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
The absolute maximum operating frequency of the
converter
is
therefore
DMIN
150ns
=
0.25
150ns
=
1.67MHz
.
The
actual operating frequency needs to be lower to allow for
modulating headroom.
The power transistor in the SC4501 is turned off every
switching period for an interval determined by the
discharge time of the oscillator ramp and the propagation
delay of the power switch. This minimum off time limits
the maximum duty cycle of the regulator at a given
switching frequency. A boost converter with high
VOUT
VIn
ratio
requires long switch on time and high duty cycle. If the
required duty cycle is higher than the attainable 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.)
The minimum off times of closed-loop boost converters set
to various output voltages were measured by lowering their
input voltages until dropout occurs. It was found that the
minimum off time of the SC4501 ranged from 80 to 110ns
at room temperature.
∆ IL
=
D(VIN
) − VCESAT
fL
(5)
where f is the switching frequency and L is the inductance.
Substituting (3) into (5) and neglecting VCESAT ,
∆IL
=
VIN
fL
1
−
VIN
VOUT +
VD
(6)
In current-mode control, the slope of the modulating
(sensed switch current) ramp should be steep enough to
lessen jittery tendency but not so steep that large flux swing
decreases efficiency. Inductor ripple current ∆IL between
25-40% of the peak inductor current limit is a good
compromise. Inductors so chosen are optimized in size
and DCR. Setting ∆IL = 0.3•(2) = 0.6A, VD=0.5V in (6),
L
=
VIN
f∆IL
1
−
VIN
VOUT +
VD
=
VIN
0.6f
1
−
VIN
VOUT + 0.5
(7)
where L is in µH and f is in MHz.
Equation (6) shows that for a given VOUT, ∆IL is the highest
when
VIN
=
(VOUT +
2
VD ) .
If VIN
varies over
a wide range,
then
choose L based on the nominal input voltage.
Beware of dropout when operating at very low input voltages
(1.5-2V) and with off times approaching 110ns. 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
SC4501 is duty-cycle and input voltage invariant and is
typically 2.8A. If the switch current limit is not at least 2A,
then the converter is likely in dropout. The switching
frequency should then be lowered to improve controllability.
Both the minimum on time and the minimum off time
reduce control range of the PWM regulator. Bench
measurement showed that reduced modulating range
started to be a problem at frequencies over 2MHz. Although
the oscillator is capable of running well above 2MHz,
controllability limits the maximum operating frequency.
Inductor Selection
The inductor ripple current ∆IL of a boost converter
operating in continuous-conduction mode is
The saturation current of the inductor should be 20-30%
higher than the peak current limit (2.8A). Low-cost powder
iron cores are not suitable for high-frequency switching
power supplies due to their high core losses. Inductors
with ferrite cores should be used.
Input Capacitor
The input current in a boost converter is the inductor
current, which is continuous with low RMS current ripples.
A 2.2-4.7µF ceramic input capacitor is adequate for most
applications.
Output Capacitor
Both ceramic and low ESR tantalum capacitors can be
used as output filtering capacitors. Multi-layer ceramic
capacitors, due to their extremely low ESR (<5mΩ), are
the best choice. Use ceramic capacitors with stable
temperature and voltage characteristics. One may be
tempted to use Z5U and Y5V ceramic capacitors for
output filtering because of their high capacitance and
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