LTC3850-1_15 Datasheet, PDF (14/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850/LTC3850-1
APPLICATIONS INFORMATION
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.
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, ∆I_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
common. Under these conditions the voltage drop across
the sense resistor’s parasitic inductance is no longer neg-
ligible. 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
inthe 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
3 illustrates the voltage waveform across a 2mΩ sense
resistor with a 2010 footprint for the 1.2V/15A converter
shown in Figure 18 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
the top switch, the value of the parasitic inductance was
determined to be 0.5nH using the equation:
ESL = ^{VESL(STEP) tON • tOFF}
∆I_L t_ON + t_OFF
If the RC time constant is chosen to be close to the parasitic
inductance divided by the sense resistor (L/R), the result-
ing waveform looks resistive again, as shown in Figure
4. For applications using low maximum sense voltages,
check the sense resistor manufacturer’s data sheet for
information about parasitic inductance. In the absence of
data, measure the voltage drop directly across the sense
resistor to extract the magnitude of the ESL step and use
the equation above to determine the ESL. However, do not
over-filter. Keep the RC time constant less than or equal
to the inductor time constant to maintain a high enough
ripple voltage on V_RSENSE.
The above generally applies to high density / high cur-
rent applications where I_(MAX) > 10A and low values of
inductors are used. For applications where I_(MAX) < 10A,
set R_F to 10 Ohms and C_F to 1000pF. This will provide a
good starting point.
The filter components need to be placed close to the IC.
The positive and negative sense traces need to be routed
as a differential pair and Kelvin connected to the sense
resistor.
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