ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 14

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3853
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. R
output current.
APPLICATIONS INFORMATION
*PLACE C1 NEAR SENSE
14
SENSE
LTC3853
–
LTC3853
SGND
SGND
PINS
Figure 2. Two Different Methods of Sensing Current
(2b) Using the Inductor DCR to Sense Current
SENSE
SENSE
SENSE
SENSE
INTV
INTV
BOOST
BOOST
PGND
PGND
(2a) Using a Resistor to Sense Current
SW
SW
V
V
BG
TG
BG
TG
CC
PLACED NEAR SENSE PINS
CC
IN
IN
+
–
+
–
FILTER COMPONENTS
+
SENSE
,
R1 || R2 • C1 =
C1*
C
F
is chosen based on the required
R
R
F
F
R2
R1
DCR
L
L1
R
SENSE(EQ)
V
INDUCTOR
IN
L
R
S
V
= DCR
R
IN
SENSE
DCR
ESL
R1 + R2
R2
3853 F02a
V
OUT
3853 F02b
The current comparator has a maximum threshold
V
comparator threshold sets the peak of the inductor cur-
rent, yielding a maximum average output current I
equal to the peak value less half the peak-to-peak ripple
current, ΔI
the equation:
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ΔV
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 ΔV
as a conservative number to start with, either for R
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 fi lter 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 fi lter consists of
two series 10Ω resistors connected to a parallel 1000pF
capacitor, resulting in a time constant of 20ns.
This same RC fi lter, with minor modifi cations, 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
SENSE(MAX)
R
SENSE
L
=
. To calculate the sense resistor value, use
determined by the I
I
V
(
MAX
SENSE MAX
)
+
(
Δ
2
SENSE
I
L
)
voltage is recommended
LIM
SENSE
setting. The current
= ΔI
L
• R
SENSE
SENSE
MAX
3853f