isl8560 Intersil Corporation, isl8560 Datasheet - Page 12

• Manufacturer Part Number: isl8560
• Description: Dc/dc Power Switching Regulator
• Manufacturer: Intersil Corporation

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.
12
ISL8560
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.
FIGURE 29. TYPICAL TRANSIENT RESPONSE
V
OUT
ΔV
ESR
ΔV
I
OUT
HUMP
ΔV
ΔV
SAG
ESL
I
tran
September 18, 2007
FN9244.6