ISL6306 Datasheet, PDF(14/33 Page) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

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ISL6306 Datasheet, PDF (14/33 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

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ISL6306

is driven by the position of the error amplifier output signal,

VCOMP, minus the current correction signal relative to the

sawtooth ramp as illustrated in Figure 7. When the modified

VCOMP voltage crosses the sawtooth ramp, the PWM output

transitions high. The MOSFET driver detects the change in

state of the PWM signal and turns off the synchronous

MOSFET and turns on the upper MOSFET. The PWM signal

will remain high until the pulse termination signal marks the

beginning of the next cycle by triggering the PWM signal low.

Current Sampling

During the forced off-time following a PWM transition low,

the associated channel current sense amplifier uses the

ISEN inputs to reproduce a signal proportional to the inductor

current (IL). This current gets sampled starting

1/6 period after each PWM goes low and continuously gets

sampled for 1/3 period, or until the PWM goes high,

whichever comes first. No matter the current sense method,

the sense current (ISEN) is simply a scaled version of the

inductor current. Coincident with the falling edge of the PWM

signal, the sample and hold circuitry samples the sensed

current signal (ISEN) as illustrated in Figure 3.

Therefore, the sample current (In) is proportional to the

output current and held for one switching cycle. The sample

current is used for current balance, load-line regulation, and

overcurrent protection.

PWM

ISEN

0.5Tsw

SAMPLE CURRENT, In

SWITCHING PERIOD

TIME

FIGURE 3. SAMPLE AND HOLD TIMING

Current Sensing

The ISL6306 supports inductor DCR sensing, MOSFET

rDS(ON) sensing, or resistive sensing techniques. The

internal circuitry, shown in Figures 4, 5, and 6, represents

one channel of an N-channel converter. This circuitry is

repeated for each channel in the converter, but may not be

active depending on the status of the PWM3 and PWM4

pins, as described in âPWM Operationâ on page 13.

INDUCTOR DCR SENSING

An inductorâs winding is characteristic of a distributed

resistance as measured by the DCR (Direct Current

Resistance) parameter. Consider the inductor DCR as a

separate lumped quantity, as shown in Figure 4. The

channel current (IL) flowing through the inductor, will also

pass through the DCR. Equation 3 shows the s-domain

equivalent voltage across the inductor VL.

VL = IL â (s â L + DCR)

(EQ. 3)

A simple RC network across the inductor extracts the DCR

voltage, as shown in Figure 4.

The voltage on the capacitor (VC) can be shown to be

proportional to the channel current (IL) see Equation 4.

------L-------

DCR

1â â

(DCR

IL)

VC = --------------------(--s-----â---R----C------+-----1----)-------------------

(EQ. 4)

If the RC network components are selected such that the RC

time constant (= R*C) matches the inductor time constant

(= L/DCR), the voltage across the capacitor (VC) is equal to

the voltage drop across the DCR (i.e., proportional to the

channel current).

VIN

IL(s)

ISL6605

DCR

INDUCTOR

VOUT

COUT

VC(s)

PWM(n)

ISL6306 INTERNAL CIRCUIT

RISEN(n)

(PTC)

SAMPLE

AND

HOLD

ISEN

---D----C-----R-----

RISEN

ISEN-(n)

ISEN+(n)

FIGURE 4. DCR SENSING CONFIGURATION

With the internal low-offset current amplifier, the capacitor

voltage (VC) is replicated across the sense resistor (RISEN).

Therefore the current out of ISEN+ pin (ISEN) is proportional

to the inductor current.

FN9226.1

May 5, 2008

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