ISL8560 View Datasheet(PDF) - Intersil

Part Name

Description

View to exact match

ISL8560

DC/DC Power Switching Regulator

Intersil 

ISL8560

Fault Protection

The ISL8560 monitors the output of the regulator for

overcurrent and undervoltage events. The ISL8560 also

provides protection from excessive junction temperatures.

Overcurrent Protection

The overcurrent function protects the switching converter from

a shorted output by monitoring the current flowing through the

upper MOSFETs.

Upon detection of overcurrent condition, the upper MOSFET

will be immediately turned off and will not be turned on again

until the next switching cycle. Upon detection of the initial

overcurrent condition, the overcurrent fault counter is set to

1 and the overcurrent condition flag is set from LOW to

HIGH. If, on the subsequent cycle, another overcurrent

condition is detected, the OC fault counter will be

incremented. If there are seven sequential OC fault

detections, the regulator will be shut down under an

overcurrent fault condition. 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 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 14% below the

internal reference voltage and the overcurrent condition flag

is LOW, then the regulator will be shut down immediately

under an undervoltage fault condition. 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 and soft-start is

attempted again.

Thermal Protection

If the ISL8560 IC junction temperature reaches a nominal

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

ISL8560 will not re-enable the regulator until the junction

temperature drops below +135°C.

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 hump, as with the sag, can be attributed

to the total amount of capacitance on the output. Figure 29

shows a typical response to a load transient.

VOUT

ΔVHUMP

ΔVESR

ΔVSAG

ΔVESL

IOUT

Itran

FIGURE 29. TYPICAL TRANSIENT RESPONSE

12

FN9244.7

September 19, 2008

Share Link: