A8286SETTR-TB ALLEGRO [Allegro MicroSystems], A8286SETTR-TB Datasheet - Page 7

• Manufacturer Part Number: A8286SETTR-TB
• Description: Dual LNB Supply and Control Voltage Regulator
• Manufacturer: ALLEGRO [Allegro MicroSystems]

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 750 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. If the ODT timer
is enabled, 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 operation. It is strongly recommended that the
ODT timer be enabled at all times. The system should provide suf-
ficient time between successive restarts to limit internal power dis-
sipation; a period of 5 s is recommended.
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
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
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.
Charge Pump.
tracking regulator output to drive the linear regulator control.
Slew Rate Control.
voltage on the BOOST or the LNB pins is changing, the output
voltage rise and fall times can be set by the value selected for the
external capacitor (C7) connected to the TCAP1 pin. Note that
during start-up, the BOOST pin is precharged to the input voltage
minus a diode drop. As a result, the slew rate control occurs from
this point.
formula:
where SR (V/s) is the required slew rate. The minimum value for
C
this calculates to a slew rate of 10.7 V/ms.
quired to limit the LNB output voltage rise and fall times, then C7
must have a value of at least 2.2 nF to minimize output noise.
of the A8286.
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.
TCAP
The rating for C7, C
This information applies to channel 2, as well as to channel 1
Modulation is unaffected by the choice of C7. If it is not re-
is 2.2 nF. Using the recommended value of C
Each generates a supply voltage above the internal
During either start-up, or when the output
C
TCAP
TCAP
In applications that have to operate at
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
, can be calculated using the following
= (I
TCAP
× 6) / SR
TCAP
, 5.6 nF,
7