ISL6444CA datasheet(9/19 Pages) INTERSIL

Part #	ISL6444CA

Description	Dual PWM Controller with DDR Memory Option for Gateway Applications

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Manufacturer	INTERSIL [Intersil Corporation]

Direct Link	http://www.intersil.com/cda/home

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9 / 19 page

9

FN9069.3

April 12, 2007

synchronous MOSFET compared to a Shottky diode. In

contrast, continuous-conduction operation in load currents

lower than the inductor critical value results in lower

efficiency. In this case, during a fraction of a switching cycle,

the direction of the inductor current changes to the opposite

actively discharging the output filter capacitor.

To maintain the output voltage in regulation, the discharged

energy should be restored during the consequent cycle of

operation by the cost of increased circulating current and

losses associated with it.

The critical value of the inductor current can be estimated by

the following expression.

To improve converter efficiency in loads lower than critical,

the switch-over to variable frequency hysteretic operation

with diode emulation is implemented into the PWM scheme.

The switch-over is provided automatically by the mode

control circuit that constantly monitors the inductor current

and alters the way the PWM signal is generated.

The voltage across the synchronous MOSFET at the

moment of time just before the upper-MOSFET turns on is

monitored for purposes of mode change. When the

converter operates in currents higher than critical, this

voltage is always positive as shown in Figure 3 and 4. In

currents lower than critical, the voltage is always negative.

The mode control circuit uses a sign of voltage across the

synchronous devices to determine if the load current is

higher or lower than the critical value.

To prevent chatter between operating modes, the circuit

looks for eight sequential matching sign signals before it

makes its decision to perform a mode change. The same

algorithm is true for both CCM-hysteretic and hysteretic-

CCM transitions.

Hysteretic Operation

When the critical inductor current is detected, the converter

enters hysteretic mode. The PWM comparator and the error

amplifier that provided control in the CCM mode are inhibited

and the hysteretic comparator is now activated. A change is

also made to the gate logic. In hysteretic mode the

synchronous rectifier MOSFET is controlled in diode

emulation mode, hence conduction in the second quadrant

is prohibited.

The hysteretic comparator initiates the PWM signal when the

output voltage gets below the lower threshold and

terminates the PWM signal when the output voltage rises

over the upper threshold. A spread or hysteresis between

these two thresholds determines the switching frequency

and the peak value of the inductor current. A transition to a

constant frequency CCM mode will happen when the load

current gets to a level higher than the critical:

Where,

ΔVhys= 15mV, is a hysteretic comparator window,

ESR is the equivalent series resistance of the output

capacitor.

Because of different control mechanisms, the value of the

load current where transition into CCM operation takes place

FIGURE 2. OUTPUT VOLTAGE PROGRAM

R2

R1

UGATE

LGATE

ISL6444

L1

Q1

Q2

C1

VOUT

VSEN

Vin

RCS

ISEN

OCSET

ROC

Cz

I

HYS

V

IN

V

O

–

() V

O

•

2F

SW

L

O

V

IN

•

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

=

(EQ. 4)

FIGURE 3. CCM -- HYSTERETIC TRANSITION

PWM

Hysteretic

1 2 3 4 5

6 7 8

VOUT

IIND

PHASE

OPERATION

MODE

OF

t

COMP

FIGURE 4. HYSTERETIC -- CCM TRANSITION

PWM

Hysteretic

1

2 3 4 5

6 7

8

VOUT

IIND

PHASE

COMP

OPERATION

MODE

OF

t

I

CCM

ΔV

hys

2

ESR

•

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

≈

(EQ. 5)

ISL6444

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