LTC3865-1_15 Datasheet, PDF (15/38 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
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.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
3865 F01
INDUCTOR OR R_SENSE
C_OUT
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
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_SENSE
=
V_SENSE(MAX)
I_(MAX)
+
ΔI_L
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
INTV_CC
BOOST
TG
LTC3865 SW
BG
PGND
SENSE⁺
SENSE^–
SGND
R_F
C_F
R_F
V_IN
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R_S ESL
C_F • 2_RF ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
FILTER COMPONENTS
PLACED NEAR SENSE PINS
3865 F02a
(2a) Using a Resistor to Sense Current
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3865
BG
PGND
SENSE⁺
SENSE^–
SGND
R1
C1* R2
INDUCTOR
L DCR
*PLACE C1 NEAR SENSE⁺,
SENSE^– PINS
R1||R2
•
C1
=
L
DCR
R_SENSE(EQ) = DCR
R2
R1 + R2
(2b) Using the Inductor DCR to Sense Current
V_OUT
3865 F02b
Figure 2. Two Different Methods of Sensing Current
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
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