ISL68127IRAZ datasheet(13/48 Pages) RENESAS

13 / 48 page

ISL68127

FN8748 Rev.2.00

Page 13 of 48

Jul 12, 2019

Automatic Phase Add and Drop

Automatic phase adding and dropping produces the most

optimal efficiency across a wide range of output loading. The

automatic phase dropping feature is optional. If automatic phase

dropping is enabled, the number of active phases at any time is

determined solely by load current. During operation, the Output 1

Phases drop beginning with the lowest phase number assigned.

Phase dropping begins with the highest assigned phase number.

Figure 8 illustrates the typical characteristic of efficiency vs load

current vs phase count.

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 three phases are active. If the load suddenly

drops to a level needing only one phase, the ISL68127 begins by

dropping a phase after 1ms. The ISL68127 then drops an

additional phase each 1ms until only one phase remains.

FAST PHASE ADD FUNCTION

The fast phase add function provides a very rapid response to

transient load conditions. The ISL68127 continuously monitors

the system regulation error and readies all dropped phases for

use if the system regulation error exceeds the user set threshold.

There is no delay if all phases are needed to support a load

transient. The fast phase add threshold is set in PowerNavigator.

You can also configure the output current threshold for adding

and dropping phases in PowerNavigator.

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. The boot refresh

frequency is programmable in PowerNavigator.

Output Voltage Configuration

Configure the output voltage set points and thresholds for each

output in PowerNavigator. You can configure parameters such as

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

thresholds.You can also adjust output voltage and margin

high/low during regulation using the PMBus commands

VOUT_COMMAND, VOUT_MARGIN_HIGH, and

VOUT_MARGIN_LOW for further tuning.

Maintain the following VOUT relationships 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 ISL68127 supports DCR, resistor, and smart power stage

current sensing. Connect to the various sense elements using 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,

PowerNavigator uses information about the effective sense

resistance and required per phase current capability 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 drops 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 on

page 14 can reproduce the hidden DCR voltage. By matching the

R-C time constant to the L/DCR time constant, you can precisely

recreate the DCR voltage across the capacitor. This means that

VDCR(t) = VC(t), 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, take care in

the PCB layout to properly place the R-C components and route

the differential lines between the controller and inductor.

Figure 9 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. Avoid routing the pair near any switching signals such

FIGURE 8. EFFICIENCY vs PHASE NUMBER

LOAD (A)

100

120

140

160

180

FIGURE 9. DCR SENSING CONFIGURATION

CSn

CSRTNn

DCR





DCR

VOUT

VPHASE

CURRENT

SENSE

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