ISL68127IRAZ datasheet(14/47 Pages) RENESAS

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ISL68127

FN8748 Rev.1.00

Page 14 of 47

Jun 26, 2017

pair near any switching signals including Phase, PWM etc. This is

the method used by Intersil on evaluation board designs.

This method senses the resistance of a metal winding where 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

ISL68127 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 10. 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 11 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 12 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 time constant is too large or too small, VC(t) will not

accurately represent real-time IOUT(t) and will worsen the

transient response. Figure 13 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 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.

SPS CURRENT SENSING

SPS current sense is accomplished by sensing each SPS IMON

output individually using VCCS as a common reference. Connect

all SPS IREF input pins and all ISL68127 CSRTNn input pins

together and tie them to VCCS, then connect the SPS IMONn

output pins to the corresponding ISL68127 CSn input pins. The

signals should be run as differential pairs from the SPS back to

the ISL68127.

FIGURE 10. SENSE RESISTOR IN SERIES WITH INDUCTOR

CSn

CSRTNn

RSENSE

ESL





RSENSE

VOUT

VPHASE

ESL

CURRENT

SENSE

FIGURE 11. VOLTAGE ACROSS R WITH AND WITHOUT RC

MISMATCHED RC

MATCHED RC

FIGURE 12. DESIRED LOAD TRANSIENT RESPONSE WAVEFORMS

IOUT

VOUT

FIGURE 13. LOAD TRANSIENT RESPONSE WHEN R-C TIME

CONSTANT IS TOO SMALL

IOUT

VOUT

FIGURE 14. LOAD TRANSIENT RESPONSE WHEN R-C TIME

CONSTANT IS TOO LARGE

IOUT

VOUT

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