SC4503 데이터 시트보기 (PDF) - Semtech Corporation
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SC4503 Datasheet PDF : 22 Pages
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
Duty Cycle
The duty cycle D of a boost converter in continuous-conduc-
tion mode (CCM) is:
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=
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−
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(3)
where VCESAT is the switch saturation voltage and VD is volt-
age drop across the rectifying diode.
where ILIM is the switch current limit.
It is worth
ratio
9,1
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noting that IOUTMAX is
and that switching
directly proportional to the
losses are neglected in its
derivation. Equation (4) therefore over-estimates the
maximum output current, however it is a useful first-order
approximation.
Using VCESAT = 0.3V, VD = 0.5V and ILIM =1.4A in (3) and (4),
the maximum output current for three V and V combi-
IN
OUT
nations are tabulated (Table 1).
Maximum Output Current
In a boost switching regulator the inductor is connected
to the input. The inductor DC current is the input current.
When the power switch is turned on, the inductor current
flows into the switch. When the power switch is off, the
inductor current flows through the rectifying diode to the
output. The output current is the average diode current.
The diode current waveform is trapezoidal with pulse width
(1 – D)T (see Figure 4). The output current available from
Inductor
Current
0
ON
OFF
ON
IIN
Switch Current
Diode Current
DT (1-D)T
ON
OFF
ON
OFF
ON
0
IOUT
Figure 4. Current Waveforms in a Boost Converter
a boost converter therefore depends on the converter oper-
ating duty cycle. The power switch current in the SC4503 is
internally limited to at least 1.4A. This is also the maximum
peak inductor or the peak input current. By estimating the
conduction losses in both the switch and the diode, an
expression of the maximum available output current of a
boost converter can be derived:
VIN (V)
3.3
3.3
5
VOUT (V)
12
5
12
D
0.754
0.423
0.615
IOUT (A)
0.34
0.80
0.53
Table 1. Calculated Maximum Output Currents
Maximum Duty-Cycle Limitation
The power transistor in the SC4503 is turned off every
switching period for 80ns. This minimum off time limits the
maximum duty cycle of the regulator. A boost converter with
high
9287
9,1
ratio requires long switch on time and high duty
cycle. If the required duty cycle is higher than the attain-
able maximum, then the converter will operate in dropout.
(Dropout is a condition in which the regulator cannot attain
its set output voltage below current limit.)
Note: dropout can occur when operating at low input volt-
ages (<3V) and with off times approaching 100ns. Shorten
the PCB trace between the power source and the device
input pin, as line drop may be a significant percentage of
the input voltage. A regulator in dropout may appear as
if it is in current limit. The cycle-by-cycle current limit of
the SC4503 is duty-cycle and input voltage invariant and
should be at least 1.4A. If the converter output is below
its set value and switch current limit is not reached (1.4A),
then the converter is likely in dropout.
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2007 Semtech Corp.
Example: Determine the highest attainable output voltage
(4) when boosting from a single Li-ion cell.
Equation (3) can be re-arranged as:
9
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