SC4508 查看數據表（PDF） - Semtech Corporation
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SC4508 Datasheet PDF : 10 Pages
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SC4508
POWER MANAGEMENT
Application Information
The SC4508 is designed to control buck (step down) or
buck-boost (inverting) converter with P-channel MOSFET
as a switch using current mode, programmable switching
frequency architecture. During steady state operation,
the switch is turned on each cycle and turned off when
the voltage across current sense resistor exceeds the
voltage level at COMP pin set by voltage loop error
amplifier. A fixed 0.5V artificial ramp is added internally
to the sensed current signal for operations when duty-
cycle is larger than 50%. In over load or output shortage
condition, if the sensed current signal reaching typical
120mV, the switch is turned off immediately in the same
cycle. If the sensed current signal further increases to
typical 160mV, not only the switch is turned off but also
the soft start capacitor is discharged by a internal MOSFET
to ground then charging back to threshold 250mV during
which the switch is held off. With the “hiccup” mode over
current protection, the thermal stress is reduced in the
faulty conditions.
Current Sense and Current Limit
The typical cycle-by-cycle current limit threshold in the
current sense pin of the SC4508 is 120mV. The over
current limit is assumed typical 120% of full load current.
Then the current sense resistor can be calculated by the
following equation:
120mV
Rs = 120% • IL (pk)
IL (pk)
=
IO
+
1 VIN − VO ( VO + VD )
2 fs • L VIN + VD
for
Buck
IL (pk) = IO
VIN + VO
VIN
+ VD
+
1
2
VIN
fs • L
(
VO + VD
VIN + VO + VD
)
for Buck − Boost
IO - full load current
VO - output voltage
VIN - input voltage
VD - diode forward voltage drop
fS - switching frequency
L - inductor
Inductor Selection
A inductor is chosen based on the required output ripple
current which is usually 20-30% of load current. The ripple
current ∆I decreases with higher inductance and
increases with higher input voltage VIN or output voltage
V
O.
L=
VIN − VO
fs • ∆IL
(
VO
VIN
+
+
VD
VD
)
for
Buck
L
=
VIN
fs • ∆IL
( VO + VD
VIN + VO + VD
)
for
Buck-Boost
Specify the maximum inductor current larger than IL(pk)
set by the current sense resistor RS to avoid the inductor
core saturation.
Input Capacitor CIN and Output Capacitor COUT Selection
Both input and output capacitors need to be sized to
handle the ripple current safely. Buck converters have
high ripple current in the input side while buck-boost
converters have high ripple current in the both input and
output capacitors. Therefore, the RMS value of the
current must be less than the high frequency ripple rating
of the capacitors. In continuous mode operation,
IRMS _INPUT _ CAP ≈ IO
VO (VIN − VO )
VIN
for Buck
IRMS _ CAP ≈ IO
capacitors.
VO + VD
VIN
for Buck-Boost input and output
The C is selected for Buck converter is by required
outputOUrTipple voltage and converter loop stability. The
output ripple is determined by:
∆VO
≈
∆IL (ESR
+
1
8fsCOUT
)
where f is converted switching frequency, C is output
capacitaSnce and ∆I is inductor ripple currenOUtT.
L
2004 Semtech Corp.
6
www.semtech.com
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