ISL62391IRTZ datasheet(14/20 Pages) INTERSIL

14 / 20 page

FN6666.4

December 22, 2008

ISL62392 monitors the phase voltage when the low-side

MOSFET is conducting inductor current to determine its

direction.

When the output load current is greater than or equal to ½

the inductor ripple current, the inductor current is always

positive, and the converter is always in CCM. The ISL62391,

ISL62392 minimizes the conduction loss in this condition by

forcing the low-side MOSFET to operate as a synchronous

rectifier.

When the output load current is less than ½ the inductor

ripple current, negative inductor current occurs. Sinking

negative inductor through the low-side MOSFET lowers

efficiency through unnecessary conduction losses. The

ISL62391, ISL62392 automatically enters DEM after the

PHASE pin has detected positive voltage and LGATE was

allowed to go high for 8 consecutive PWM switching cycles.

The ISL62391, ISL62392 will turn off the low-side MOSFET

once the phase voltage turns positive, indicating negative

inductor current. The ISL62391, ISL62392 will return to CCM

on the following cycle after the PHASE pin detects negative

voltage, indicating that the body diode of the low-side

MOSFET is conducting positive inductor current.

Efficiency can be further improved with a reduction of

unnecessary switching losses by reducing the PWM

frequency. It is characteristic of the R3 architecture for the

PWM frequency to decrease while in diode emulation. The

extent of the frequency reduction is proportional to the

reduction of load current. Upon entering DEM, the PWM

frequency makes an initial step-reduction because of a 33%

step-increase of the window voltage VW.

Because the switching frequency in DEM is a function of

load current, very light load conditions can produce

frequencies well into the audio band. This can be

problematic if audible noise is coupled into audio amplifier

circuits. To prevent this from occurring, the ISL62391,

ISL62392 allows the user to float the FCCM input. This will

allow DEM at light loads, but will prevent the switching

frequency from going below ~28kHz to prevent noise

injection to the audio band. A timer is reset each PWM

pulse. If the timer exceeds 30µs, LGATE is turned on,

causing the ramp voltage to reduce until another UGATE is

commanded by the voltage loop.

Overcurrent Protection

The overcurrent protection (OCP) setpoint is programmed

with resistor, ROCSET, that is connected across the OCSET

and PHASE pins.

Figure 26 shows the overcurrent-set circuit for SMPS1. The

inductor consists of inductance L and the DC resistance

(DCR). The inductor DC current IL creates a voltage drop

across DCR, which is given by Equation 6:

The ISL62391, ISL62392 sinks a 10µA current into the

OCSET1 pin, creating a DC voltage drop across the resistor

ROCSET, which is given by Equation 7:

Resistor RO is connected between the OUT1 pin and the

actual output voltage of the converter. During normal

operation, the OUT1 pin is a high impedance path, therefore

there is no voltage drop across RO. The DC voltage

difference between the OCSET1 pin and the OUT1 pin can

be established using Equation 8:

The ISL62391, ISL62392 monitors the OCSET1 pin and the

OUT1 pin voltages. Once the OCSET1 pin voltage is higher

than the OUT1 pin voltage for more than 10µs, the ISL62391,

ISL62392 declares an OCP fault. The value of ROCSET is

then written as Equation 9:

Where:

-ROCSET (Ω) is the resistor used to program the

overcurrent setpoint

-IOC is the output current threshold that will activate the

OCP circuit

- DCR is the inductor DC resistance

For example, if IOC is 20A and DCR is 4.5mΩ, the choice of

ROCSET is ROCSET = 20A x 4.5mΩ/10µA = 9kΩ.

Resistor ROCSET and capacitor CSEN form an R-C network

to sense the inductor current. To sense the inductor current

correctly, not only in DC operation but also during dynamic

operation, the R-C network time constant ROCSET-CSEN

FIGURE 26. OVERCURRENT-SET CIRCUIT

PHASE1

ROCSET

CSEN

OCSET1

OUT1

ISL62391,

DCR

10¬µ

VDCR

VROCSET

ISL62392

DCR

L DC R

•

(EQ. 6)

ROCSET

μAR

OCSET

•

(EQ. 7)

OCSET1 V

–

OUT1

L DC R

•

μAR

OCSET

•

–

(EQ. 8)

(EQ. 9)

OCSET

OC DCR

•

μA

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ISL62391, ISL62392

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