A8296SESTR-T Allegro Microsystems Inc, A8296SESTR-T Datasheet - Page 7

• Manufacturer Part Number: A8296SESTR-T
• Description: IC REG LNB SGL SUPPLY 16-QFN
• Manufacturer: Allegro Microsystems Inc

Specifications of A8296SESTR-T

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

A8296
Protection
The A8296 has a wide range of protection features and fault diag-
nostics which are detailed in the Status Register section.
Boost Converter/Linear Regulator
The A8296 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 A8296 is not exceeded.
The boost converter operates at 352 kHz typical: 16 times the
internal 22 kHz tone frequency. All the loop compensation,
current sensing, and slope compensation functions are provided
internally.
The A8296 has internal pulse-by-pulse current limiting on 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 and the IC will be shut
down if the overcurrent condition lasts for more than 48 ms. If
this occurs, the A8296 must be reenabled for normal operation.
The system should provide sufficient time between successive
restarts to limit internal power dissipation; a minimum of 5 s is
recommended.
At extremely light loads, the boost converter operates in 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.
In the case that two or more set top box LNB outputs are con-
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 A8296 output
will auto-recover to their programmed levels.
Single LNB Supply and Control Voltage Regulator
LNB
, the linear regulator must
Functional Description
BOOST
,
Charge Pump.
tracking regulator output to drive the linear regulator control.
LNB and BOOST Current Limits.
I
ISET pin to GND as shown in the applications schematic. The
LNB current limit can be set from 300 to 500 mA, correspond-
ing to an R
current limit is exceeded for more than the Overcurrent Disable
Time (t
set, as shown in figure 1. The LNB output current limit can be
set as high as 600 mA (R
not to exceed the thermal limit of the package or thermal shut-
down (TSD) will occur. The A8296 is guaranteed to support I
≥ 425 mA continuously at 70ºC ambient with V
V
according to the following equation:
where I
the ISET pin is 0 V (that is, shorted to GND), I
be clamped to a moderately high value (approximately 1.5 A).
Care should be taken to ensure that ISET is not inadvertently
grounded. If no resistor is connected to the ISET pin (that is, if
ISET is open-circuit), I
0 A and the A8296 will not support any load (OCP will occur
prematurely).
The BOOST pulse-by-pulse current limit, I
matically scaled along with the LNB output current limit. The
typical BOOST current limit is set according to the following
equation:
where both I
Automatically scaling the BOOST current limit allows the
designer to choose the lowest possible saturation current of the
boost inductor, reducing its physical size and PCB area, thus
minimizing cost.
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-
OUT(MAX)
OUT
= 20 V. The typical LNB output current limit can be set
DIS
OUT(MAX)
) then the A8296 will be shut down and the OCP bit
can be set by connecting a resistor (RSET) from the
SET
I
BOOST(MAX)
BOOST(MAX)
value of 100 to 60.4 kΩ, respectively. If the LNB
Generates a supply voltage above the internal
is in mA and R
I
OUT(MAX)
During either start-up, or when the output
OUT(MAX)
= 3.0 × I
SET
and I
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Worcester, Massachusetts 01615-0036 U.S.A.
= 49.9 kΩ) but care should be taken
= 29,925 / R
OUT(MAX)
TCAP
SET
OUT(MAX)
will be set to approximately
The LNB output current limit,
or C7 in the Applications
is in kΩ. If the voltage at
are in mA.
SET
+ 900 mA ,
BOOST(MAX)
,
OUT(MAX)
IN
= 10 V and
, is auto-
will
OUT
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