ISL6307A Datasheet, PDF(14/33 Page) Intersil Corporation – Ultra-high bandwidth 6-Phase PWM Controller with 8 Bit VID Code Capable of Precision RDS(ON) or DCR Differential Current Sensing

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ISL6307A Datasheet, PDF (14/33 Pages) Intersil Corporation – Ultra-high bandwidth 6-Phase PWM Controller with 8 Bit VID Code Capable of Precision RDS(ON) or DCR Differential Current Sensing

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ISL6307A

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 ISL6307A 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 the PWM Operation section.

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 R-C 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.

VC = -ï£ï£«--s-----â---D----------C-L-------(-R----s-----+-â---R-1---Cï£¸ï£¶-----â-+--(--D-1----)C-----R------â---I--L---)

(EQ. 4)

If the R-C 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

ISL6609

PWM(n)

IL(s)

DCR

INDUCTOR

VC(s)

VOUT

COUT

ISL6307A INTERNAL CIRCUIT

SAMPLE

HOLD

ISEN

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

RISEN

ISEN-(n)

ISEN+(n)

RISEN(n)

(PTC)

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.

Equation 5 shows that the ratio of the channel current to the

sensed current ISEN is driven by the value of the sense

resistor and the DCR of the inductor.

ISEN

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

RISEN

(EQ. 5)

FN9236.0

February 6, 2006

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