SC4605 データシートの表示（PDF） - Semtech Corporation
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SC4605 Datasheet PDF : 19 Pages
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SC4605
POWER MANAGEMENT
Applications Information (Cont.)
The inductor value can be determined according to its
operating point and the switching frequency as follows:
L
=
VO ⋅ (VI − VO )
VI ⋅ fs ⋅ ∆I ⋅ IOMAX
Where:
fs = switching frequency and
DI = ratio of the peak to peak inductor current to the
maximum output load current.
The peak to peak inductor current is:
IP−P = ∆I • IOMAX
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.
IPEAK
= IOMAX
+
Ip −p
2
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:
PCOPPER
=
I2
LRMS
⋅ RWINDING
Where:
ILRMS is the RMS current in the inductor. This current
can be calculated as follows:
ILRMS = IOMAX ⋅
1+
1
3
⋅
∆I2
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
SC4605 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
SC4605 applications.
Boost Capacitor Selection
The boost capacitor selection is based on its discharge
ripple voltage, worst case condition time and boost cur-
rent. The worst case conduction time T can be estimated
as follows:
W
Tw
=
1
fs
⋅ Dmax
Where:
f = the switching frequency and
DSmax = maximum duty ratio, 0.97 for the SC4605.
The required minimum capacitance for boost capacitor
will be:
Cboost
=
IB
VD
⋅ TW
Where:
I = the boost current and
VB = discharge ripple voltage
D
With f = 300kH, V = 0.3V and I = 50mA, the required
capacSitance for theD boost capaciBtor is:
Cboost
=
IB
VD
⋅
1
fs
⋅ Dmax
=
0.05
0.3
⋅
1
300k
⋅
0.97
=
540nF
Input Capacitor Selection
The input capacitor selection is based on its ripple cur-
rent level, required capacitance and voltage rating. This
capacitor must be able to provide the ripple current by
the switching actions. For the continuous conduction
2004 Semtech Corp.
8
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