A8285SLB datasheet(6/17 Pages) ALLEGRO

6 / 17 page

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A8285/A8287

LNB Supply and Control Voltage Regulator

Boost Converter/Linear Regulator. A current-mode

boost converter provides the tracking regulator a supply

voltage that tracks the requested LNB output voltage. The

converter operates at 16 times the internal tone

frequency, 352 kHz nominal.

The tracking regulator provides minimum power dissipation

across the range of output voltages, assuming the input volt-

age is less than the output voltage, by adjusting the BOOST

pin voltage 600 mV nominal above the LNB output voltage

selected. Under conditions where the input voltage is greater

than the output voltage, the tracking regulator must drop the

differential voltage. When operating in this condition, care

must be taken to ensure that the safe operating temperature

range of the A8285/A8287 is not exceeded. For additional

information, see Power Dissipation in the Application Infor-

mation section.

Note: To conserve power at light loads, the boost converter

operates in a pulse-skipping mode.

Overcurrent Protection.

The A8285/A8287 is protected

against both overcurrent and short circuit conditions by lim-

iting the output current to I

LIM . In the event of an overcur-

rent, the current limit can be applied indeﬁnitely. Alterna-

tively, if the ODT feature is enabled, and the fault current

appears for longer than the disable time t

DIS, then the device

is turned off. The device can be enabled again via the I2C

interface. If the overcurrent is removed before the disable

time has elapsed, the device remains functioning. These set-

tings are made in the Control register and the Status register.

Charge Pump. Generates a supply voltage above the

internal tracking regulator output to drive the linear regula-

tor control.

Slew Rate Control. During either start-up or when the

output voltage on the BOOST pin is being changed, the

output voltage rise and fall times can be programmed by an

external capacitor located on the TCAP pin. Note that during

start-up, the BOOST pin is precharged to the input voltage

minus a diode drop. As a result, the slew rate control occurs

from this point.

The value for TCAP can be calculated using the following

formula:

TCAP = (I

CAP

× 8) / (ΔV/s)

where ΔV/s is required slew rate. The smallest value for

TCAP is 2.2 nF.

Modulation is unaffected by the choice of TCAP. If limit-

ing LNB output voltage rise and fall times is not required,

the TCAP terminal must have a value of at least a 2.2 nF to

minimize output noise.

External Tone Modulation. To improve design ﬂexibil-

ity and to allow implementation of proposed LNB remote

control standards, the logic modulation input pin EXTM

is provided. The logic signal supplied to this pin creates a

650 mV±250 mV tone signal on the TOUT pin by control-

ling a 40 mA current pull-down device through the

DiSEqC™ ﬁlter. The shape of the tone waveform depends

on the ﬁlter components used and the LNB/cable capacitance.

Tone Detection

. A 22 kHz tone envelope detector is pro-

vided in the A8287 solution. The detector extracts the tone

signal and provides it as an open-collector signal on the TDO

pin. The maximum tone out error is ±1 tone cycle, and the maxi-

mum tone out delay with respect to the input is ±1 tone cycle.

Control Register. The main functions of the A8285/

A8287 are controlled via the I2C interface by writing to the

control register. The power-up states for the control func-

tions are all zero. Control functions include the following:

• Internal Tone Modulation Enable (ENT). When the

ENT bit is set to 1, the internal tone generator controls

a 40 mA pull-down device, thus creating the tone signal

after the DiSEqC™ ﬁlter in a way identical to the EXTM

scheme. The internal oscillator is factory-trimmed to

provide a tone of 22 ±2 kHz. No further adjustment is

required. Burst coding of the 22 kHz tone is accomplished

due to the fast response of the serial command and rapid

tone response. This allows implementation of the

DiSEqC™ 2.0 protocols.

• Select Output Voltage Amplitude (VSEL0, VSEL1,

VSEL2, VSEL3). The LNB output voltage can be pro-

grammed to a particular voltage according to the Output Volt-

age Amplitude Selection table shown on the following page.

• Enable (ENB). When set to 1, the LNB output is enabled.

When reset to 0, the LNB output is disabled.

• Overcurrent Limit (I

LIM). Selects the output overcurrent

limit. When set to 0, the limit is 500 mA. When set to 1, the

limit is 700 mA.

• Overcurrent Disable Time (ODT). When set to 1, in the

event of an overcurrent occuring for a duration exceeding

the disable time, the device is turned off. When set to 0,

Functional Description

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