a8292 Allegro MicroSystems, Inc., a8292 Datasheet - Page 7

• Manufacturer Part Number: a8292
• Description: Dual Lnb Supply And Control Voltage Regulator
• Manufacturer: Allegro MicroSystems, Inc.

A8292
Protection
The A8292 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
than the output voltage, V
differential voltage. When operating in these conditions, care
must be taken to ensure that the safe operating temperature range
of the A8292 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 600 mA typical, and the IC will
be shut down if the overcurrent condition lasts for more than 48
ms. If this occurs, the A8292 must be re-enabled for normal opera-
tion. The system should provide sufficient time between succes-
sive restarts to limit internal power dissipation; a period of 2 s is
recommended.
16 times the internal 22 kHz tone frequency. All the loop com-
pensation, current sensing, and slope compensation functions are
provided internally.
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 A8292 output will auto-
recover to their programmed levels.
Charge Pump.
tracking regulator output to drive the linear regulator control.
The A8292 has internal pulse-by-pulse current limit on the boost
Each of the boost converters operates at 352 kHz typical:
At extremely light loads, the boost converters operate in a
In the case that two or more set top box LNB outputs are con-
Each generates a supply voltage above the internal
LNB
, the linear regulator must drop the
Dual LNB Supply and Control Voltage Regulator
BOOST
Functional Description
, is greater
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 TCAP pin to GND (C
Schematic). 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 situa-
tion is when two set-top boxes are connected in parallel. If this is
the case, the following formula can be used to calculate C
maximum value of C
cause the voltage transition specification 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 to 13 V in a
reasonable amount of time.
ODT (Overcurrent Disable Time)
If the LNB output current exceeds 600 mA, typical, for more than
48 ms, then the LNB output will be disabled and the OCP bit will
be set.
Short Circuit Handling
If the LNB output is shorted to ground, the LNB output current
will be clamped to 600 mA, typical. If the short circuit condition
lasts for more than 48 ms, the A8292 will be disabled and the
OCP bit will be set.
Auto-Restart
After a short circuit condition occurs, the host controller should
periodically re-enable the A8292 to check if the short circuit has
been removed. Consecutive startup attempts should allow at least
2 s of delay between restarts.
The value of C
The minimum value of C
C
C
TCAP
TCAP
TCAP
≥ (I
≥ (10 μA × 6)(2 × 100 μF) / 500 mA = 24 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
however too large a value will probably
In applications that have to operate at
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
= (I
TCAP
TCAP
TCAP
is 2.2 nF. There is no theoretical
BOOST
or C7 in the Applications
× 6) / SR ,
TCAP
) / I
TCAP
, 10 nF, should provide
LIMLNB
.
TCAP
,
to avoid
TCAP
:
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