8284 Datasheet, PDF (5/8 Pages) Allegro MicroSystems – TWO-OUTPUT LNB SUPPLY AND CONTROL-VOLTAGE REGULATOR

8284
TWO-OUTPUT LNB SUPPLY AND
CONTROL-VOLTAGE REGULATOR
FUNCTIONAL DESCRIPTION
The ENT (Tone Enable) terminal activates the internal
tone signal, modulating the dc output with a ±0.3 V, 22
kHz symmetrical waveform. The internal oscillator is
factory trimmed to provide a tone of 22 kHz ± 2 kHz. No
further adjustment is required. The internal oscillator
operates the buck converter at 16 times the tone fre-
quency.
Burst coding of the 22 kHz tone can be accomplished,
due to the fast response of the ENT input and rapid tone
response. This allows implementation of the DiSEqC™
protocols.
To improve design flexibility and to allow implemen-
tation of proposed LNB remote control standards, an
analog modulation input terminal is available (EXTM).
An appropriate dc blocking capacitor must be used to
couple the modulating signal source to the EXTM termi-
nal. If external modulation is not used, the EXTM termi-
nal can be left open.
The output linear regulators will sink and source
current. This feature allows full modulation capability
into capacitive loads as high as 0.25 µF.
The programmed output voltage rise and fall times
can be set by an internal 25 kΩ resistor and an external
capacitor located on the TCAP terminal. Although any
value of capacitor is permitted, practical values are
typically 0.001 µF to 0.02 µF. This feature only affects
the turn on and programmed voltage rise and fall times.
Modulation is unaffected by the choice of TCAP. This
terminal can be left open if voltage rise and fall time
control is not required.
Two terminals are dedicated to the over-current
protection/monitoring: SENSE and OLF. The LNB output
is current limited. The short-circuit protection threshold is
set by the value of an external resistor, R_S, between
terminals 3 and 8. R_S = V_OM(th)/I_OM where V_OM(th) is the
current-limiting threshold voltage and I_OM is the desired
current limit value. The minimum recommended value
for R_S is 0.17 Ω.
In operation, the short-circuit protection produces
current fold-back at the input due to the tracking con-
verter. If the output is shorted the linear regulator will
limit the output current to I_OM. The tracking converter
will maintain a constant voltage drop of 0.8 V across the
linear regulator. This condition results in typically 550
mW dissipation (I_OM • 0.8 V). Short-circuit or thermal
shutdown activation will cause the OLF terminal, an open-
drain diagnostic output flag, to go LOW.
Thermal resistance:
DIP — R_θJA = 40°C/W, R_θJT = 6°C/W,
SOIC — R_θJA = 55°C/W, R_θJT = 6°C/W.
The device junction temperature should be kept below
150°C. Thermal shut-down circuitry turns off the device
if junction temperature exceeds +165°C typically.
The products described here are manufactured under one or more
U.S. patents or U.S. patents pending.
Allegro MicroSystems, Inc. reserves the right to make, from time to
time, such departures from the detail specifications as may be
required to permit improvements in the performance, reliability, or
manufacturability of its products. Before placing an order, the user is
cautioned to verify that the information being relied upon is current.
Allegro products are not authorized for use as critical components
in life-support devices or systems without express written approval.
The information included herein is believed to be accurate and
reliable. However, Allegro MicroSystems, Inc. assumes no responsi-
bility for its use; nor for any infringement of patents or other rights of
third parties which may result from its use.
www.allegromicro.com
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