LTC3811_15 Datasheet, PDF (25/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
pin. For applications requiring maximum current limit ac-
curacy, a higher peak sense voltage (e.g., 85mV) should
be chosen. An additional benefit of a higher peak SENSE
pin threshold is a slight reduction in the minimum on-
time of the controller. That is, for a given ripple current
in the inductor, a higher peak sense voltage results in
higher SENSE pin dV/dt, speeding up the input stage of
the current comparator slightly. For applications where
high efficiency and tight current limit accuracy are both
important, the peak current sense voltage can be reduced
to as low as 24mV.
In multiphase applications, only one error amplifier is used
to control all of the phase current comparators. As a result,
in multiphase applications all of the RNG pins should all be
tied to the same potential, in order to program the same
power stage g_m for each phase.
SENSE⁺ and SENSE^– Pins
The common mode input voltage range of the current
comparators is 0V to 3.5V. Continuous linear operation is
provided throughout this range, allowing output voltages
between 0.6V (the reference input to the error amplifiers)
and 3.3V. The SENSE⁺ and SENSE^– pins are also the inputs
to the voltage positioning current sense g_m amplifier.
Under normal operation, a small current of about 1.5μA
flows out of the SENSE inputs and represents the total
base current of the two vertical PNP input stages (one in
the current comparator and one in the voltage positioning
current sense amplifier). When the common mode voltage
is lower than about 0.4V, the current flowing out of the
SENSE pins increases, up to about 2.2μA at V_SENSE = 0V.
Figure 11 illustrates the change in the SENSE pin current
as a function of common mode voltage.
Sensing Techniques Using Low Value Resistors
For previous generation current mode controllers, the
maximum sense voltage was high enough (e.g., 75mV
for the LTC1628 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
24mV. In addition, inductor ripple currents greater than 50%
with operation up to 1MHz are becoming more common.
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 12. 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.
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
COMMON MODE VOLTAGE (V)
3811 G33
Figure 11. SENSE Pin Input Bias Current vs
Common Mode (Output) Voltage
V_IN
DRV_CC
LTC3811
BOOST
TG
SW
BG
PGND
SENSE⁺
SENSE^–
SGND
R_F
C_F
R_F
V_IN
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R_S ESL
C_F • 2R_F = ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
FILTER COMPONENTS
PLACED NEAR SENSE PINS
3811 F12
Figure 12. Using a Resistor to Sense Current with the LTC3811
3811f
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