ISL8502

FN6389 Rev 2.00

Page 14 of 19

June 29, 2010

restart in a hiccup mode with the delay between restarts being

4 soft-start periods. At the end of the fourth soft-start wait

period, the fault counters are reset, the EN pin is released, and

soft-start is attempted again. If the overcurrent condition goes

away prior to the OC Fault Counter reaching a count of four,

the Overcurrent Condition Flag will set back to LOW.

If the Overcurrent Condition Flag is HIGH and the Overcurrent

Fault Counter is less than four and an undervoltage event is

detected, the regulator will be shut down immediately.

UNDERVOLTAGE PROTECTION

If the voltage detected on the FB pin falls 18% below the internal

reference voltage and the overcurrent condition flag is LOW, then

the regulator will be shutdown immediately under an

Undervoltage Fault Condition and the EN pin will be pulled LOW.

An Undervoltage Fault Condition will result with the regulator

attempting to restart in a hiccup mode with the delay between

restarts being 4 soft-start periods. At the end of the fourth soft-

start wait period, the fault counters are reset, the EN pin is

released, and soft-start is attempted again.

THERMAL PROTECTION

If the ISL8502 IC junction temperature reaches a nominal

temperature of +150°C, the regulator will be disabled. The

ISL8502 will not re-enable the regulator until the junction

temperature drops below +130°C.

SHOOT-THROUGH PROTECTION

A shoot-through condition occurs when both the upper and lower

MOSFETs are turned on simultaneously, effectively shorting the

input voltage to ground. To protect from a

shoot-through condition, the ISL8502 incorporates specialized

circuitry, which insures that the complementary MOSFETs are not

ON simultaneously.

Application Guidelines

Operating Frequency

The ISL8502 can operate at switching frequencies from

500kHz to 1.2MHz. A resistor tied from the FS pin to ground is

used to program the switching frequency Equation 3.

Output Voltage Selection

The output voltage of the regulator can be programmed via an

external resistor divider that is used to scale the output voltage

relative to the internal reference voltage and feed it back to the

inverting input of the error amplifier. Refer to Figure 34.

the value chosen for the feedback resistor and the desired

output voltage of the regulator. The value for the feedback

resistor is typically between 1k

 and 10k.

Output Capacitor Selection

An output capacitor is required to filter the inductor current and

supply the load transient current. The filtering requirements are a

function of the switching frequency and the ripple current. The

load transient requirements are a function of the slew rate (di/dt)

and the magnitude of the transient load current. These

requirements are generally met with a mix of capacitors and

careful layout.

High frequency capacitors initially supply the transient and slow

the current load rate seen by the bulk capacitors. The bulk filter

capacitor values are generally determined by the ESR (Effective

Series Resistance) and voltage rating requirements rather than

actual capacitance requirements.

High frequency decoupling capacitors should be placed as close

to the power pins of the load as physically possible. Be careful not

to add inductance in the circuit board wiring that could cancel the

usefulness of these low inductance components. Consult with the

manufacturer of the load on specific decoupling requirements.

The shape of the output voltage waveform during a load transient

that represents the worst case loading conditions will ultimately

determine the number of output capacitors and their type. When

this load transient is applied to the converter, most of the energy

required by the load is initially delivered from the output

capacitors. This is due to the finite amount of time required for the

inductor current to slew up to the level of the output current

required by the load. This phenomenon results in a temporary dip

in the output voltage. At the very edge of the transient, the

Equivalent Series Inductance (ESL) of each capacitor induces a

spike that adds on top of the existing voltage drop due to the

Equivalent Series Resistance (ESR).

After the initial spike, attributable to the ESR and ESL of the

capacitors, the output voltage experiences sag. This sag is a

direct consequence of the amount of capacitance on the output.

During the removal of the same output load, the energy stored in

the inductor is dumped into the output capacitors. This energy

dumping creates a temporary hump in the output voltage. This



48000



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

=

(EQ. 3)



–

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

=

(EQ. 4)

FIGURE 33. TYPICAL TRANSIENT RESPONSE