LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 40

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3838
APPLICATIONS INFORMATION
Set the inductor value to give 40% ripple current at maxi-
mum V
Select 0.56μH which is the nearest standard value.
The resulting maximum ripple current is:
Often in a high power application, DCR current sensing is
preferred over R
order to determine the DCR filter values, first the inductor
manufacturer has to be chosen. For this design, the Vishay
IHLP-4040DZ-01 model is chosen with a value of 0.56μH
and a DCR
The maximum sense voltage, V
range that LTC3838 can handle without any additional
scaling. Therefore, the DCR filter can use a simple RC
filter across the inductor. If the C is chosen to be 0.1μF ,
then the R can be calculated as:
The resulting V
This voltage can be generated with a resistive divider from
the INTV
the maximum load current of 15A can be supplied under
all conditions, such as lower INTV
and account for the range of LTC3838’s own V
variation within specification, a higher V
used to provide margin.
A better and the recommended way to set V
tie the V
40
V
L =
R
ΔI
V
SENSE(MAX)
RNG
DCR
L
⎛
⎝ ⎜
=
IN
RNG
350kHz • 40% • 15A
CC
=
=
⎛
⎝ ⎜
using the adjusted operating frequency:
350kHz • 0.56µH
MAX
DCR • C
pin to signal ground (SGND). To make sure that
V
pin to SGND for an equivalent of V
SENSE(MAX)
0.05
=1.8mΩ. This implies that:
RNG
= 1.8mΩ • [1 + (100°C – 25°C) • 0.4%/°C]
1.2V
L
SENSE
1.2V
• (15A – 5.8A/2) = 28mV
DCR
pin voltage is:
in order to maximize efficiency. In
=
= 28mV • 20 = 0.56V
1.8mΩ • 0.1µF
⎞
⎠ ⎟
⎞
⎠ ⎟
0.56µH
⎛
⎝ ⎜
⎛
⎝ ⎜
1–
1–
SENSE(MAX)
1.2V
24V
CC
1.2V
24V
due to a lower V
⎞
⎠ ⎟
⎞
⎠ ⎟
= 3.1kΩ
= 5.8A
RNG
RNG
= 0.54µH
, is within the
is to simply
SENSE(MAX)
RNG
should be
= 0.6V,
IN
,
while using an additional resistor in the DCR filter, as
discussed in DCR Inductor Current Sensing, to scale the
V
lower limit of the LTC3838’s own V
so that the maximum output current can be guaranteed.
In this design example, a 3.57k and 15k resistor divider is
used. The previously calculated V
from 28mV to 22.6mV, which is close to the lower limit of
LTC3838’s V
the equivalent R
than the 3.1k calculated above for a matched R
and L-DCR network. The resulted mismatch allows for a
slightly higher ripple in V
Remember to check the peak inductor current, considering
the upper spec limit of V
lowest operating temperature, is not going to saturate the
inductor or exceed the rating of power MOSFETs.
For the external N-channel MOSFETs, Renesas
RJK0305DBP (R
= 4.5V, θ
MOSFET (main switch). RJK0330DBP (R
max, V
the bottom MOSFET (synchronous switch). The power dis-
sipation for each MOSFET can be calculated for V
typical T
The resulted junction temperatures at an ambient tem-
perature T
P
P
BOT
TOP
SENSE(MAX)
T
T
J(TOP)
J(BOT)
= 1.2W
= 0.54W
=
=
+ 24V
GS
⎛
⎝ ⎜
⎛
⎝ ⎜
24V – 1.2V
1.2V
(
JA
24V
J
= 4.5V, θ
= 125°C:
A
= 75°C + (0.54W)(40°C/W) = 97°C
= 75°C + (1.2W)(40°C/W) = 123°C
= 40°C/W, T
24V
= 75°C are:
SENSE(MAX)
⎞
⎠ ⎟
)
down by a comfortable margin below the
2
(
15A
⎛
⎝ ⎜
15A
DS(ON)
DCR
JA
2
⎞
⎠ ⎟
)
2
= 40°C/W, T
(
15A
⎞
⎠ ⎟
(
13mΩ
(
= 3.57k//15k = 2.9k, slightly lower
J(MAX)
150pF
= 13mΩ max, C
)
specification at V
2
SENSE(MAX)
SENSE
(
3.9mΩ
)
⎡ ⎣
)
= 150°C) is chosen for the top
1+ 0.4% 125°C – 25°C
⎡
⎢
⎣
5.3V – 3V
J(MAX)
.
SENSE(MAX)
2.5Ω
SENSE(MAX)
)
⎡ ⎣
1+ 0.4% 125°C – 25°C
= 150°C) is chosen for
(
and the DCR
MILLER
+
1.2Ω
RNG
DS(ON)
3V
(
is scaled down
specification,
= 150pF , V
= 0.6V. Note
IN
⎤
⎥ 350kHz
⎦
(
DCR
= 24V and
= 3.9mΩ
)
(MIN)
⎤ ⎦
-C
3838fa
DCR
)
GS
)
at
⎤ ⎦