LTC3853_15 Datasheet, PDF (14/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
APPLICATIONS INFORMATION
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.
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3853
R_SENSE
L1
R_S ESL
BG
PGND
SENSE⁺
SENSE^–
SGND
R_F
C_F R_F
FILTER COMPONENTS
PLACED NEAR SENSE PINS
3853 F02a
(2a) Using a Resistor to Sense Current
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3853
BG
PGND
SENSE⁺
SENSE^–
SGND
R1
C1* R2
INDUCTOR
L DCR
V_OUT
*PLACE C1 NEAR SENSE⁺,
SENSE^– PINS
R1||R2
•
C1
=
L
DCR
R_SENSE(EQ) = DCR
R2
R1 + R2
3853 F02b
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
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 )}
+
∆I _L
2
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ∆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, a 15mV ∆V_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.
3853fc
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