ISL68134_16 Datasheet, PDF (13/50 Pages) Intersil Corporation – Digital Dual Output, 7-Phase Configurable PWM

ISL68134
Switching Frequency
Switching frequency is user configurable over a range of 200kHz
to 1MHz.
Current Sensing
The ISL68134 supports DCR, resistor and smart power stage
current sensing. Connection to the various sense elements is
accomplished via the CS and CSRTN pins. Current sensing inputs
are high impedance differential inputs to reject noise and ground
related inaccuracies.
To accommodate a wide range of effective sense resistance,
information about the effective sense resistance and required
per phase current capability is utilized by the GUI to properly
configure the current sense circuitry.
INDUCTOR DCR SENSING
DCR sensing takes advantage of the fact that an inductor
winding has a resistive component (DCR) that will drop a voltage
proportional to the inductor current. Figure 8 on page 13 shows
that the DCR is treated as a lumped element with one terminal
inaccessible for measurement. Fortunately, a simple R-C network
as shown in Figure 9 is capable of reproducing the hidden DCR
voltage. By simply matching the R-C time constant to the L/DCR
time constant, it is possible to precisely recreate the DCR voltage
across the capacitor. This means that VDCR(t) = VC(t), thus
preserving even the high frequency characteristic of the DCR
voltage.
L
DCR
V_PHASE
VOUT
R
L
DCR
 R C
IC
CSRTNn
C
CSn
CURRENT
SENSE
FIGURE 8. DCR SENSING CONFIGURATION
Modern inductors often have such low DCR values that the
resulting signal is <10mV. To avoid noise problems, care must be
taken in the PCB layout to properly place the R-C components and
route the differential lines between controller and inductor.
Figure 8 graphically shows one PCB design method that places
the R component near the inductor V_PHASE and the C component
very close to the IC pins. This minimizes routing of the noisy
V_PHASE and maximizes filtering near the IC. Route the lines
between the inductor and IC as a pair on a single layer directly to
the controller. Care must be taken to avoid routing the pair near
any switching signals including Phase, PWM etc. This is the
method used by Intersil on evaluation board designs.
This method is sensing the resistance of a metal winding where
the DCR value will increase with temperature. This must be
compensated or the sensed (and reported) current will increase
with temperature. In order to compensate the temperature effect,
the ISL68134 provides temperature sensing options and an
internal methodology to apply the correction.
RESISTIVE SENSING
For more accurate current sensing, a dedicated current sense
resistor R_SENSE in series with each output inductor can serve as the
current sense element. This technique, however, reduces the overall
converter efficiency due to the additional power loss on the current
sense element R_SENSE.
V_PHASE
R_{SENSE ESL}
V_OUT
R
ESL
RSENSE
 RC
IC
CSRTNn
C
CSn
CURRENT
SENSE
FIGURE 9. SENSE RESISTOR IN SERIES WITH INDUCTOR
A current sensing resistor has a distributed parasitic inductance,
known as ESL (Equivalent Series Inductance, typically less than
1nH). Consider the ESL as a separate lumped quantity, as shown
in Figure 9. The phase current I_L, flowing through the inductor,
will also pass through the ESL. Similar to DCR sensing described
previously, a simple R-C network across the current sense
resistor extracts the R_SENSE voltage. Simply match the
ESL/R_SENSE time constant to the R-C time constant.
Figure 10 shows the sensed waveforms with and without
matching RC when using resistive sense. PCB layout should be
treated similar to that described for DCR sense.
MATCHING RC
NO MATCHING RC
FIGURE 10. VOLTAGE ACROSS R WITH AND WITHOUT RC
L/DCR OR ESL/R_SEN MATCHING
Assuming the compensator design is correct, Figure 11 on
page 14 shows the expected load transient response waveforms if
L/DCR or ESL/R_SEN is matching the R-C time constant. When the
load current I_OUT has a square change, the output voltage V_OUT
also has a square response, except for the potential overshoot at
load release. However, there is always some uncertainty in the true
parameter values involved in the time constant matching and
therefore fine-tuning is generally required.
If the R-C timing constant is too large or too small, V_C(t) will not
accurately represent real-time I_OUT(t) and will worsen the
transient response. Figure 12 on page 14 shows the load
transient response when the R-C timing constant is too small. In
this condition, V_OUT will sag excessively upon load insertion and
may create a system failure or early overcurrent trip. Figure 13
on page 14 shows the transient response when the R-C timing
constant is too large. V_OUT is sluggish in drooping to its final
value. Use these general guides if fine-tuning is needed.
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FN8817.0
September 28, 2016