LTC3853_15 datasheet(14/36 Pages) LINER

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LTC3853

3853fc

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APPLICATIONS INFORMATION

Figure 2. Two Different Methods of Sensing Current

(2a) Using a Resistor to Sense Current

(2b) Using the Inductor DCR to Sense Current

VIN

INTVCC

BOOST

PGND

FILTER COMPONENTS

PLACED NEAR SENSE PINS

SENSE+

SENSE–

SGND

LTC3853

ESL

3853 F02a

RSENSE

VIN

INTVCC

BOOST

PGND

*PLACE C1 NEAR SENSE+,

SENSE– PINS

INDUCTOR

DCR

SENSE+

SENSE–

SGND

LTC3853

VOUT

3853 F02b

C1*

R1||R2 • C1 =

DCR

RSENSE(EQ) = DCR

R1 + R2

the information at the sense terminals and making the

programmed current limit unpredictable. If DCR sensing

is used (Figure 2b), sense resistor R1 should be placed

closetotheswitchingnode,topreventnoisefromcoupling

into sensitive small-signal nodes. The capacitor C1 should

be placed close to the IC pins.

Low Value Resistors Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 2a. RSENSE is chosen based on the required

output current.

The current comparator has a maximum threshold

VSENSE(MAX) determined by the ILIM setting. The current

comparatorthresholdsetsthepeakoftheinductorcurrent,

yielding a maximum average output current IMAX equal to

the peak value less half the peak-to-peak ripple current,

DIL.Tocalculatethesenseresistorvalue,usetheequation:

RSENSE =

VSENSE(MAX)

I(MAX) +

∆IL

Because of possible PCB noise in the current sensing loop,

the AC current sensing ripple of

DVSENSE = DIL • 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 15mV

DVSENSE voltage is recommended

as a conservative number to start with, either for RSENSE

or DCR sensing applications.

For previous generation current mode controllers, the

maximum sense voltage was high enough (e.g., 75mV for

theLTC1628/LTC3728family)thatthevoltagedropacross

the parasitic inductance of the sense resistor represented

a relatively small error. For today’s highest current density

solutions, however, 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 1MHz are becoming more com-

mon. Under these conditions the voltage drop across the

senseresistor’sparasiticinductanceisnolongernegligible.

A typical sensing circuit using a discrete resistor is shown

inFigure2a.Inpreviousgenerationsofcontrollers,asmall

RC filter placed near the IC was commonly used to reduce

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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