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| 型号： | A8292SETTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 7/20页 |
| 文件大小： | 0K |
| 描述： | IC REG LNB SUPPLY/CONTROL 28-MLP |
| 标准包装： | 1 |
| 应用： | 转换器，模拟和数字式卫星信号接收器 |
| 输入电压： | 8 V ~ 16 V |
| 输出数： | 2 |
| 输出电压： | 12.7 V ~ 20.4 V |
| 工作温度： | -20°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-VFQFN 裸露焊盘 |
| 供应商设备封装： | 28-QFN/MLP 裸露焊盘（5x5） |
| 包装： | 标准包装 |
| 产品目录页面： | 1141 (CN2011-ZH PDF) |
| 其它名称： | 620-1187-6 |
A8292
Dual LNB Supply and Control Voltage Regulator
Functional Description
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 BOOST , is greater
than the output voltage, V LNB , the linear regulator must drop the
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.
The A8292 has internal pulse-by-pulse current limit 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 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.
Each of the boost converters operates at 352 kHz typical:
16 times the internal 22 kHz tone frequency. All the loop com-
pensation, current sensing, and slope compensation functions are
provided internally.
At extremely light loads, the boost converters operate 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 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. Each generates a supply voltage above the internal
tracking regulator output to drive the linear regulator control.
Slew Rate Control. During either start-up, or when the output
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 TCAP or C7 in the Applications
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.
The value of C TCAP can be calculated using the following for-
mula:
C TCAP = ( I TCAP × 6) / SR ,
where SR is the required slew rate of the LNB output voltage,
in V/s, and I TCAP is the TCAP pin current specified in the data
sheet. The recommended value for C TCAP , 10 nF, should provide
satisfactory operation for most applications. However, in some
cases, it may be necessary to increase the value of C TCAP to avoid
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 TCAP :
C TCAP ≥ (I TCAP × 6)(2 × C BOOST ) / I LIMLNB ,
C TCAP ≥ (10 μ A × 6)(2 × 100 μ F) / 500 mA = 24 nF .
The minimum value of C TCAP is 2.2 nF. There is no theoretical
maximum value of C TCAP however too large a value will probably
cause the voltage transition specification to be exceeded. Tone
generation is unaffected by the value of C TCAP .
Pull-Down Rate Control. In applications that have to operate at
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.
Allegro MicroSystems, LLC
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
7
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