Datasheet

LTC3839
37
3839fa
APPLICATIONS INFORMATION
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 LTC3839 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 (5.3V) or RT pin (1.2V) to signal ground
(SGND). To make sure that the maximum load current can
be supplied under all conditions, such as lower INTV
CC
due to a lower V
IN
, and account for the range of IC’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 sim-
ply 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 IC’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 less than half of the lower
limit of LTC3839’s statistical 2-channel-sum V
SENSE(MAX)
at V
RNG
= 0.6V (see Current Limit Programming section
for details). 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 individual channel’s 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
These numbers show that careful attention should be paid
to proper heat sinking when operating at higher ambient
temperatures. For higher frequency, higher input voltage,
and/or higher load current applications, the LTC3839 IC’s
junction temperature should be estimated to make sure
the maximum junction temperature rating is not exceeded,
as discussed in the DRV
CC
Regulator and EXTV
CC
Power
section.