a8291 Allegro MicroSystems, Inc., a8291 Datasheet - Page 7

• Manufacturer Part Number: a8291
• Description: A8291 Single Lnb Supply And Control Voltage Regulator
• Manufacturer: Allegro MicroSystems, Inc.

A8291
Protection
The A8291 has a wide range of protection features and fault diag-
nostics which are detailed in the Status Register section.
Boost Converter/Linear Regulator
The A8291 solution 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
is greater than the output voltage, V
drop the differential voltage. When operating in these conditions,
care must be taken to ensure that the safe operating temperature
range of the A8291 is not exceeded.
the internal 22 kHz tone frequency. All the loop compensation,
current sensing, and slope compensation functions are provided
internally.
the boost 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 A8291 must be reenabled
for normal operation. The system should provide sufficient time
between successive restarts to limit internal power dissipation; a
minimum of 2 s is recommended.
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 pos-
sible 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 programmed voltage). The output with the highest voltage
will effectively turn off the other outputs. As soon as this voltage
is reduced below the value of the other outputs, the A8291 output
will auto-recover to their programmed levels.
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
The boost converter operates at 352 kHz typical: 16 times
The A8291 has internal pulse-by-pulse current limiting on
At extremely light loads, the boost converter operates in a
In the case that two or more set top box LNB outputs are con-
Generates a supply voltage above the internal
During either start-up, or when the output
Single LNB Supply and Control Voltage Regulator
LNB
, the linear regulator must
Functional Description
BOOST
,
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
ably 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 V 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 A8291 will be disabled and the
OCP bit will be set.
Auto-Restart
After a short circuit condition occurs, the host controller should
periodically reenable the A8291 to check if the short circuit has
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
C
TCAP
TCAP
can be calculated using the following for-
× 6)(2 × C
In applications that have to operate at
however too large a value will prob-
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
, 10 nF, should provide
LIMLNB
TCAP
TCAP
.
,
to avoid
TCAP
:
7