
= 0.47µH
Set the channel 2 VTT inductor value L2 to give 35% ripple
current at the maximum load to 10A for the maximum V
IN
of 14V using the adjusted operating frequency.
L1=
V
OUT
f
(
)
I
RIPPLE
(
)
1−
V
OUT
V
IN(MAX)
=
0.75V
400kHz
(
)
37.5% t 10A
(
)
1−
0.75
14
= 0.47µH
⎛
⎝
⎜
⎛
⎝
⎜
⎞
⎠
⎟
⎞
⎠
⎟
Choose the nearest standard value of 0.47µH for L2, which
will result in 37.5% ripple current.
The resulting channel 1 VDDQ maximum ripple current is:
∆I
L1
=
1.5V
400kHz
(
)
0.47µH
(
)
1−
1.5V
14V
= 7.12A
⎛
⎝
⎜
⎞
⎠
⎟
The resulting channel 2 VTT maximum ripple current is:
∆I
L2
=
0.75V
400kHz
(
)
0.47µH
(
)
1−
0.75V
14V
= 3.78A
⎛
⎝
⎜
⎞
⎠
⎟
For Figure 11, standard demo board the current limit is set
using sense resistors. The V
RNG
is grounded which results
in a maximum V
SENSE
voltage across the R
SENSE
resistor
of 30mV. If we assume 50% over the nominal output of
20A this gives a starting point of 1m for channel 1 VDDQ
and 2m for channel 2 VTT.
Channel 1 VDDQ current limit for 1m R
SENSE
.
ILIMIT
VDDQ
=
V
SENSE
R
SENSE
+
∆I
L
2
=
30mV
1mΩ
+
7.12A
2
= 33.5A
Channel 2 VTT current limit for 2m R
SENSE
.
ILIMIT
VTT
=
V
SENSE
R
SENSE
+
∆I
L
2
=
30mV
2mΩ
+
3.78A
2
= 16.9A
In high power applications, DCR current sensing is often
preferred to R
SENSE
in order to maximize efficiency. The
inductor model is selected based on its inductor and DCR
value. The Würth WE7443330047 with a rated current of
20A, a saturation current of 47A and DCR of 0.8m is
chosen for channel 1 VDDQ. The Würth WE7443340047
with a rated current of 19A, a saturation current of 32A
and DCR of 1.72m is chosen for channel 2 VTT. The DCR
demo board design is Figure 13.
In this design example V
RNG
was grounded to produce an
internal default value of 30mV on V
SENSE
.
Channel 1 VDDQ DCR Current limit:
ILIMIT
VDDQ
=
V
SENSE
R
SENSE
+
∆I
L
2
=
30mV
0.8mΩ •(1+(100
°
C–25
°
C)•0.4% /
°
C)
+
7.12A
2
=32.4A
Channel 2 VTT DCR Current Limit:
ILIMIT
VTT
=
V
SENSE
R
SENSE
+
∆IL
2
=
30mV
1.72mΩ •(1+(100
°
C–25
°
C)•0.4% /
°
C)
+
3.78A
2
=15.3A
APPLICATIONS INFORMATION