LTC3853 datasheet(14/36 Pages) LINER

14 / 36 page

LTC3853

3853f

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

close to the switching node, to prevent noise from coupling

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

comparator threshold sets the peak of the inductor cur-

rent, yielding a maximum average output current IMAX

equal to the peak value less half the peak-to-peak ripple

current,

ΔIL. To calculate the sense resistor value, use

the equation:

SENSE

SENSE MAX

MAX

()

Because of possible PCB noise in the current sensing loop,

the AC current sensing 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 15mV

ΔVSENSE 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

the LTC1628 / LTC3728 family) that the voltage drop across

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

common. Under these conditions the voltage drop across

the sense resistor’s parasitic inductance is no longer neg-

ligible. A typical sensing circuit using a discrete resistor is

shown in Figure 2a. In previous generations of controllers,

a small RC ﬁlter placed near the IC was commonly used to

reduce the effects of capacitive and inductive noise coupled

inthe 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. For example,

Figure 3 illustrates the voltage waveform across a 2mΩ

sense resistor with a 2010 footprint for the 1.2V/15A

converter operating at 100% load. The waveform is the

LTC3853 Datasheet(PDF) 14 Page - Linear Technology

LTC3853 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL