LTC7802 datasheet(17/34 Pages) AD

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LTC7802-3.3

Rev. 0

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inductors. In a high current application requiring such

an inductor, power loss through a sense resistor would

cost several points of efficiency compared to inductor

DCR sensing.

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 applications 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 temperature; consult

the manufacturers’ data sheets for detailed information.

Using the maximum inductor current (IL(MAX)) and ripple

current (ΔIL) from the Inductor Value Calculation sec-

tion, the target sense resistor value is calculated from the

nominal inductance, C1 value, and DCR:

RSENSE(EQUIV) ≤

VSENSE(MAX)

IL(MAX)+

� IL

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, 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 target equivalent resistance R1||R2 is calculated from

the nominal inductance, C1 value, and DCR:



R1 R2 =

(DCR at 20

°C) • C1

The sense resistor values are:



R1 R2

; R2

R1• RD

� RD

APPLICATIONS INFORMATION

Figure 2. Current Sensing Methods

(2a) Using a Resistor to Sense Current

(2b) Using the Inductor DCR to Sense Current

RSENSE ESL

LTC7802-3.3

BOOST

SENSE1,2+

SENSE1,2–

VOUT1,2

VIN1,2

PLACE RF AND CF NEAR SENSE PINS

SENSE RESISTOR

WITH PARASITIC

INDUCTANCE

RF*CF = ESL/RSENSE

POLE-ZERO

CANCELLATION

780233 F02a

780233 F02b

LTC7802-3.3

BOOST

SENSE1, 2+

SENSE1, 2–

GND

VIN1,2

VOUT1,2

C1*

*PLACE C1 NEAR SENSE PINS

RSENSE(EQ) = DCR(R2/(R1+R2))

DCR

INDUCTOR

(R1||R2) • C1 = L/DCR

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