Datasheet

LTC3838
40
3838fa
APPLICATIONS INFORMATION
Set the inductor value to give 40% ripple current at maxi-
mum V
IN
using the adjusted operating frequency:
L =
1.2V
350kHz • 40% •15A
⎛
⎝
⎜
⎞
⎠
⎟
1–
1.2V
24V
⎛
⎝
⎜
⎞
⎠
⎟
= 0.54µH
Select 0.56µH which is the nearest standard value.
The resulting maximum ripple current is:
ΔI
L
=
1.2V
350kHz • 0.56µH
⎛
⎝
⎜
⎞
⎠
⎟
1–
1.2V
24V
⎛
⎝
⎜
⎞
⎠
⎟
= 5.8A
Often in a high power application, DCR current sensing is
preferred over R
SENSE
in order to maximize efficiency. In
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
MAX
=1.8m. This implies that:
V
SENSE(MAX)
= 1.8m • [1 + (100°C – 25°C) • 0.4%/°C]
• (15A – 5.8A/2) = 28mV
The maximum sense voltage, V
SENSE(MAX)
, is within the
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:
R
DCR
=
L
DCR • C
DCR
=
0.56µH
1.8mΩ• 0.1µF
= 3.1kΩ
The resulting V
RNG
pin voltage is:
V
RNG
=
V
SENSE(MAX)
0.05
= 28mV • 20 = 0.56V
This voltage can be generated with a resistive divider from
the INTV
CC
pin to signal ground (SGND). To make sure that
the maximum load current of 15A can be supplied under
all conditions, such as lower INTV
CC
due to a lower V
IN
,
and account for the range of LTC3838’s own V
SENSE(MAX)
variation within specification, a higher V
RNG
should be
used to provide margin.
A better and the recommended way to set V
RNG
is to simply
tie the V
RNG
pin to SGND for an equivalent of V
RNG
= 0.6V,
while using an additional resistor in the DCR filter, as
discussed in DCR Inductor Current Sensing, to scale the
V
SENSE(MAX)
down by a comfortable margin below the
lower limit of the LTC3838’s own V
SENSE(MAX)
specification,
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
SENSE(MAX)
is scaled down
from 28mV to 22.6mV, which is close to the lower limit of
LTC3838’s V
SENSE(MAX)
specification at V
RNG
= 0.6V. Note
the equivalent R
DCR
= 3.57k//15k = 2.9k, slightly lower
than the 3.1k calculated above for a matched R
DCR
-C
DCR
and L-DCR network. The resulted mismatch allows for a
slightly higher ripple in V
SENSE
.
Remember to check the peak inductor current, considering
the upper spec limit of V
SENSE(MAX)
and the DCR
(MIN)
at
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
DS(ON)
= 13m max, C
MILLER
= 150pF, V
GS
= 4.5V, θ
JA
= 40°C/W, T
J(MAX)
= 150°C) is chosen for the top
MOSFET (main switch). RJK0330DBP (R
DS(ON)
= 3.9m
max, V
GS
= 4.5V, θ
JA
= 40°C/W, T
J(MAX)
= 150°C) is chosen for
the bottom MOSFET (synchronous switch). The power dis-
sipation for each MOSFET can be calculated for V
IN
= 24V and
typical T
J
= 125°C:
P
TOP
=
1.2V
24V
⎛
⎝
⎜
⎞
⎠
⎟
15A
()
2
13mΩ
()
1+0.4% 125°C–25°C
()
⎡
⎣
⎤
⎦
+ 24V
()
2
15A
2
⎛
⎝
⎜
⎞
⎠
⎟
150pF
()
2.5Ω
5.3V – 3V
+
1.2Ω
3V
⎡
⎣
⎢
⎤
⎦
⎥
350kHz
()
= 0.54W
P
BOT
=
24V – 1.2V
24V
⎛
⎝
⎜
⎞
⎠
⎟
15A
()
2
3.9mΩ
()
1+0.4% 125°C–25°C
()
⎡
⎣
⎤
⎦
=1.2W
The resulted junction temperatures at an ambient tem-
perature T
A
= 75°C are:
T
J(TOP)
= 75°C + (0.54W)(40°C/W) = 97°C
T
J(BOT)
= 75°C + (1.2W)(40°C/W) = 123°C