SC104EVB 查看數據表（PDF） - Semtech Corporation
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SC104EVB Datasheet PDF : 12 Pages
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SC104
POWER MANAGEMENT
Applications Information (Cont.)
Step 8: (Cont.)
inductor current ramps beyond the current limit trip point.
Since ILIM increases as RLIM decreases, this value is a
maximum. The maximum current limit resistor, RLIM(C), for
continuous mode is therefore:
( ) RLIM(C)
=
VLIM(MIN) • Ratio(MIN)
ILIM( C )
−
tplh
•
VIN − VCE(SAT )
L(C)
Where:
VLIM(MIN) = the minimum value of the current limit voltage
reference = 385mV
Ratio = the minimum value of the I to I ratio
(MIN)
LX
LIM
= 7100 A/A
tplh = propagation delay from reaching the current limit
trip point to the power switch turning off = 200ns
Schottky diode: any schottky diode rated for the
average and peak currents being seen in the circuit will
suffice. However a higher current rated schottky diode
will result in lower forward voltage drops and hence higher
efficiency. The selection of the schottky will depend upon
the optimum choice between efficiency, board space, and
cost.
Inductor: similarly, any inductor rated for the average and
peak currents required by the design and capable of
operating at the fixed off-time of 760ns will suffice, but
inductors with lower series resistance will result in lower
losses. The selection of the inductor will depend upon
the optimum choice between efficiency, board space, com-
ponent height, and cost. Toko D62LCB and D63LCB se-
ries work very well.
LED Dimming
Selecting the calculated value or the next lower
standard value is recommended.
Component Selection - General
Capacitor Selection: the SC104 has been designed to
be used with ceramic input and output capacitors. The
input to the device should be bypassed using a 4.7µF
ceramic capacitor rated for the maximum input voltage.
The output capacitor should be a ceramic capacitor in
the range of 0.22µF to 1µF. Care should be taken that
the voltage rating of this capacitor meets the output
voltage requriements, and if the part is going to be run
open circuit during assembly testing using the OVP
feature, then the capacitor should be rated 50V.
Dimming the LEDs (i.e. reducing the output current from
the set level to reduce brightness) can be achieved a
couple of ways:
1) PWMing the ADJ pin using an open drain or open
collector (with no pull-up). Using a PWM signal at this pin
will reduce the output current by alternating between OFF
(ADJ < 100mV) and switching normally. The lower the
duty cycle, the lower the output current. A PWM frequency
of 1kHz maximum is recommended.
2) analog voltage applied to the ADJ pin: apply a DC
voltage between 0V to 0.1V (OFF) and 1V (full current) to
the ADJ pin. This pin should not be pulled above 1V
under normal operation, and should never exceed the
absolute maximum rating. Using a DC voltage will result
in the modulation frequency of the inductor current ramp
reducing and care must be taken to ensure that this does
not become audible in sensitive applications.
2005 Semtech Corp.
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