a8286 Allegro MicroSystems, Inc., a8286 Datasheet - Page 7

• Manufacturer Part Number: a8286
• Description: Dual Lnb Supply And Control Voltage Regulator
• Manufacturer: Allegro MicroSystems, Inc.

A8286
Protection
The A8286 has a wide range of protection features and fault diag-
nostics which are detailed in the Status Register section.
Boost Converter/Linear Regulator
Each channel contains a tracking current-mode boost converter
and linear regulator. The boost converter tracks the requested
LNB voltage to within 800 mV, to minimize power dissipation.
Under conditions where the input voltage, V
the output voltage, V
ferential voltage. When operating in these conditions, care must
be taken to ensure that the safe operating temperature range of the
A8286 is not exceeded.
converter, and DC current limiting on the LNB output, to protect
the IC against short circuits. When the LNB output is shorted, the
LNB output current is limited to 900 mA typical, and the IC will
be shut down if the overcurrent condition lasts for more than 48
ms. If this occurs, the A8286 must be re-enabled for normal opera-
tion. The system should provide sufficient time between successive
restarts to limit internal power dissipation; a period of 5 s is recom-
mended.
16 times the internal 22 kHz tone frequency. All the loop compen-
sation, current sensing, and slope compensation functions are pro-
vided internally.
a pulse-skipping mode. Pulse skipping occurs when the BOOST
voltage rises to approximately 450 mV above the BOOST target
output voltage. Pulse skipping stops when the BOOST voltage
drops 200 mV below the pulse skipping level.
nected together by the customer (e.g., with a splitter), it is possible
that one output could be programmed at a higher voltage than the
other. This would cause a voltage on one output that is higher than
its programmed voltage (e.g., 19 V on the output of a 13 V pro-
grammed voltage). The output with the highest voltage will effec-
tively turn off the other outputs. As soon as this voltage is reduced
below the value of the other outputs, the A8286 output will auto-
recover to their programmed levels.
The A8286 has internal pulse-by-pulse current limit on the boost
Each of the boost converters operates at 352 kHz typical:
At extremely light loads, and the boost converters operate in
In the case that two or more set top box LNB outputs are con-
LNB
, the linear regulator must drop the dif-
Dual LNB Supply and Control Voltage Regulator
BOOST
Functional Description
, is greater than
Charge Pump.
tracking regulator output to drive the linear regulator control.
Slew Rate Control.
voltage at the LNB pin is transitioning, the output voltage rise and
fall times can be set by the value of the capacitor connected from
the TCAP1 pin to GND (C
matic). Note that during start-up, the BOOST pin is pre-charged to
the input voltage minus a voltage drop. As a result, the slew rate
control for the BOOST pin occurs from this voltage.
mula:
where SR is the required slew rate of the LNB output voltage,
in V/s, and I
sheet. The recommended value for C
satisfactory operation for most applications. However, in some
cases, it may be necessary to increase the value of C
activating the current limit of the LNB output.
One such situation is when two set-top boxes are connected in
parallel. If this is the case, the following formula can be used to
calculate a larger value for C
maximum value of C
bly cause the voltage transition specifications to be exceeded. Tone
generation is unaffected by the value of C
Pull-Down Rate Control.
very light loads and that require large load capacitances (in the
order of tens to hundreds of microfarads), the output linear stage
provides approximately 40 mA of pull-down capability. This
ensures that the output volts are ramped from 18 V to 13 V in a
reasonable amount of time.
The value of C
The minimum value of C
C
C
TCAP
TCAP
TCAP
Each generates a supply voltage above the internal
≥ (I
≥ (10 μA × 6)(2 × 100 μF) / 800 mA = 15 nF .
is the TCAP pin current specified in the data-
TCAP
TCAP
During either start-up, or when the output
C
TCAP
TCAP
can be calculated using the following for-
× 6)(2 × C
In applications that have to operate at
TCAP
, however, too large a value will proba-
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
= (I
TCAP
TCAP
TCAP
or C7 in the Applications Sche-
:
is 2.2nF. There is no theoretical
BOOST
TCAP
× 6) / SR ,
, 10 nF, should provide
) / I
TCAP
LIMLNB
.
TCAP
,
to avoid
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