ISL68134IRAZ datasheet(13/50 Pages) RENESAS

13 / 50 page

ISL68134

FN8817 Rev.1.00

Page 13 of 50

Jun 19, 2017

Switching Frequency

The 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 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. 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 in

which the DCR value will increase with temperature. This must be

compensated or the sensed (and reported) current will increase

with temperature. 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 RSENSE 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 RSENSE.

A current sensing resistor has a distributed parasitic inductance,

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

1nH. 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

resistor extracts the RSENSE voltage. Simply match the

ESL/RSENSE 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.

L/DCR OR ESL/RSEN MATCHING

Assuming the compensator design is correct, Figure 11 on

page 14 shows the expected load transient response waveforms if

L/DCR or ESL/RSEN is matching the R-C time constant. When the

load current IOUT has a square change, the output voltage VOUT

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, VC(t) will not

accurately represent real-time IOUT(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, VOUT 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. VOUT is sluggish in drooping to its final

value. Use these general guides if fine-tuning is needed.

FIGURE 8. DCR SENSING CONFIGURATION

CSn

CSRTNn

DC R





DCR

VOUT

VPHASE

CURRENT

SENSE

FIGURE 9. SENSE RESISTOR IN SERIES WITH INDUCTOR

CSn

CSRTNn

RSENSE

ESL





RSENSE

VOUT

VPHASE

ESL

CURRENT

SENSE

FIGURE 10. VOLTAGE ACROSS R WITH AND WITHOUT RC

NOMATCHINGRC

MATCHINGRC

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