feature

LNB selection and backup functions; built-in tone oscillator factory-trimmed to 22 kHz for convenience; DiSEqC 8482 ; Auxiliary modulation input; internal thermal protection; reverse current protection; cable length compensation ( A8282SLB only); these devices incorporate patent-pending features.

Always order by the full part number, eg A8282SLB. Used in analog and digital satellite receivers, these Low Noise Block Converter Regulators (LNBRs) are monolithic linear switching regulators specially designed for voltage regulators that provide power and interface to LNB downconverters over coax Signal. If the unit is in standby mode (EN terminal low), the regulator output is disabled, allowing antenna downconverters to be provided or controlled by other satellite receivers sharing the same coaxial cable. In this mode, the device will limit the output reverse current. Set the A8281SLB output to 13 or 18 V via the VSEL terminal. It is available in a 16-lead SOIC power tag package. The power label is at ground potential and does not require electrical isolation. Set the A8282SLB output to 12, 13, 18 or 20 V termination via VSEL. In addition, you can pass 1-V to compensate for voltage drops in coax (LLC termination high). It is supplied in a 24-lead SOIC power tag package. The power label is at ground potential and does not require electrical isolation. The A8282SLB is an improved version of the A8283SLB without the bypass switch.

Function description

Buck regulator. A current-mode buck converter provides the linear regulator with a supply voltage that tracks the selected LNB output voltage. The buck converter operates at 16 times the internal tone frequency, nominally 352khz.

The tracking regulator provides minimal power dissipation over the entire output voltage range by regulating the sense terminal voltage (nominally 900 mV above the LNB output voltage). The tracking modifier also provides plenty of room for tonal injection.

Linear regulator. --- The output linear regulator will sink or source current. This allows modulation of the tone into a 0.1µF capacitive load over an output current range of 12mA to 750mA .

Slew rate control. ---- Programmable output voltage rise and fall times can be set by external capacitors (with internal 25kΩ resistors) located on the TCAP terminals. Acceptable capacitance values range from 4.7nF to 47nF. This feature only affects turn-on and programmed voltage rise and fall times. Modulation is not affected by capacitors. If the LNB output voltage rise and fall time limits are not required, the TCAP termination should use 100nF ceramic as the default 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 adjuster. The LNB output current is limited. The short-circuit protection threshold is set by the value of the external resistor RS together with the internal 135 mV reference voltage (VILNB(th)).

RS = 0.135/ILNBM

where ILNBM is the desired current limit. The sense resistor should be selected based on the maximum DC plus AC (tone) load current, internal VILNB (TH) tolerance, and sense resistor accuracy. For 750 mA applications, precision 140-mΩ resistors are recommended. For 500mA applications, the resistor value can be increased to 200mA.

In operation, short-circuit protection creates a current limit at the input due to the tracking converter. If the output is shorted, the linear regulator limits the output current to ILNBM.

fault output. A short circuit or thermal shutdown will cause the OLF terminal (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 650mV peak-to-peak trapezoidal waveform. The internal oscillator is factory trimmed to provide a 22 kHz tone. No further adjustment is required. Burst encoding of tones is possible due to the fast response of the ENT input and the fast tone response. This allows the implementation of the DiSEqC™ protocol.

External tone modulation. To increase design flexibility and allow implementation of the proposed LNB remote control standard, analog modulation input terminals are available (EXTM). The modulating signal source must be coupled to the EXTM terminal with an appropriate DC blocking capacitor. The peak-to-peak input amplitude should be kept between 100 mV and 125 mV to ensure the DiSEqC amplitude specification over the output current range. If external modulation is not used, a 0.1µF ceramic capacitor should be used to separate the EXTM terminal from ground.

application information

Component selection:

Input capacitor, CIN. Electrolytic capacitors should be placed as close as possible to the VIN terminals of the device. The input current is a square wave with fast rise and fall times, so the capacitor must be able to handle the rms current without excessive temperature rise. The value of this capacitor is not as important as the ESR. The worst case currents are maximum load current, minimum VIN and maximum VLNB (highest switching duty cycle). Choose a ripple current rating greater than:

Buck Inductor, L1. 100µH Power Inductor for all operating conditions. The saturation current rating of the inductor must be greater than 1.3-A. To maximize efficiency, the DC resistance should be less than 350-MΩ.

Clamping Diode, D1. Switch node LX requires a Schottky diode. The current rating of this diode should be 1.5 times the maximum load current.

