Datasheet

LTC3838-1
21
38381f
For more information www.linear.com3838-1
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
+
LTC3838-1
SENSE
–
C
F
38381 F03a
V
OUT
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R
SENSE
38381 F03b
Figure 3a. R
SENSE
Current Sensing
Figure 3b. Sense Lines Placement with Sense Resistor
R
SENSE
method offers more precise control of the current
limit but the resistor will dissipate loss. The DCR method
saves the cost of the sense resistors and may offer bet-
ter efficiency, especially in high current applications, but
tolerance and the variation over temperature in the DCR
value usually requires larger design margins.
R
SENSE
Inductor Current Sensing
The LTC3838-1 can be configured to sense the inductor
currents through either current sensing resistors (R
SENSE
)
or inductor DC resistance (DCR). The current sensing
resistors provide the most accurate 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 the V
RNG
pin, 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)
–
∆I
L(MAX)
2
Conversely, given R
SENSE
and I
OUT(MAX)
, V
SENSE(MAX)
(set
by the V
RNG
pin) 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 the ICs 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 1mΩ 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 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 4a illustrates the voltage waveform
across a 2
mΩ sense resistor with a 2010 footprint for a
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
scope probes and waveform math to obtain a differential
measurement. Based on additional measurements of the
inductor ripple current and the on-time and off-time of