ISL68124IRAZ datasheet(13/46 Pages) RENESAS

13 / 46 page

ISL68124

FN8796 Rev.2.00

Page 13 of 46

June 16, 2017

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

ISL68124 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), which is typically

less than 4nH. 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 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 12 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 shows the transient

response when the R-C time 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 ISL68124 CSRTNx input pins

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

output pins to the corresponding ISL68124 CSx input pins. The

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

the ISL68124.

Temperature Sensing

The ISL68124 supports temperature sensing through BJT or

smart power stage sense elements. Support for BJT sense

elements uses the well known delta Vbe method and allows up

to two sensors (MMBT3906 or similar) on each temperature

sense input, TMON0 and TMON1. Support for smart power stage

uses a linear conversion algorithm and allows one sensor reading

per pin. The conversion from voltage to temperature for smart

power stage sensing is user-programmable through the

PowerNavigator GUI.

The SPS temperature sensing measures the

temperature-dependent voltage output on the SPS TMON pin. All

of the SPS devices attached to the Output 0 rail have their TMON

pins connected to the ISL68124 TMON0 pin. All of the SPS

devices attached to the Output 1 rail have their TMON pins

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

MISMATCHED RC

MATCHED RC

FIGURE 11. DESIRED LOAD TRANSIENT RESPONSE WAVEFORMS

FIGURE 12. LOAD TRANSIENT RESPONSE WHEN R-C TIME

CONSTANT IS TOO SMALL

FIGURE 13. LOAD TRANSIENT RESPONSE WHEN R-C TIME

CONSTANT IS TOO LARGE

IOUT

VOUT

IOUT

VOUT

IOUT

VOUT

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