Output capacitor, CBULK. Low ESR (<200-mΩ) electrolytic capacitors are recommended to reduce ripple voltage. Below 50 mV peak-to-peak is a reasonable target.

where iriple(max) = VBULK(min) x (1 – [VBULK(min)/VIN(max)]/(L1 x 352 kHz).

Output capacitor, CLNB. Increasing the output capacitor, CLNB, will attenuate the noise. However, this is limited by the requirement of low cable capacitance for 22khz audio transmission.

Also, since the sink current of the linear regulator is limited, a high value output capacitor coupled with a low level of output current can cause overshoot in the 22 kHz tone. The operating point above the straight line in the figure below will not have excessive overshoot.

Layout Notes:

1. The printed circuit board should use a heavy-duty plane. The ideal is a double-sided board, with ground planes on both sides of the board. Several copper vias under the device can be used to connect the ground plane and improve thermal performance.

2. For best electrical and thermal performance, the device should be soldered directly to the circuit board.

3. Keep the sense resistance traces as short and wide as possible to reduce trace resistance.

4. Connect the bypass capacitor as close as possible to the equipment. Low value ceramic capacitors should be placed closer to the device than electrolytic capacitors. The supply voltage (VIN) should be separated from electrolytic capacitors placed as close as possible to the device.

5. Place the TCAP capacitor as close to the device as possible.

ground. Use a star ground method at the equipment ground terminal. This keeps the analog and power grounds separate from the device on the PWB.

Noise immunity. LNB systems can have a peak specification of 50 mV for noise on coax. With proper layout and following a few guidelines, this is easy to achieve:

1. Use low ESR capacitors for VBULK. The recommended maximum value is 200 meters.

2. The LNB output is sensitive to the TCAP reference terminal. Keep the PWB trace short and the CTCAP positioned close to the device. This terminal is a high impedance node and can induce noise from close proximity to an unshielded inductor. If the inductor cannot be placed far enough to avoid noise pickup, it must be ensured 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. Make sure to place a 1 to 10µF capacitor on the internal reference VINT (A8282 only).

4. Bypass EXTM to ground with a 0.1 micron ceramic capacitor.

5. Increasing the output capacitor will reduce the noise. However, this has to be weighed against the requirement of low cable capacitance for 22 kHz tone transmission.

DirecTV. Using the A8282, the LNB output voltage of 440 mV can be increased from the nominal 13-V setting to comply with DirecTV requirements. This is done by connecting an 8-volt resistor between the S-S and C-V terminals, sourcing approximately 2.76-1 A to the node. The LNB output voltage is approximately six times the setting of the voltage-selective DAC as shown.

DiSEqC™. The 22 kHz tone is specified to be compatible with the EULTELSAT coaxial cable bus standard.

The LNB output will be able to drive the DiSEqC termination network. The inductor must pass DC current with minimal losses, while the parallel resistor provides the recommended source impedance of 22 kHz. Unidirectional communication systems such as DeSEQC 1 do not require such termination, and the LNB can be connected directly to the coaxial cable.

13V to 18V conversion. --As a method of receiving LNB communication, the LNB output can be quickly toggled between high and low settings. The TCAP capacitor will control the slew rate based on RC charging.

When the expected transition time is less than milliseconds, use a smaller TCAP value. In this case, the minimum rise time is limited by the charging time of the switching regulator output capacitor. This depends on the LNB load current, peak current limit in the buck switch and output amplitude variation. tr=CBULK(VLNB(H)–VLNB(L))/I(AV), where I(AV) is the average current available to charge the output capacitor, which can be estimated by I(AV)=1.4–ILNB. Note that this is only a limitation, as the output capacitor can be charged on low to high LNB voltage changes. For high-to-low transitions, the output voltage will be subject to TCAP. The minimum value of CTCAP is 4.7-nF.

power consumption. The power dissipation and operating junction temperature of the device can be estimated to ensure that the device operates within the desired thermal budget.

The total device power consumption (PD) consists of three components:

In the formula, PD(bias)=VIN(IIN–0.004), PD(lin)=VBUCK x ILNB, PD(buck)=ILNB2 x rDS(on) x VBULK/VIN

In the formula, VBULK=ΔVBUCK+(ILNB x RS)+VLNB. The device junction temperature can be estimated as:

or

where TT is the power connector temperature (lead 4 or 13 for A8281SLB, leads 6, 7, 18 or 19 for A8282SLB) and Rθ is 6°C/W.