ISL99360 datasheet(21/98 Pages) RENESAS

21 / 98 page

ISL68220

4. Operating the Device

R16DS0015EU0100 Rev.1.00

Page 21 of 97

Oct.1.20

4.10.3 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),

which is typically less than 4nH. Consider the ESL as a separate lumped quantity, as shown in Figure 10. The

phase current, IL, flowing through the inductor, also passes through the ESL. Similar to DCR sensing described

previously, a simple R-C network across the current-sense resistor extracts the RSENSE voltage. Match the

ESL/RSENSE time constant to the R-C time constant.

Figure 11 shows the sensed waveforms with and without matching RC when using resistive sense. PCB layout for

resistive sense should follow the guidance described for DCR sense.

4.10.4 L/DCR or ESL/RSEN Matching

Assuming the compensator design is correct, Figure 12 shows the expected load transient response waveforms if

L/DCR or ESL/RSEN matches the R-C time constant for a droop application. 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 so that fine-tuning is generally required.

If the R-C timing constant is too large or too small, VC(t) does not accurately represent real-time IOUT(t) and

worsens the transient response. Figure 13 on page 22 shows the load transient response when the R-C time

constant is too small. In this condition, VOUT sags excessively on load insertion and can create a system failure or

early overcurrent trip.

Figure 10. Sense Resistor in Series with Inductor

Figure 11. Voltage Across R with and without RC

Figure 12. Desired Load Transient Response Waveforms

IOUT

VOUT

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