Datasheet

LTC3838
21
3838fa
APPLICATIONS INFORMATION
R
F
R ESL
R
SENSE
RESISTOR
AND
PARASITIC INDUCTANCE
C
F
• 2R
F
≤ ESL/R
S
POLE-ZERO
CANCELLATION
FILTER COMPONENTS
PLACED NEAR SENSE PINS
R
F
SENSE
+
LTC3838
SENSE
–
C
F
3838 F03a
V
OUT
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R
SENSE
3838 F03b
Figure 3a. R
SENSE
Current Sensing
Figure 3b. Sense Lines Placement with Sense Resistor
R
SENSE
Inductor Current Sensing
The LTC3838 can be configured to sense the inductor
currents through either low value series current sensing
resistors (R
SENSE
) or inductor DC resistance (DCR). The
choice between the two current sensing schemes is largely
a design trade-off between cost, power consumption and
accuracy. DCR sensing is becoming popular because it
saves expensive current sensing resistors and is more
power efficient, especially in high current applications.
However, current sensing resistors provide the most ac-
curate current limits for the controller.
A typical R
SENSE
inductor current sensing scheme is shown
in Figure 3a. The filter components (R
F
, C
F
) need to be
placed close to the IC. The positive and negative sense
traces need to be routed as a differential pair close to-
gether and Kelvin (4-wire) connected underneath the sense
resistor, as shown in Figure 3b. Sensing current elsewhere
can effectively add parasitic inductance to the current sense
element, degrading the information at the sense terminals
and making the programmed current limit unpredictable.
R
SENSE
is chosen based on the required maximum output
current. Given the maximum current, I
OUT(MAX)
, maximum
sense voltage, V
SENSE(MAX)
, set by V
RNG
, and maximum
inductor ripple current ∆I
L(MAX)
, the value of R
SENSE
can
be chosen as:
R
SENSE
=
V
SENSE(MAX)
I
OUT(MAX)
–
ΔI
L(MAX)
2
Conversely, given R
SENSE
and I
OUT(MAX)
, V
SENSE(MAX)
and
thus V
RNG
voltage can be determined from the above equa-
tion. To ensure the maximum output current, sufficient
margin should be built in the calculations to account for
variations of LTC3838 under different operating conditions
and tolerances of external components.
Because of possible PCB noise in the current sensing
loop, the current sensing voltage ripple ∆V
SENSE
= ∆I
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, 10mV of ∆V
SENSE
is recommended as
a conservative number to start with, either for R
SENSE
or
Inductor DCR sensing applications.
For today’s highest current density solutions the value
of the sense resistor can be less than 1m and the
peak sense voltage can be as low as 20mV. In addition,
inductor ripple currents greater than 50% with operation
up to 2MHz are becoming more common. Under these
conditions, the voltage drop across the sense resistor’s
parasitic inductance becomes more relevant. A small RC
filter placed near the IC has been traditionally used to re-
duce the effects of capacitive and inductive noise coupled
in the sense traces on the PCB. A typical filter consists of
two series 10 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.