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A8281SLB

P1-P3P4-P6P7-P9P10-P12P13-P14
8281
AND
8282
LNB SUPPLY AND
CONTROL-VOLTAGE REGULATORS
7
www.allegromicro.com
Grounding. Use a star ground approach at the
device ground terminals. This allows the analog and
power grounds to be kept separate on the PWB up to
the device.
Noise immunity. LNB systems can have a 50-mV
peak specification for noise on the coaxial cable.
This is easily achievable with proper layout and
following a few guidelines:
1. Use a low-ESR capacitor for V
BULK
. A maximum
of 200-mΩ is recommended.
2. The LNB output is sensitive to the TCAP refer-
ence terminal. Keep the PWB traces short and loca-
tion of C
TCAP
close to the device. This terminal is a
high-impedance node and noise can be induced from
proximity to an unshielded inductor. If the inductor
can not be placed far enough away to avoid noise
pickup, it is important to ensure that the induced
voltage is out of phase with the switching node LX.
Rotating the inductor can change the phase of the
induced voltage.
3. Be sure to place a 1-µF to 10-µF capacitor on
internal reference V
INT
(A8282 only).
4. Bypass EXTM with a 0.1-µF ceramic capacitor to
ground.
5. Increasing the output capacitance will attenuate
noise. However, this must be traded off with the
requiremnent for low cable capacitance for 22-kHz-
tone transmission.
APPLICATIONS INFORMATION (cont’d)
DirecTV®. With the A8282, it is possible to raise
the LNB output voltage 440-mV from the nominal
13-V setting to comply with DirecTV requirements.
This is accomplished by connecting a 1-MΩ resistor
between the VINT and TCAP terminals, sourcing
approximately 2.76-µA into the TCAP node. The
LNB output voltage is approximately six times the
setting of the voltage-select DAC as shown in the
figure.
–
+
VINT
LNB
VOLTAGE
REG.
25 kΩ
TCAP
1 MΩ
VOLTAGE
SELECT
X6
Dwg. EP-074
DiSEqC™. The 22-kHz tone is specified to be
compatible with EUTELSAT coaxial cable bus
standards.
The LNB output will be able to drive the DiSEqC
termination network. The inductor must pass the dc
current with minimal loss while the parallel resistor
provides the recommended source impedance at
22-kHz. Unidirectional communication systems such
as DiSEqC 1.0 do not need this termination and the
LNB can be directly connected to the coaxial cable.
13-V to 18-V transition. The LNB output can be
rapidly switched between a high and a low setting as
a method of receiver-to-LNB communication. The
TCAP capacitor will control the slew rate based on
the RC charging.
t
r
or t
f
= 25 x 10
3
x C
TCAP
ln(V
LNB(H)
/V
LNB(L)
)
8281
AND
8282
LNB SUPPLY AND
CONTROL-VOLTAGE REGULATORS
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
8
Small values of TCAP are used when the desired
transition time is less than a millisecond. In this case,
the minimum rise time is limited by the charge time
of the switching regulator output capacitor. This is
dependent on the LNB load current, peak current limit
in the buck switch, and the output amplitude change.
t
r
= C
BULK
(V
LNB(H)
– V
LNB(L)
)/I
(AV)
where I
(AV)
is the average current available to charge
the output capacitor and can be estimated by I
(AV)
=
1.4 – I
LNB
. Note that this is only a limitation due to
the ability to charge the output capacitor on a low-to-
high change of the LNB voltage. For high-to-low
transitions, the output voltage will be slew limited by
TCAP.
The minimum value for C
TCAP
is 4.7-nF.
Power dissipation. The power dissipated, and
operating junction temperature of the device, can be
estimated to ensure that the device is operating within
the desired thermal budget.
The total device power dissipation (P
D
) is com-
prised of three components:
P
D
= P
D(bias)
+ P
D(lin)
+ P
D(buck)
where P
D(bias)
= V
IN
(I
IN
– 0.004),
P
D(lin)
= ∆V
BUCK
x I
LNB
,
P
D(buck)
= I
LNB
2
x r
DS(on)
x V
BULK
/V
IN
where V
BULK
= ∆V
BUCK
+ (I
LNB
x R
S
) + V
LNB
.
The device junction temperature can then be
estimated as
T
J
= (P
D
x R
θJA
) + T
A
or
T
J
= (P
D
x R
θJT
) + T
T
APPLICATIONS INFORMATION (cont’d)
where T
T
is the power tab temperature (leads 4 or 13
for the A8281SLB or leads 6, 7, 18, or 19 for the
A8282SLB) and R
θJT
is 6°C/W.
Package thermal resistances, R
θJA
, measured on
JEDEC standard “high-K” four layer board:
A8281SLB..................................... 38°C/W
A8282SLB..................................... 35°C/W
measured on two-sided PWB with 3 square inches
(1935 mm
2
) copper ground area on each side:
A8281SLB..................................... 48°C/W
A8282SLB..................................... 45°C/W
8281
AND
8282
LNB SUPPLY AND
CONTROL-VOLTAGE REGULATORS
9
www.allegromicro.com
APPLICATIONS INFORMATION (cont’d)
Typical application
23
17
8
1
2
3
4
5
9
12
16
14
13
24
22
21
20
11
9
CHARGE
PUMP
REG.
BUCK
Dwg. EP-072
15
DiSEqC TERMINATION
ENT
ENB
VSEL0
VSEL1
LLC
OLF
TCAP
VIN
VIN
+30 V
C1
C2
C3
R1
C
INT

0.22 μF
180 μH
+
C
BULK
L1
R
S
C
BYP
D1
C
IN
+
C
TCAP
C
LNB
D2
POWER
GROUND
ANALOG
GROUND
L2
A8282SLB
10
+5 V
N
6
7
19
18
VOLTAGE
CONTROL
P1-P3P4-P6P7-P9P10-P12P13-P14

A8281SLB

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