SC4614 데이터 시트보기 (PDF) - Semtech Corporation
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SC4614 Datasheet PDF : 14 Pages
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SC4614
POWER MANAGEMENT
Applications Information (Cont.)
termined by the output inductance and output capaci-
tance. The values of output inductor and output bulk
capacitors have to be properly selected so that the soft
start peak current does not exceed the load trip point of
the short circuit protection.
Internal LDO for Gate Drive
An internal LDO is designed in the SC4614 to lower the
12V supply voltage for gate drive. A 1uF external ce-
ramic capacitor connected in between DRV pin to the
ground is needed to support the LDO. The LDO output is
connected to the low gate drive internally, and has to be
connected to the high gate drive through an external
bootstrap circuit. The LDO output voltage is set at 8.2V.
The manufacture data and bench tested results show
that, for low Rdson MOSFETs run at applied load current,
the optimum gate drive voltage is around 8.2V, where
the total power losses of power MOSFETs are minimized.
COMPONENT SELECTION
General design guideline of switching power supplies can
be applied to the component selection for the SC4614.
Inductor and MOSFETs
The selection of inductor and MOSFETs should meet ther-
mal requirements because they are power loss dominant
components. Pick an inductor with as high inductance
as possible without adding extra cost and size. The higher
inductance, the lower ripple current, the smaller core loss
and the higher efficiency will be. However, too high in-
ductance slows down output transient response. It is rec-
ommended to choose the inductance that creates an
inductor ripple current of approximate 20% of maximum
load current. So choose inductor value from:
L
=
IO
5
× fosc
×VO
× (1- VO
VIN
)
The MOSFETs are selected by their Rdson, gate charge,
and package specifications. The SC4614 provides 1.5A
gate drive current and gives 50nC/1.5A=33ns switching
time for driving a 50nC gate charge MOSFET. The switch-
ing time ts contributes to the top MOSFET switching loss:
duction losses of the top and bottom MOSFETs are given
by:
PC _ TOP
=
I
2
O
× Rdson
×
D
PC _ BOT
=
I
2
O
× Rdson × (1- D)
If the requirement of total power losses for each MOSFET
is given, the above equations can be used to calculate
the values of Rdson and gate charge, then the devices
can be determined accordingly. The solution should en-
sure the MOSFET is within its maximum junction tem-
perature at highest ambient temperature.
Output Capacitor
The output capacitors should be selected to meet both
output ripple and transient response criteria. The output
capacitor ESR causes output ripple VRIPPLE during the
inductor ripple current flowing in. To meet output ripple
criteria, the ESR value should be:
RESR
<
L × fOSC ×VRIPPLE
VO
× (1-
VO
VIN
)
The output capacitor ESR also causes output voltage tran-
sient VT during a transient load current IT flowing in. To
meet output transient criteria, the ESR value should be:
RESR
<
VT
IT
To meet both criteria, the smaller one of above two ESRs
is required.
The output capacitor value also contributes to load tran-
sient response. Based on a worst case where the induc-
tor energy 100% dumps to the output capacitor during
the load transient, the capacitance then can be calcu-
lated by:
C
>
L
×
IT2
VT2
PS = IO ×VIN × tS × fOSC
There is no significant switching loss for the bottom
MOSFET because of its zero voltage switching. The con-
Input Capacitor
The input capacitor should be chosen to handle the RMS
ripple current of a synchronous buck converter. This value
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