ISL68124IRAZ datasheet(12/46 Pages) RENESAS

12 / 46 page

ISL68124

FN8796 Rev.2.00

Page 12 of 46

June 16, 2017

Automatic Phase Add and Drop

To produce the most optimal efficiency across a wide range of

output loading, the modulator supports automatic dropping or

adding of phases. Use of automatic phase dropping is optional. If

automatic phase dropping is enabled, the number of active

phases at any time is determined solely by load current. During

operation, phases of Output 1 will drop beginning with the lowest

phase number assigned. Phase dropping begins with the highest

assigned phase number. Figure 7 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 ISL68124 will begin

by dropping a phase after 1ms. An additional phase will be

dropped each 1ms thereafter until only one 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, 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 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. The frequency of

the boot refresh is programmable through PowerNavigator.

Output Voltage Configuration

Output voltage set points and thresholds for each output can be

configured with PowerNavigator GUI. Parameters such as output

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

can be configured with the GUI. Additionally, output voltage and

margin high/low can be adjusted during regulation using 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 ISL68124 supports DCR, resistor, and smart power stage

current sensing. Connection to the various sense elements is

accomplished 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,

information about the effective sense resistance and required

per phase current capability is used 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 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 including Phase, PWM etc. This is the method

used by Intersil on evaluation board designs.

LOAD (A)

FIGURE 7. EFFICIENCY vs PHASE NUMBER

3-PHASE

2-PHASE

1-PHASE

FIGURE 8. DCR SENSING CONFIGURATION

CSn

CSRTNn

DC R





DCR

VOUT

VPHASE

CURRENT

SENSE

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