ISL6336DIRZ datasheet(16/30 Pages) RENESAS

16 / 30 page

ISL6336D

FN8320 Rev 0.00

Page 16 of 30

Oct 6, 2014

The voltage on the capacitor VC, can be shown to be proportional

to the channel current IL. See Equation 8.

If the R-C network components are selected such that the RC

time constant matches the ESL-RSEN time constant (R*C =

ESL/RSEN), the voltage across the capacitor VC is equal to the

voltage drop across the RSEN, i.e., proportional to the channel

current. As an example, a typical 1mΩ sense resistor can use

R = 348 and C = 820pF for the matching. Figures 6 and 7 show

the sensed waveforms without and with matching RC when using

resistive sense.

Because of the internal filter at the ISENn- pin, one capacitor, CT,

is needed to match the time delay between the ISENn- and

ISENn+ signals. Select the proper CT to keep the time constant of

RISEN and CT (RISEN x CT) close to 27ns.

Equation 9 shows that the ratio of the channel current to the

sensed current, ISEN, is driven by the value of the sense resistor

and the RISEN.

L/DCR OR ESL/RSEN MATCHING

Assuming the compensator design is correct, Figure 8 shows the

expected load transient response waveforms if L/DCR or

ESL/RSEN is matching the R-C time constant. When the load

current has a square change, the IMON voltage (VIMON) without

a decoupling capacitor also has a square response. However,

there is always some PCB contact impedance of current sensing

components between the two current sensing points; it hardly

accounts into the L/DCR or ESL/RSEN matching calculation. Fine

tuning the matching is necessarily done in the board level to

improve overall transient performance and system reliability.

If the R-C timing constant is too large or too small, VC(s) will not

accurately represent real-time IOUT(s) and will worsen fault

response at the transient event. Figure 9 shows the IMON

transient voltage response when the R-C timing constant is too

small. VIMON will sag excessively upon load insertion and may

create a system failure or early overcurrent trip. Figure 10 shows

the transient response when the R-C timing constant is too large.

VIMON is sluggish in reaching its final value. The excessive delay

on current sensing will not provide a fast OCP response and hurt

system reliability.

Channel-Current Balance

The sensed current In from each active channel is summed

together and divided by the number of active channels. The

resulting average current IAVG provides a measure of the total

load current. Channel current balance is achieved by comparing

the sensed current of each channel to the average current to

make an appropriate adjustment to the PWM duty cycle of each

channel with Intersil’s patented current-balance method.

Channel current balance is essential in achieving the thermal

advantage of multiphase operation. With good current balance,

the power loss is equally dissipated over multiple devices and a

greater area.

Voltage Regulation

The compensation network shown in Figure 11 assures that the

steady-state error in the output voltage is limited only to the error

in the reference voltage (output of the DAC) and offset errors in

the OFS current source, remote-sense and error amplifiers.

Intersil specifies the guaranteed tolerance of the ISL6336D to

include the combined tolerances of each of these elements.

The output of the error amplifier, VCOMP, is compared to

sawtooth waveforms to generate the PWM signals. The PWM

signals control the timing of the Intersil MOSFET drivers and

regulate the converter output to the specified reference voltage.

The internal and external circuitry, which control voltage

regulation, are illustrated in Figure 11.

VC s



ESL

RSEN

----------------







RSEN IL







sRC 1





-------------------------------------------------------------------------

(EQ. 8)

FIGURE 6. VOLTAGE ACROSS R WITHOUT RC

FIGURE 7. VOLTAGE ACROSS C WITH MATCHING RC

ISEN

RSEN

RISEN

------------------



(EQ. 9)

FIGURE 8. DESIRED LOAD TRANSIENT RESPONSE WAVEFORMS

LOAD

VIMON

FIGURE 9. LOAD TRANSIENT RESPONSE WHEN R-C TIME

CONSTANT IS TOO SMALL

LOAD

VIMON

FIGURE 10. LOAD TRANSIENT RESPONSE WHEN R-C TIME

CONSTANT IS TOO LARGE

LOAD

VIMON

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