SC4501 查看數據表（PDF） - Semtech Corporation
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SC4501 Datasheet PDF : 21 Pages
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SC4501
POWER MANAGEMENT
Application Information
Duty Cycle
The duty cycle D of a boost converter is:
D
=
1
1
−
−
VIN
VOUT + VD
VCESAT
VOUT + VD
(3)
where VCESAT is the switch saturation voltage and VD is
voltage drop across the rectifying diode.
It is worth noting that IOUTMAX is directly proportional to the
ratio
VIN
VOUT
.
Equation
(4)
over-estimates
the
maximum
output current at high frequencies (>1MHz) since
switching losses are neglected in its derivation.
Nevertheless it is a useful first-order approximation.
Using VCESAT = 0.3V, VD = 0.5V and ILIM = 2A in (3) and (4),
the maximum output currents for three VIN and VOUT
combinations are shown in Table 1.
Maximum Output Current
In a boost switching regulator the inductor is connected
to the input. The DC inductor 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 (Figure 5). The output current available from a
boost converter therefore depends on the converter
operating duty cycle. The power switch current in the
SC4501 is internally limited to 2A. This is also the maximum
inductor or the 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:
( ) IOUTMAX
=
ILIMVIN
VOUT
1
−
D
45
−
VD
−D
VD − VCESAT
VIN
(4)
where ILIM is the switch current limit.
Inductor Current
ON
OFF
ON
IIN
Switch Current
VIN ( V )
2.5
3.3
5
VOUT ( V )
12
5
12
D
0.820
0.423
0.615
IOUTMAX ( A )
0.35
1.14
0.76
Table 1. Calculated Maximum Output Current [ Equation (4)]
Considerations for High Frequency Operation
The operating duty cycle of a boost converter decreases as
VIN approaches VOUT. The PWM modulating ramp in a
current-mode switching regulator is the sensed current ramp
of the control switch. This current ramp is absent unless
the switch is turned on. The intersection of this ramp with
the output of the voltage feedback error amplifier
determines the switch pulse width. The propagation delay
time required to immediately turn off the switch after it
is turned on is the minimum switch on time. Regulator
closed-loop measurement shows that the SC4501 has
a minimum on time of about 150ns at room temperature.
The power switch in the SC4501 is either not turned on
at all or for at least 150ns. If the required switch on time
is shorter than the minimum on time, the regulator will
either skip cycles or it will start to jitter.
Diode Current
DT (1-D)T
ON
OFF
ON
OFF
ON
I OUT
Figure 5. Current Waveforms in a Boost Regulator
Example: Determine the maximum operating frequency
of a Li-ion cell to 5V converter using the SC4501.
Assuming that VD=0.5V, VCESAT=0.3V and VIN=2.6 - 4.2V,
the minimum duty ratio can be found using (3).
DMIN
=
1−
1−
4.2
5 0+.03.5
5 + 0.5
=
0.25
© 2005 Semtech Corp.
9
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