Datasheet

LTC3853
14
3853fa
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
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE
+
SENSE
–
SGND
LTC3853
R
S
R
F
R
F
C
F
ESLL1
3853 F02a
R
SENSE
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
*PLACE C1 NEAR SENSE
+
,
SENSE
–
PINS
INDUCTOR
DCRL
SENSE
+
SENSE
–
SGND
LTC3853
V
OUT
3853 F02b
R1
R2C1*
R1
||
R2 • C1 =
L
DCR
R
SENSE(EQ)
= DCR
R2
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. R
SENSE
is chosen based on the required
output current.
The current comparator has a maximum threshold
V
SENSE(MAX)
determined by the I
LIM
setting. The current
comparator threshold sets the peak of the inductor current,
yielding a maximum average output current I
MAX
equal to
the peak value less half the peak-to-peak ripple current,
DI
L
. To calculate the sense resistor value, use the equation:
R
SENSE
=
V
SENSE(MAX)
I
(MAX)
+
DI
L
2
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of DV
SENSE
= DI
L
• R
SENSE
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 DV
SENSE
voltage is recommended
as a conservative number to start with, either for R
SENSE
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 com-
mon. Under these conditions the voltage drop across the
sense resistor’s parasitic inductance is no longer negligible.
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 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. 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
superposition of a purely resistive component and a
purely inductive component. It was measured using two