LTC7803 datasheet(20/42 Pages) AD

20 / 42 page

LTC7811

Rev. 0

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APPLICATIONS INFORMATION

applications. For a buck converter, this error is propor-

tional to the input voltage and may degrade line regulation

or cause loop instability. An RC filter into the sense pins,

as shown in Figure 2a, can be used to compensate for this

error. Set the RC filter time constant RF • CF = ESL/RSENSE

for optimal cancellation of the ESL. In general, select CF

to be in the range of 1nF to 10nF and calculate the cor-

responding RF. Surface mount sense resistors in low ESL

wide footprint geometries are recommended to minimize

this error. If not specified on the manufacturer's data

sheet, the ESL can be approximated as 0.4nH for a resis-

tor with a 1206 footprint and 0.2nH for a 1225 footprint.

Inductor DCR Current Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC7811 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 2b. 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 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 section,

the target sense resistor value is:

RSENSE(EQUIV) ≤

VSENSE(MAX)

IL(MAX)+

ΔIL

(b) Using the Inductor DCR to Sense Current

(a) Using a Resistor to Sense Current

Figure 2. Current Sensing Methods

7811 F02a

LTC7811

INTVCC

BOOST

SENSE1,2+

(SENSE3–)

SENSE1, 2–

(SENSE3+)

VOUT1,2

(VIN3)

VIN1,2

(VOUT3)

ESL

PLACE RF AND CF NEAR SENSE PINS

RSENSE

SENSE RESISTOR

WITH PARASITIC

INDUCTANCE

RF *CF = ESL/RSENSE

POLE-ZERO

CANCELLATION

7811 F02b

LTC7811

INTVCC

BOOST

SENSE1, 2+

(SENSE3–)

SENSE1, 2–

(SENSE3+)

GND

VIN1,2

(VOUT3)

VOUT1,2

(VIN3)

C1*

*PLACE C1 NEAR SENSE PINS

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

DCR

INDUCTOR

(R1||R2) • C1 = L/DCR

RSENSE

LTC7811

SENSE3+

SENSE3-

VIN

PLACE RF AND CF NEAR SENSE PINS

VOUT3

7811 F02c

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