LTC3838-2_15 Datasheet, PDF (23/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
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 LTC3838-2 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.
LTC3838-2
SENSE⁺
SENSE^–
R_F
C_{F RF}
R_SENSE RESISTOR
AND
PARASITIC INDUCTANCE
R
ESL
C_F • 2R_F ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
38382 F03a
FILTER COMPONENTS
PLACED NEAR SENSE PINS
Figure 3a. R_SENSE Current Sensing
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R_SENSE ^{38382 F03b}
C_OUT
Figure 3b. Sense Lines Placement with Sense Resistor
R_SENSE is chosen based on the required maximum output
current. Given the maximum current, I_OUT(MAX), maximum
sense voltage, V_SENSE(MAX), 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)
can be determined from the above equation. 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 2mΩ 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
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38382f
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