ISL6324A datasheet(13/39 Pages) RENESAS

13 / 39 page

ISL6324A Hybrid SVI/PVI with I2C

FN6880 Rev 1.00

Page 13 of 39

May 14, 2010

The PWM signals command the MOSFET driver to turn on/off

the channel MOSFETs.

For 4-channel operation, the channel firing order is 1-2-3-4:

PWM3 pulse happens 1/4 of a cycle after PWM4, PWM2

output follows another 1/4 of a cycle after PWM3, and PWM1

delays another 1/4 of a cycle after PWM2. For 3-channel

operation, the channel firing order is 1-2-3.

Connecting ISEN4- to VCC selects three channel operation

and the pulse times are spaced in 1/3 cycle increments. If

ISEN3- is also connected to VCC, 2-Channel operation is

selected and the PWM2 pulse happens 1/2 of a cycle after

PWM1 pulse. If ISEN2- is also connected to VCC, 1-Channel

operation is selected.

Continuous Current Sampling

In order to realize proper current-balance, the currents in each

channel are sampled continuously every switching cycle.

During this time, the current-sense amplifier uses the ISEN

inputs to reproduce a signal proportional to the inductor

current, IL. This sensed current, ISEN, is simply a scaled

version of the inductor current.

The ISL6324A supports Inductor DCR current sensing to

continuously sample each channel’s current for channel-current

balance. The internal circuitry, shown in Figure 6 represents

Channel N of an N-Channel converter. This circuitry is repeated

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

depending on how many channels are operating.

Inductor windings have a characteristic distributed resistance

or DCR (Direct Current Resistance). For simplicity, the inductor

DCR is considered as a separate lumped quantity, as shown in

Figure 6. The channel current ILn, flowing through the inductor,

passes through the DCR. Equation 5 shows the S-domain

equivalent voltage, VL, across the inductor.

A simple R-C network across the inductor (R1, R2 and C)

extracts the DCR voltage, as shown in Figure 6. The voltage

across the sense capacitor, VC, can be shown to be

proportional to the channel current ILn, shown in Equation 6.

Where:

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

time constant matches the inductor L/DCR time constant (see

Equation 8), then VC is equal to the voltage drop across the

DCR multiplied by the ratio of the resistor divider, K. If a

resistor divider is not being used, the value for K is 1.

The capacitor voltage VC, is then replicated across the effective

internal sense resistor, RISEN. This develops a current through

RISEN which is proportional to the inductor current. This

current, ISEN, is continuously sensed and is then used by the

controller for load-line regulation, channel-current balancing,

and overcurrent detection and limiting. Equation 9 shows that

the proportion between the channel current, IL, and the sensed

FIGURE 4. CONTINUOUS CURRENT SAMPLING

TIME

PWM

ISEN

SWITCHING PERIOD

VL s



sL DCR







(EQ. 5)

(EQ. 6)

VC s





DCR

------------- 1





R1 R2





R1 R2

------------------------ C









-------------------------------------------------------- KDCR IL





R2 R1

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

(EQ. 7)

FIGURE 5. INDUCTOR DCR CURRENT SENSING

CONFIGURATION

ISENn-

ISL6323 INTERNAL CIRCUIT

VIN

UGATE(n)

DCR

INDUCTOR

VOUT

COUT

VC(s)

VL(s)

ISEN

VC(s)

ISENn+

LGATE(n)

MOSFET

DRIVER

RSET

SAMPLE

RISEN

VCC

CSET

DCR

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

R1 R2



R1 R2

--------------------- C



(EQ. 8)

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