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ISL8502

2A Synchronous Buck Regulator with Integrated MOSFETs

Intersil 

ISL8502

Overcurrent Fault Condition and the EN pin will be pulled

LOW. An Overcurrent 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. 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.

RT[kΩ] = -f-O-----S4----8C---0-[--k0----0H-----z---]-

(EQ. 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 output voltage programming resistor, R4, will depend on

the value chosen for the feedback resistor and the desired

output voltage of the regulator. The value for the feedback

resistor is typically between 1kΩ and 10kΩ.

R4

=

-----R----1-----×----0----.-6----V-------

VOUT – 0.6V

(EQ. 4)

If the output voltage desired is 0.6V, then R4 is left unpopulated.

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).

VOUT

DVHUMP

DVESR

DVSAG

DVESL

IOUT

ITRAN

FIGURE 33. TYPICAL TRANSIENT RESPONSE

14

FN6389.2

June 29, 2010

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