LTC3839 Datasheet, PDF (37/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
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 =1.8mΩ. This implies that:
V_SENSE(MAX) = 1.8mΩ • [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) = 13mΩ 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) = 3.9mΩ
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.
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