ISL68144IRAZ datasheet(13/48 Pages) RENESAS

13 / 48 page

ISL68144

FN8888 Rev. 3.00

Page 13 of 48

Feb 8, 2018

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 if it exceeds the user set threshold,

all dropped phases will be readied for use. In this way, there is no

delay if all phases are needed to support a load transient. The

fast phase add threshold is set in PowerNavigator. The 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 disable period, a

boot refresh circuit turns on the low-side MOSFET of each

dropped phase to refresh the boot capacitor. The frequency of

the boot refresh is programmable using PowerNavigator.

Output Voltage Configuration

Output voltage set points and thresholds for each output can be

configured using PowerNavigator. Parameters such as output

voltage, VOUT margin high/low and VOUT OV/UV fault thresholds

can be configured with GUI. Additionally, output voltage and

margin high/low can be adjusted during regulation through the

PMBus commands VOUT_COMMAND, VOUT_MARGIN_HIGH, and

VOUT_MARGIN_LOW for further tuning. The following VOUT

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 VOUT commands are

bounded by VOUT_MAX and VOUT_MIN to provide protection

against incorrect set points being sent to the device.

Switching Frequency

The switching frequency is user-configurable over a range of

200kHz to 1MHz.

Current Sensing

The ISL68144 supports DCR, resistor, and smart power stage

current sensing. Connection to the various sense elements is

accomplished through 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 used by PowerNavigator 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 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.

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 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. The lines between the inductor

and IC should be routed as a pair on a single layer directly to the

controller. Care must be taken to avoid routing the pair near any

switching signals such as Phase or PWM. This is the method used

by Renesas on evaluation board designs.

This method senses the resistance of a metal winding in which

the DCR value increases with temperature. This must be

compensated or the sensed (and reported) current will increase

with temperature. To compensate for the temperature effect, the

ISL68144 provides temperature sensing options and an internal

methodology to apply the correction.

RESISTIVE SENSING

For more accurate current sensing, a dedicated current sense

resistor, RSENSE, in series with each output inductor can serve as

the current sense element. However, this technique reduces the

overall converter efficiency due to the additional power loss on the

current sense element, RSENSE.

A current sensing resistor has a distributed parasitic inductance,

known as Equivalent Series Inductance (ESL), typically less than

4nH. Consider the ESL as a separate lumped quantity, as shown

in Figure 9. The phase current IL, 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

FIGURE 8. DCR SENSING CONFIGURATION

FIGURE 9. SENSE RESISTOR IN SERIES WITH INDUCTOR

CSn

CSRTNn

DC R





DCR

VOUT

VPHASE

CURRENT

SENSE

CSn

CSRTNn

RSENSE

ESL





RSENSE

VOUT

VPHASE

ESL

CURRENT

SENSE

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