ISL8502 Intersil Corporation, ISL8502 Datasheet - Page 15

• Manufacturer Part Number: ISL8502
• Description: 2.5A Synchronous Buck Regulator
• Manufacturer: Intersil Corporation

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 through the
following Equation 3.
If the FS pin is left unconnected, the ISL8502 will default to a
500kHz switching frequency.
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 36.
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.
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.
R
4
R
T
=
[
kΩ
----------------------------------
V
R
OUT
]
1
=
×
–
0.6V
----------------------------- -
f
OSC
0.6V
48000
[
kHz
]
15
(EQ. 3)
(EQ. 4)
ISL8502
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 35 shows a
typical response to a load transient.
V
FIGURE 35. TYPICAL TRANSIENT RESPONSE
OUT
Δ V
ESR
Δ V
I
OUT
HUMP
Δ V
Δ V
SAG
ESL
I
tran
January 17, 2007
FN6389.0