LTC3869-2 datasheet(21/54 Pages) LINER

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LTC3838

3838fa

APPLICATIONS INFORMATION

ESL

RSENSE RESISTOR

AND

PARASITIC INDUCTANCE

CF • 2RF ≤ ESL/RS

POLE-ZERO

CANCELLATION

FILTER COMPONENTS

PLACED NEAR SENSE PINS

SENSE+

LTC3838

SENSE–

3838 F03a

VOUT

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

RSENSE

3838 F03b

Figure 3a. RSENSE Current Sensing

Figure 3b. Sense Lines Placement with Sense Resistor

RSENSE Inductor Current Sensing

The LTC3838 can be configured to sense the inductor

currents through either low value series current sensing

resistors (RSENSE) or inductor DC resistance (DCR). The

choice between the two current sensing schemes is largely

a design trade-off between cost, power consumption and

accuracy. DCR sensing is becoming popular because it

saves expensive current sensing resistors and is more

power efficient, especially in high current applications.

However, current sensing resistors provide the most ac-

curate current limits for the controller.

A typical RSENSEinductorcurrentsensingschemeisshown

in Figure 3a. The filter components (RF, CF) need to be

placed close to the IC. The positive and negative sense

traces need to be routed as a differential pair close to-

gether and Kelvin (4-wire) connected underneath the sense

resistor, as shown in Figure 3b. Sensing current elsewhere

can effectively add parasitic inductance to the current sense

element, degrading the information at the sense terminals

and making the programmed current limit unpredictable.

RSENSE is chosen based on the required maximum output

current. Given the maximum current, IOUT(MAX), maximum

sense voltage, VSENSE(MAX), set by VRNG, and maximum

inductor ripple current

∆IL(MAX), the value of RSENSE can

be chosen as:

SENSE =

SENSE(MAX)

OUT(MAX) –

ΔI

L(MAX)

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

thus VRNG voltage can be determined from the above equa-

tion. To ensure the maximum output current, sufficient

margin should be built in the calculations to account for

variations of LTC3838 under different operating conditions

and tolerances of external components.

Because of possible PCB noise in the current sensing

loop, the current sensing voltage ripple

∆VSENSE = ∆IL •

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, 10mV of

∆VSENSE is recommended as

a conservative number to start with, either for RSENSE or

Inductor DCR sensing applications.

For today’s highest current density solutions the value

of the sense resistor can be less than 1mΩ and the

peak sense voltage can be as low as 20mV. In addition,

inductor ripple currents greater than 50% with operation

up to 2MHz 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.

This same RC filter, with minor modifications, can be

used to extract the resistive component of the current

sense signal in the presence of parasitic inductance.

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