Datasheet

LTC3865/LTC3865-1
15
3865fb
APPLICATIONS INFORMATION
Filter components mutual to the sense lines should be
placed close to the LTC3865/LTC3865-1, and the sense
lines should run close together to a Kelvin connection
underneath the current sense element (shown in Figure 1).
Sensing current elsewhere can effectively add parasitic
inductance and capacitance to the current sense element,
degrading the information at the sense terminals and mak-
ing 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.
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R
SENSE
3865 F01
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
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 fi lter placed near the IC was commonly used to reduce
the effects of capacitive and inductive noise coupled in
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 input
common mode range of the current comparator is 0V
to 5V. 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, ΔI
L
. To calculate the sense resistor
value, use the equation:
R
V
I
I
SENSE
SENSE MAX
MAX
L
=
+
()
()
Δ
2
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ΔV
SENSE
= ΔI
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 ΔV
SENSE
voltage is recommended
as a conservative number to start with, either for R
SENSE
or DCR sensing applications.
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE
+
SENSE
–
SGND
LTC3865
V
OUT
3865 F02a
C
F
• 2
RF
≤ ESL/R
S
POLE-ZERO
CANCELLATION
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R
S
ESL
C
F
R
F
R
F
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
*PLACE C1 NEAR SENSE
+
,
SENSE
–
PINS
INDUCTOR
DCRL
SENSE
+
SENSE
–
SGND
LTC3865
V
OUT
3865 F02b
R1
R2C1*
R1
||
R2 • C1 =
L
DCR
R
SENSE(EQ)
= DCR
R2
R1 + R2
(2a) Using a Resistor to Sense Current
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current