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LTC3838

3838fa

(OPT)

DCR

INDUCTOR

L/DCR = (R1||R2) C1

C1 NEAR SENSE PINS

SENSE+

LTC3838

SENSE–

3838 F05

VOUT

COUT

Figure 5. DCR Current Sensing

APPLICATIONS INFORMATION

< 10A, set RF to 10Ω and CF to 1000pF. This will provide

a good starting point.

The filter components need to be placed close to the IC.

The positive and negative sense traces need to be routed

as a differential pair and Kelvin (4-wire) connected to the

sense resistor.

DCR Inductor Current Sensing

For applications requiring higher efficiency at high load

currents, the LTC3838 is capable of sensing the voltage

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. With the ability to program

current limit through the VRNG pin, R2 may be optional. 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.

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 TL 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, VSENSE(MAX) is easily calculated as:

SENSE(MAX) = DCRMAX(25

°C)

•1

+ 0.4% T

L(MAX) –25°C

()

⎡

⎣

⎤

⎦

•I

OUT(MAX) –

ΔI

⎛

⎝⎜

⎞

⎠⎟

If VSENSE(MAX) iswithinthemaximumsensevoltage(30mV

to 100mV) of the LTC3838 as programmed by the VRNG

pin, then the RC filter only needs R1. If VSENSE(MAX) is

higher, then R2 may be used to scale down the maximum

sense voltage so that it falls within range.

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