LTC3838-2_15 Datasheet, PDF (25/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
DCR Inductor Current Sensing
For applications requiring higher efficiency at high load
currents, the LTC3838-2 is capable of sensing the volt-
age drop across the inductor DCR, as shown in Figure 5.
The DCR of the inductor represents the small amount of
DC winding resistance, which can be less than 1mΩ for
today’s low value, high current inductors.
In a high current application requiring such an inductor,
conduction loss through a sense resistor would cost several
points of efficiency compared to DCR sensing.
The inductor DCR is sensed by connecting an RC filter
across the inductor. This filter typically consists of one or
two resistors (R1 and R2) and one capacitor (C1) as shown
in Figure 5. If the external (R1||R2) • C1 time constant is
chosen to be exactly equal to the L/DCR time constant, the
voltage drop across the external capacitor is equal to the
voltage drop across the inductor DCR multiplied by R2/
(R1 + R2). Therefore, R2 may be used to scale the voltage
across the sense terminals when the DCR is greater than
the target sense resistance. C1 is usually selected in the
range of 0.01µF to 0.47µF. This forces R1||R2 to around
2k to 4k, reducing error that might have been caused by
the SENSE pins’ input bias currents.
LTC3838-2
SENSE⁺
SENSE^–
INDUCTOR
L DCR
L/DCR = (R1||R2) C1
R1
V_OUT
C_OUT
C1
R2
(OPT)
38382 F05
C1 NEAR SENSE PINS
Figure 5. DCR Current Sensing
Resistor R1 should be placed close to the switching node,
to prevent noise from coupling into sensitive small-signal
nodes. Capacitor C1 should be placed close to the IC pins.
The first step in designing DCR current sensing is to
determine the DCR of the inductor. Where provided, use
the manufacturer’s maximum value, usually given at 25°C.
Increase this value to account for the temperature coef-
ficient of resistance, which is approximately 0.4%/°C. A
conservative value for inductor temperature T_L is 100°C.
The DCR of the inductor can also be measured using a good
RLC meter, but the DCR tolerance is not always the same
and varies with temperature; consult the manufacturers’
data sheets for detailed information.
From the DCR value, V_SENSE(MAX) is easily calculated as:
V_SENSE(MAX) = DCR_MAX(25°C)
( ) ^{•1+ 0.4% T}L(MAX) ^{– 25°C }
• ^{}I_{OUT(MAX )}
–
∆I_L
2


If V_SENSE(MAX) is within the maximum sense voltage
(30mV typical) of the LTC3838-2, then the RC filter only
needs R1. If V_SENSE(MAX) is higher, then R2 may be used
to scale down the maximum sense voltage so that it falls
within range.
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
( ) _{PLOSS (R1) =}
V_IN(MAX) – V_OUT
R1
• V_OUT
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
R_SENSE sensing. Light load power loss can be modestly
higher with a DCR network than with a sense resistor due
to the extra switching losses incurred through R1. How-
ever, DCR sensing eliminates a sense resistor, reduces
conduction losses and provides higher efficiency at heavy
loads. Peak efficiency is about the same with either method.
For more information www.linear.com/3838-2
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