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A8293 Schematic ( PDF Datasheet ) - Allegro MicroSystems

Teilenummer A8293
Beschreibung Single LNB Supply and Control Voltage Regulator
Hersteller Allegro MicroSystems
Logo Allegro MicroSystems Logo 




Gesamt 19 Seiten
A8293 Datasheet, Funktion
A8293
Single LNB Supply and Control Voltage Regulator
Features and Benefits
Description
2-wire serial 2IC™ -compatible interface: control (write) and Intended for analog and digital satellite receivers, this single
status (read)
low noise block converter regulator (LNBR) is a monolithic
LNB voltages (8 programmable levels) compatible with
linear and switching voltage regulato,rspecifically designed to
all common standards including domestic Japan models
provide the power and the interface signals to an LNB down
Tracking switch-mode power converter for lowest dissipation converter via coaxial cable. The A8293 requires few external
Integrated converter switches and current sensing
components, with the boost switch and compensation circuitry
Provides up to 700 mA load current
integrated inside of the device.A high switching frequency is
Static current limit circuit allows full current at startup and chosen to minimize the size of the passive filtering components,
1318V output transition; reliably starts wide load range further assisting in cost reductionT. he high levels of component
Push-pull output stage minimizes 1318V and 1813 V integration ensure extremely low noise and ripple figures.
output transition times for highly capacitive loads
Adjustable rise/fall time via external timing capacitor
Built-in tone oscillator, factory-trimmed to 22 kHz
facilitates DiSEqC™ tone encoding, even at no-load
Four methods of 22 kHz tone generation, via I2C™ data
bits and/or external pin
The A8293 has been designed for high ef ficiency, utilizing
the Allegro® advanced BCD process. The integrated boost
switch has been optimized to minimize both switching and
static losses. To further enhance ef ficiency, the voltage drop
across the tracking regulator has been minimized.
Auxiliary modulation input
For DiSEqC™ communications, several schemes are available
LNB overcurrent with timer
for generating tone signals, all the way down to no-load, and
Diagnostics for output voltage level, input supply UVLO using either the internal clock or an external time source.
Packages:
20-contact, 4 × 4 mm
MLP/QFN (suffix ES)
28 contact, 5 × 5 mm
MLP/QFN (suffix ET)
Continued on the next page…
VS
VREG
C3
220 nF
VDD
R1 R2 R3 R4
EXTM
SDA
SCL
ADD
Functional Block Diagram
L1
33 μH
D1
C1
100 nF
C2
100 μF
VIN
C5
100 μF
C6
1 μF
C4
100 nF
LX GNDLX BOOST VCP
Regulator
DAC
I2 C™-
Compatible
Interface
Charge
Pump
D3
A
Boost
Converter
fsw
LNB
Voltage
Control
EXTM
TMode
Wave
Shape
Linear
Stage
TCAP
Fault Monitor
TGate
fsw
OCP
PNG
TSD
VUV
Clock
Divider 22 kHz
Oscillator
LNB
D2
C8
100 nF
C10
220 nF
VOUT
D4
C9
10 nF
A
TCAP
C7
10 nF (or 22 nF)
IRQ
PAD
For recommended external components, refer to table 7
8293-DS, Rev. 4
GND
A D3 and D4 are used for surge protection.
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A8293 Datasheet, Funktion
A8293
Single LNB Supply and Control Voltage Regulator
Functional Description
Protection
The A8293 has a wide range of protection features and fault diag-
nostics which are detailed in the Status Register section.
Boost Converter/Linear Regulator
The A8293 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, VBOOST,
is greater than the output voltage, VLNB, 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 A8293 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 A8293 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 to 700 mA typical,
and the IC will be shut down if the overcurrent condition lasts
for more than 48 ms. If this occurs, the A8293 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.
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 A8293 output
will auto-recover to their programmed levels.
Charge Pump. Generates a supply voltage above the internal
tracking regulator output to drive the linear regulator control.
and fall times can be set by the value of the capacitor connected
from the TCAP pin to GND (CTCAP 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 CTCAP can be calculated using the following for-
mula:
CTCAP = (ITCAP × 6) / SR ,
where SR is the required slew rate of the LNB output voltage,
in V/s, and ITCAP is the TCAP pin current specified in the data
sheet. The recommended value for CTCAP, 10 nF, should provide
satisfactory operation for most applications. However, in some
cases, it may be necessary to increase the value of CTCAP 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 CTCAP:
C TCAP (ITCAP × 6)(2 × CBOOST) / ILIMLNB ,
C TCAP (10 μA × 6)(2 × 100 μF) / 500 mA = 24 nF .
The minimum value of CTCAP is 2.2 nF. There is no theoretical
maximum value of CTCAP however too large a value will prob-
ably cause the voltage transition specification to be exceeded.
Tone generation is unaffected by the value of CTCAP .
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 V to 13 V in a
reasonable amount of time.
ODT (Overcurrent Disable Time)
If the LNB output current exceeds 700 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 700 mA, typical. If the short circuit condition
lasts for more than 48 ms, the A8293 will be disabled and the
OCP bit will be set.
Auto-Restart
Slew Rate Control. During either start-up, or when the output
voltage at the LNB pin is transitioning, the output voltage rise
After a short circuit condition occurs, the host controller should
periodically reenable the A8293 to check if the short circuit has
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6

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A8293 pdf, datenblatt
A8293
Single LNB Supply and Control Voltage Regulator
Table 3. Output Voltage Amplitude Selection
VSEL3
VSEL2
VSEL1
VSEL0
LNB (V)
0 0 0 0 12.709
0 0 0 1 13.042
0 0 1 0 13.375
0 1 0 0 14.042
0 1 1 1 15.042
1 0 0 0 18.042
1 0 1 0 18.709
1 0 1 1 19.042
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
12

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