Datasheet

LTC3883/LTC3883-1
38
3883fa
For more information www.linear.com/LTC3883
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
I
SENSE
+
I
SENSE
–
SGND
LTC3883
V
OUT
3883 F018b
C
F
• 2
RF
≤ ESL/R
S
POLE-ZERO
CANCELLATION
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R
S
ESL
C
F
R
F
R
F
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
*PLACE C1 NEAR SENSE
+
, SENSE
–
PINS
INDUCTOR
DCR
R3
OPTIONAL
C2
>1µF
L
I
SENSE
+
I
SENSE
–
SGND
LTC3883
V
OUT
3883 F18a
R1
R2C1*
((R1+ R3)||R2) × C1 =
R3 = R1
L
2 × DCR
IOUT_CAL_GAIN = DCR
R2
R1 + R2 + R3
APPLICATIONS INFORMATION
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
Figure 18a. Inductor DCR Current Sense Circuit
Figure 18b. Resistor Current Sense Circuit
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.