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LTC7862

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

Inductor DCR Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC7862 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 3b. The DCR of the inductor represents the small

amount of DC winding resistance of the copper, which

can be less than 1mΩ for today’s low value, high current

inductors. In a high current application requiring such

an inductor, power loss through a sense resistor would

cost several points of efficiency and higher power loss

compared to inductor DCR sensing.

(3a) Using a Resistor to Sense Current

(3b) Using the Inductor DCR to Sense Current

Figure 3. Current Sensing Methods

7862 F03a

LTC7862

BOOST

GND

SENSE–

SENSE+

VIN

VOUT

RSENSE

CAP

PLACED NEAR SENSE PINS

7862 F03b

LTC7862

VIN

VOUT

C1*

*PLACE C1 NEAR SENSE PINS RSENSE(EQ) = DCR(R2/(R1+R2))

DCR

INDUCTOR

(R1||R2) • C1 = L/DCR

BOOST

GND

SENSE–

SENSE+

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 drop

across the inductor DCR multiplied by R2/(R1 + R2). R2

scales the voltage across the sense terminals for appli-

cations where the DCR is greater than the target sense

resistor value. To properly dimension the external filter

components, the DCR of the inductor must be known. It

can be measured using a good RLC meter, but the DCR

tolerance is not always the same and varies with tempera-

ture; consult the manufacturers’ data sheets for detailed

information.

Using the inductor ripple current value from the Inductor

Value Calculation section, the target sense resistor value is:

RSENSE(EQUIV) =

VSENSE(MAX)

IMAX +

ΔIL

To ensure that the application will deliver full load cur-

rent over the full operating temperature range, choose

the minimum value for VSENSE(MAX) in the Electrical

Characteristics table.

Next, determine the DCR of the inductor. When provided,

use the manufacturer’s maximum value, usually given at

20°C. Increase this value to account for the temperature

coefficient of copper resistance, which is approximately

0.4%/°C. A conservative value for TL(MAX) is 100°C.

To scale the maximum inductor DCR to the desired sense

resistor value (RD), use the divider ratio:

RD =

RSENSE(EQUIV)

DCRMAX at TL(MAX)

C1 is usually selected to be in the range of 0.1μF to

0.47μF. This forces R1|| R2 to around 2k, reducing error

that might have been caused by the SENSE+ pin’s ±1μA

current.

The equivalent resistance R1||R2 is scaled to the tempera-

ture inductance and maximum DCR:

R1|| R2 =

(DCR at 20°C) • C1

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