LTC3883 Datasheet, PDF (38/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Applications Information
coupling into sensitive small-signal nodes. The capacitor
C1 should be placed close to the IC pins. This impedance
difference can result in loss of accuracy in the current
reading of the ADC. The current reading accuracy can be
improved by matching the impedance of the two pins. To
accomplish this add a series resistor between V_OUT and
I_SENSE– equal to R1. A capacitor of 1µF or greater should
be placed in parallel with this resistor. If the peak voltage
is <75mV at room temperature, R2 is not required.
Low Value Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 18b. R_SENSE is chosen based on the required
output current.
V_IN
INTV_CC
V_IN
BOOST
TG
SW
LTC3883
BG
PGND
I_SENSE+
I_SENSE–
SGND
R1
C1* R2
INDUCTOR
L DCR
C2
>1µF
R3
OPTIONAL
V_OUT
((R1+ R3)||R2) × C1 =
R3 = R1
L
2 × DCR
IOUT_CAL_GAIN
=
DCR
R1
+
R2
R2
+
R3
*PLACE C1 NEAR SENSE⁺, SENSE^– PINS
Figure 18a. Inductor DCR Current Sense Circuit
3883 F18a
V_IN
INTV_CC
BOOST
TG
SW
LTC3883
BG
PGND
I_SENSE+
I_SENSE–
SGND
R_F
C_F
R_F
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R_S ESL
C_F • 2_RF ≤ ESL/R_S
POLE-ZERO
CANCELLATION
FILTER COMPONENTS
PLACED NEAR SENSE PINS
Figure 18b. Resistor Current Sense Circuit
38
V_IN
V_OUT
3883 F018b
The current comparator has a maximum threshold
V_SENSE(MAX) determined by the I_LIMIT setting. The input
common mode range of the current comparator is 0V to
5.5V (if V_IN is greater than 6V). 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
Due to 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 minimum ∆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. In the new highest current density
solutions; however, the value of the sense resistor can be
less than 1mΩ and the peak sense voltage can be less than
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 negligible. A
typical sensing circuit using a discrete resistor is shown in
Figure 18b. In previous generations of controllers, a small
RC filter placed near the IC was commonly used to reduce
the effects of the capacitive and inductive noise coupled
in the sense traces on the PCB. A typical filter consists of
two series 100Ω resistors connected to a parallel 1000pF
capacitor, resulting in a time constant of 200ns.
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 19 illustrates the voltage waveform across
a 2mΩ resistor with a 2010 footprint. The waveform is
the superposition of a purely resistive component and a
3883f