ISL68127IRAZ datasheet(14/48 Pages) RENESAS

14 / 48 page

ISL68127

FN8748 Rev.2.00

Page 14 of 48

Jul 12, 2019

as Phase and PWM. This is the method used by Renesas on

evaluation board designs.

This method senses the resistance of a metal winding in which

the DCR value increases with temperature. The temperature

must be compensated or the sensed (and reported) current

increases 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, also passes

through the ESL. Similar to “Inductor DCR Sensing” on page 13, 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. The PCB layout should

be similar to that described in “Inductor DCR Sensing”.

L/DCR OR ESL/RSEN MATCHING

If 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) does not

accurately represent real-time IOUT(t) and reduces the transient

response. Figure 13 shows the load transient response when the

R-C timing constant is too small.

In this condition, VOUT sags excessively at load insertion and can

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 is sensed by sensing each SPS IMON output

individually using VCCS as a common reference. Complete the

following steps to sense SPS current.

1. Connect all SPS IREF input pins and all ISL68127 CSRTNn

input pins together and tie them to VCCS.

2. 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 R-C

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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