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LTC3839

3839fa

Figure 5. DCR Current Sensing

APPLICATIONS INFORMATION

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 LTC3839 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 LTC3839 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.

The maximum power loss in R1 is related to duty cycle,

and will occur in continuous mode at the maximum input

voltage:

LOSS R1

()=

IN(MAX) –VOUT

()•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

RSENSE 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. However,

DCR sensing eliminates a sense resistor, reduces conduc-

tion losses and provides higher efficiency at heavy loads.

Peak efficiency is about the same with either method.

(OPT)

DCR

INDUCTOR

L/DCR = (R1||R2) C1

C1 NEAR SENSE PINS

SENSE+

LTC3839

SENSE–

3839 F05

VOUT

COUT

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