ISL8560 查看數據表（PDF） - Renesas Electronics

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ISL8560

DC/DC Power Switching Regulator

Renesas Electronics 

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

FN9244 Rev 7.00

September 19, 2008

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

VHUMP

VESR

VSAG

VESL

IOUT

Itran

FIGURE 29. TYPICAL TRANSIENT RESPONSE

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