Analysis

LNB voltage regulator ICs for satellite receiver applications

29th January 2007
ES Admin
The new A8290 and A8291 from Allegro MicroSystems Europe are single low noise block (LNB) convertor regulator ICs for use in analogue and digital satellite receivers.
These monolithic linear and switching voltage regulators are specifically designed to provide power and interface signals to an LNB downconvertor via coaxial cable. The A8290/91 require few external components because the boost switch and compensation circuitry is integrated inside the device.

A high switching frequency is chosen to minimise the size of the passive filtering components, further assisting in cost reduction. The high levels of component integration ensure extremely low noise and ripple figures.

The A8290/91 ICs have been designed for high efficiency, utilising Allegro's proprietary advanced BCD process. The integrated boost switch has been optimised to minimise both switching and static losses. To further enhance efficiency, the voltage drop across the tracking regulator has been
minimised.

The new devices have integrated tone detection capability to support full two-way DiSEqC(tm) communications. The built-in tone oscillator is factory-trimmed to 22 kHz to facilitate DiSEqC(tm) tone encoding, even under no-load conditions. Several schemes are available for generating tone
signals, all the way down to no load, using either the internal clock or an external time source. A DiSEqC(tm) filter bypass switch is also integrated, to minimise the output impedance during tone generation.

Each device features 16 programmable LNB voltage levels, with a two-wire serial I2C interface providing easy programming and status readout. The A8290 provides up to 650 mA continuous load current while the A8291 provides up to 500 mA, making them ideal for newer DirecTV loads. The A8290 also has an output current limit of 900 mA typical (600 mA for the A8291) with a 48
ms overcurrent timer which allows output up to current limit for a short period if needed.

A static current limit circuit allows full current at startup and a 13 V/18 V output transition to reliably start a wide range of loads. The push-pull output stage minimises the 13 V/18 V and 18 V/13 V output transition times for highly capacitive loads. Rise and fall times are adjustable via an
external timing capacitor so that transition rates can be set as required - in symmetrical fashion, for example.

These devices offer a high level of diagnostic capabilities, including detection of output voltage level, input supply undervoltage lockout, and DiSEqC(tm) tone output. A cable disconnect diagnostic feature enables the devices to troubleshoot for boxes with disconnected cable connections.

The A8290 and A8291 are supplied in a small (5 x 5 mm) QFN/MLP package, offering a very small footprint for a highly integrated solution.

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