ISL68137_16 Datasheet, PDF (14/53 Pages) Intersil Corporation – Digital Dual Output, 7-Phase Configurable PWM

ISL68137
Phases are dropped one at a time with a user programmed drop
delay between drop events. As an example, suppose the delay is
set to 1ms and 3 phases are active. Should the load suddenly
drop to a level needing only 1 phase, the ISL68137 will begin by
dropping a phase after 1ms. An additional phase will be dropped
each 1ms thereafter until only 1 phase remains.
In addition to the described load current add/drop thresholds,
the fast phase add function provides a very rapid response to
transient load conditions. This feature continuously monitors the
system regulation error and should it exceed the user set
threshold, all dropped phases will be readied for use. In this way,
there is no delay should all phases be needed to support a load
transient. The fast phase add threshold is set in the
PowerNavigator™ GUI. Output current threshold for adding and
dropping phases can also be configured.
To ensure dropped phases have sufficient boot capacitor charge
to turn on the high-side MOSFET after a long period of disable, a
boot refresh circuit turns on the low-side MOSFET of each
dropped phase to refresh the boot capacitor. Frequency of the
boot refresh is programmable via PowerNavigator™.
Output Voltage Configuration
Output voltage set points and thresholds for each output can be
configured with PowerNavigator™ GUI. Parameters such as
output voltage, V_OUT margin high/low and V_OUT OV/UV fault
thresholds can be configured with GUI. Additionally, output
voltage and margin high/low can be adjusted during regulation
via PMBus command VOUT_COMMAND, VOUT_MARGIN_HIGH
and VOUT_MARGIN_LOW for further tuning. The following V_OUT
relationships must be maintained for correct operation:
VOUT_OV_FAULT_LIMIT > VOUT_COMMAND
(VOUT_MARGIN_HIGH and VOUT_MARGIN_LOW, if used) >
VOUT_UV_FAULT_LIMIT. Additionally, the V_OUT commands are
bounded by VOUT_MAX and VOUT_MIN to provide protection
against incorrect set points being sent to the device. The
ISL68137 also incorporates AVSBus functionality for high speed
changes to the V_OUT target.
Switching Frequency
Switching frequency is user configurable over a range of 200kHz
to 1MHz.
Current Sensing
The ISL68137 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 9 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 10 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.
V_PHASE
L
DCR
R
L
DCR

R
C
V_OUT
IC
CSRTNn
C
CSn
CURRENT
SENSE
FIGURE 9. 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 9 graphically shows one PCB design method that places
the R component near the inductor VPHASE and the C component
very close to the IC pins. This minimizes routing of the noisy
VPHASE 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 ISL68137 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.
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FN8757.0
September 27, 2016