a8291 Allegro MicroSystems, Inc., a8291 Datasheet - Page 8

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

Table 1. Detection Thresholds for Tone Generation Options
A8291
been removed. Consecutive startup attempts should allow at least
2 s of delay between restarts.
In-Rush Current
At start-up or during an LNB reconfiguration event, a tran-
sient surge current above the normal DC operating level can be
provided by the A8291. This current increase can be as high as
600 mA, typical, for as long as required, up to a maximum of
48 ms.
Tone Detection
A 22 kHz tone detector is provided in the A8291 solution. The
detector extracts the tone signal and provides it as an open-drain
signal on the TDO pin. The maximum tone out error is ±1 tone
cycle, and the maximum tone out delay with respect to the input
is 1 tone cycle. Detection thresholds are given in table 1.
Tone Generation
The A8291 solution offers four options for tone generation,
providing maximum flexibility to cover every application. The
EXTM pin (external modulation), in conjunction with the I
control bits: TMODE (tone modulation) and TGATE (tone gate),
provide the necessary control. The TMODE bit controls whether
the tone source is either internal or external (via the EXTM pin).
Both the EXTM pin and TGATE bit determine the 22 kHz con-
trol, whether gated or clocked.
that when using option 4, when EXTM stops clocking, the LNB
volts park at the LNB voltage, either plus or minus half the tone
signal amplitude, depending on the state of EXTM. For example,
if the EXTM is held low, the LNB dc voltage is the LNB pro-
grammed voltage minus 325 mV (typical).
TDET is set in the status register. When the internal tone is used
Guaranteed
Threshold
Threshold
Detection
Rejection
TMODE
TGATE
(Fig. 1)
(mV
(mV
Option
EXTM
Four options for tone generation are shown in figure 1. Note
With any of the four options, when a tone signal is generated,
PP
PP
)
)
Control
400
100
0/1
1
1
1
Control
400
100
0/1
2
1
1
Transmit
logic signal,
continuous
Control
22 kHz
300
100
0/1
3
0
Single LNB Supply and Control Voltage Regulator
Control
22 kHz
signal
gated
logic
300
100
4
0
1
tone transmission
At least one must
be 0 to prevent
n.a.
300
100
0
Receive
n.a.
400
100
2
1
C™
(options 1 or 2), the minimum tone detect amplitude is 400 mV,
and when an external tone is used (options 3 or 4), the minimum
tone detection amplitude is 300 mV.
DiSEqC™ Bypass MOSFET
A pair of N-channel MOSFETs are connected in parallel (source
to drain and drain to source) to provide a low source output
impedance during tone transmission.
tive low) or by setting the BFC2 bit to 1 in the Control register.
When the BFC pin is used instead of I
latched; a logic high or low turns the FET off or on. When the
Figure 1. Options for tone generation
Option 1 – Use internal tone, gated by the TGATE bit.
Option 2 – Use internal tone, gated by the EXTM pin.
Option 3 – Use external tone, gated by the TGATE bit.
Option 4 – Use external tone.
The MOSFETs are enabled either via the BFC input pin (ac-
TMODE
TGATE
Tone
(LNB Ref)
TMODE
TMODE
TMODE
EXTM
EXTM
TGATE
Tone
(LNB Ref)
EXTM
TGATE
Tone
(LNB Ref)
EXTM
TGATE
Tone
(LNB Ref)
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
LNB (V)
LNB (V)
LNB (V)
LNB (V)
2
C™ control, it is not
8