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參數(shù)資料
| 型號(hào)： | 8281 |
| 廠商： | Allegro MicroSystems, Inc. |
| 英文描述： | LNB SUPPLY AND CONTROL-VOLTAGE REGULATORS |
| 中文描述： | LNB電源與控制，電壓調(diào)節(jié)器 |
| 文件頁數(shù)： | 5/14頁 |
| 文件大?。?/td> | 375K |
| 代理商： | 8281 |
8281
AND
8282
LNB SUPPLY AND
CONTROL-VOLTAGE REGULATORS
5
www.allegromicro.com
Buck regulator.
A current-mode buck converter
provides the linear regulator a supply voltage that
tracks the selected LNB output voltage. The buck
converter operates at 16 times the internal tone fre-
quency, nominally 352
-
kHz.
The tracking regulator provides minimum power
dissipation across the range of output voltages by
adjusting the SENSE terminal voltage, nominally
900
-
mV above the LNB output voltage. The tracking
regulator also provides adequate headroom for tone
injection.
Linear regulator.
The output linear regulator will
sink or source current. This allows tone modulation
into a capacitive load of 0.1
-
μ
F over the output
current range of 12
-
mA to 750
-
mA.
Slew rate control.
The programmed output volt-
age rise and fall times can be set by an external
capacitor (with an internal 25
-
k
resistor) located on
the TCAP terminal. The range of acceptable capaci-
tor values is 4.7
-
nF to 47
-
nF. This feature only
affects the turn-on and programmed voltage rise and
fall times. Modulation is unaffected by the capacitor.
If LNB output voltage rise and fall time limiting is
not required, the TCAP terminal should use a 100
-
nF
ceramic as a default value to minimize output noise.
If a small value capacitor is used, the rise time will be
limited by the time required to charge the VBULK
capacitor.
Short-circuit limit regulator.
The LNB output is
current limited. The short-circuit protection threshold
is set by the value of an external resistor, R
S
, in
conjunction with an internal 135
-
mV reference
voltage (V
ILNB(th)
).
R
S
= 0.135/I
LNBM
where I
LNBM
is the desired current-limit value. The
sense resistor should be chosen based on the maxi-
mum dc plus ac (tone) load current required, internal
V
ILNB(th)
tolerance, and sense resistor accuracy. For
750
-
mA applications, a precision 140
-
m
resistor is
recommended. For 500
-
mA applications, the resistor
value can be raised to 200
-
m
.
In operation, the short-circuit protection produces
current limiting at the input due to the tracking con-
verter. If the output is shorted, the linear regulator
will limit the output current to I
LNBM
.
Fault output.
Short-circuit or thermal shutdown
will cause the OLF terminal, an open-drain diagnostic
output flag, to go LOW.
Internal tone modulation.
The ENT (tone enable)
terminal activates the internal tone signal, modulating
the dc output with a 650
-
mV peak-to-peak trapezoidal
waveform. The internal oscillator is factory trimmed
to provide a tone of 22
-
kHz. No further adjustment is
required. Burst coding of the tone can be accom-
plished, due to the fast response of the ENT input and
rapid tone response. This allows implementation of
the DiSEqC protocols.
External tone modulation.
To improve design
flexibility and to allow implementation of proposed
LNB remote control standards, an analog modulation
input terminal is available (EXTM). An appropriate
dc-blocking capacitor must be used to couple the
modulating signal source to the EXTM terminal. The
peak-to-peak input amplitude should stay within
100
-
mV to 125
-
mV to ensure the DiSEqC amplitude
specification over the output current range. If exter-
nal modulation is not used, the EXTM terminal
should be decoupled to ground with a 0.1
-
μ
F ceramic
capacitor.
FUNCTIONAL DESCRIPTION
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