SC4607
POWER MANAGEMENT
Application Information (Cont.)
The peak to peak inductor current is:
current. As shown in Figure 4, t
top MOSFET off to the bottom MOSFET on is adaptive by
the delay from the
d1,
Ip−p = ∆I• IOMAX
detecting the voltage of the phase node. t , the delay
from the bottom MOSFET off to the top MOSFET on is
d2
fixed, is 40ns for the SC4607. This control scheme guar-
antees avoidance of cross conduction or shoot through
between the upper and lower MOSFETs and also mini-
mizes the conduction loss in the body diode of the bot-
tom MOSFET for high efficiency applications.
After the required inductor value is selected, the proper
selection of the core material is based on the peak in-
ductor current and efficiency requirements. The core
must be able to handle the peak inductor current IPEAK
without saturation and produce low core loss during the
high frequency operation is:
Ip−p
2
IPEAK = IOMAX
+
TOP MOSFET Gate Drive
BOTTOM MOSFET Gate Drive
The power loss for the inductor includes its core loss and
copper loss. If possible, the winding resistance should
be minimized to reduce inductor’s copper loss. The core
loss can be found in the manufacturer’s datasheet. The
inductor’ copper loss can be estimated as follows:
Ground
Phase node
td2
td1
PCOPPER = I2
RWINDING
LRMS
Figure 4. Timing Waveforms for Gate Drives and Phase
Node
Where:
ILRMS is the RMS current in the inductor. This current can
be calculated as follow is:
Inductor Selection:
1
3
ILRMS = IOMAX 1+
∆I2
The factors for selecting the inductor include its cost,
efficiency, size and EMI. For a typical SC4607 applica-
tion, the inductor selection is mainly based on its value,
saturation current and DC resistance. Increasing the in-
ductor value will decrease the ripple level of the output
voltage while the output transient response will be de-
graded. Low value inductors offer small size and fast tran-
sient responses while they cause large ripple currents,
poor efficiencies and more output capacitance to smooth
out the large ripple currents. The inductor should be able
to handle the peak current without saturating and its
copper resistance in the winding should be as low as
possible to minimize its resistive power loss. A good trade-
off among its size, loss and cost is to set the inductor
ripple current to be within 15% to 30% of the maximum
output current.
Output Capacitor Selection:
Basically there are two major factors to consider in se-
lecting the type and quantity of the output capacitors.
The first one is the required ESR (Equivalent Series Re-
sistance) which should be low enough to reduce the volt-
age deviation from its nominal one during its load changes.
The second one is the required capacitance, which should
be high enough to hold up the output voltage. Before the
SC4607 regulates the inductor current to a new value
during a load transient, the output capacitor delivers all
the additional current needed by the load. The ESR and
ESL of the output capacitor, the loop parasitic inductance
between the output capacitor and the load combined
with inductor ripple current are all major contributors to
the output voltage ripple. Surface mount speciality poly-
mer aluminum electrolytic chip capacitors in UE series
from Panasonic provide low ESR and reduce the total
capacitance required for a fast transient response.
POSCAP from Sanyo is a solid electrolytic chip capacitor
that has a low ESR and good performance for high fre-
quency with a low profile and high capacitance. Above
mentioned capacitors are recommended to use in
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The inductor value can be determined according to its
operating point and the switching frequency as follows:
Vout (V − Vout
)
in
L =
V
fs ∆I IOMAX
in
Where:
fs = switching frequency and
∆I = ratio of the peak to peak inductor current to the
maximum output load current.
2005 Semtech Corp.
10
SEMTECH [ SEMTECH CORPORATION ]
