LTC3613 datasheet(16/36 Pages) LINER

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LTC3613

3613fa

APPLICATIONS INFORMATION

RSENSE Inductor Current Sensing

A typical RSENSE inductor current sensing scheme is

shown in Figure 3. RSENSE is chosen based on the required

maximum output current. Given the maximum current,

IOUT(MAX),maximumsensevoltage,VSENSE(MAX),setbythe

VRNG pin, and maximum inductor ripple current, ΔIL(MAX),

the value of RSENSE can be chosen as:

RSENSE=

VSENSE(MAX)

IOUT(MAX)–

ΔI

L(MAX)

Conversely, given RSENSE and IOUT(MAX), VSENSE(MAX)

and thus the VRNG voltage could be determined from the

above equation. To assure that the maximum rated output

current can be supplied for different operating conditions

and component variations, sufficient design margin should

be built into these calculations.

Because of possible PCB noise in the current sensing loop,

the current ripple of ΔVSENSE = ΔIL • RSENSE also needs

to be checked in the design to get a good signal-to-noise

ratio. In general, for a reasonably good PCB layout, a

10mV ΔVSENSE voltage is recommended as a conservative

number to start with, either for RSENSE or DCR sensing

applications.

For today’s highest current density solutions the value of

the sense resistor can be less than 1mΩ and the maxi-

mum sense voltage can be as low as 30mV. In addition,

inductor ripple currents greater than 50% with operation

up to 1MHz are becoming more common. Under these

conditions, the voltage drop across the sense resistor’s

parasitic inductance becomes more relevant. A small RC

filter placed near the IC has been traditionally used to re-

duce the effects of capacitive and inductive noise coupled

in the sense traces on the PCB. A typical filter consists of

two series 10Ω resistors connected to a parallel 1000pF

capacitor, resulting in a time constant of 20ns.

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 LTC3613 is capable of sensing the voltage

drop across the inductor DCR, as shown in Figure 4. The

DCR of the inductor represents the small amount of DC

winding resistance, which can be less than 1mΩ for to-

day’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.

ESL

RSENSE RESISTOR

AND

PARASITIC INDUCTANCE

FILTER COMPONENTS

PLACED NEAR SENSE PINS

SENSE+

LTC3613

SENSE–

3613 F03

VOUT

Figure 3. RSENSE Current Sensing

(OPT)

DCR

INDUCTOR

L/DCR = (R1||R2) C1

C1 NEAR SENSE PINS

SENSE+

LTC3613

SENSE–

3613 F04

VOUT

COUT

Figure 4. DCR Current Sensing

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