How to shrink satellites, telemetry subsystems
Article By : Allan Robinson
Find out how using a 32-channel driver IC and a multiplexer can help satellite manufacturers reduce the size of next-generation satellite command and telemetry subsystems.
Nearly 1,400 satellites are in orbit in 2017. These satellites serve military and commercial purposes with missions spanning a wide range of functions such as reconnaissance, surveillance, imaging, signal intelligence, meteorology, navigation, television, and voice communications. As the number of missions grows, the demand from customers for more functionality also grows. This requires larger satellite payloads, forcing manufacturers to increase the size, weight and power (SWaP) of their satellite subsystems.
This article discusses how using the ISL72813SEH 32-channel driver IC and the ISL71841SEH multiplexer shrink the size of next-generation satellite command and telemetry subsystems. Examining these components reveals that they reduce footprint size by up to 50%, while doubling the number of telemetry data inputs and command outputs. Also, we’ll discuss the basic operation of the command output circuit, and its role in the overall telemetry system. And we’ll compare the 32-channel driver IC to an 8-channel device, and show how it significantly decreases circuit size.
Figure 1 shows a block diagram for a next-generation satellite command and telemetry subsystem that typically has hundreds of telemetry inputs and command outputs. The 32-channel driver array controls the relays and solenoids used to turn on and off the propulsion system thrusters, to change orbital altitude, configure payload waveguide and coax switches, or turn on a motor to tilt the solar panels.
Figure 1: Block diagram of next-generation command and telemetry subsystems.
The ISL72813SEH 32-channel driver circuit IC drives the coils of relays, waveguide and coaxial switches that can require up to 500mA of current and −28V across their coils. Relays, waveguide switches and coaxial switches are common components used in spacecraft and satellite command subsystems. The logic inputs to the ISL72813SEH are TTL/CMOS compatible allowing easy interface to CPUs, FPGAs or µProcessors.
Operation of the control command output circuit
Figure 2 shows the block diagram of the satellite’s command output circuit.
Figure 2: Block diagram of command output circuit.
The circuit consists of six ISL72813SEH driver ICs that allows for 192 command outputs. Each output goes to the coil of either a relay or a solenoid in a waveguide or coaxial switch that require −28Vdc across their coils to activate their main circuit. A relay coil draws less than 100mA of current while a waveguide/coaxial solenoid can draw up to 500mA current. The commands from these devices are used to control the communications payload and other subsystems in the spacecraft.
Power for the relay and switch coils is provided by a −30V dc power source connected at the common-emitter (VEE) of the six drivers. The −30V dc source provides up to 500mA to power the ISL72813SEH outputs.
An ISL72813SEH device has five logic inputs (A0 – A4) to select one of the 32 driver channels and an enable pin that can be used to disable all of the 32 channels when driven low. Each of the (A0 – A4) logic pins of the six ISL72813SEH drivers are connected in parallel, as shown in Figure 2, and connected to five of the FPGAs logic outputs. Each enable pin of the six drivers goes to a separate FPGA output logic pin. Only one channel of an ISL72813SEH’s 32 channels can be activated at a time. However, it is possible to turn on up to six channels simultaneously by driving all six of the enable pins high.
When a specific driver channel is commanded by the flight computer to be activated, the FPGA will apply the appropriate logic signals to select and enable the channel. It then holds the enable pin high for a specific time, which generates a −28V at the output of the selected channel to momentarily drive the channel’s load.
The application only requires 12 logic outputs from the FPGA to control the 192 command outputs. The logic inputs of the six ISL72813SEH devices respond to TTL logic levels and the FPGA drives them directly with 3.3V or 5V logic.
Advantages of the new design
Older designs use discrete current driver arrays to build the command output subsystem. A single driver array IC contains at most eight Darlington current drivers in a 20-lead LCC package. To use the driver array ICs in an application requires an external blocking diode at each of the Darlington inputs and 16 level shifting circuits shared between Darlington inputs. The diodes at the input side of the Darlington drivers are required due to a leakage path inside the driver. The level shifting circuits allow 5V logic to control the driver array ICs when their ground is connected to −30V. Each level shifting circuit consisted of three resistors and one transistor.
Figure 3 shows the components and circuitry replaced by one ISL72813SEH part, which replaced four discrete current driver array ICs. Also eliminated were 32 blocking diodes and 16 level shifting circuits, which consisted of 48 resistors, 16 transistors, and a pair of decoder ICs that are used to select one of the 16 level shifting circuits. Note that for a circuit card with 192 command outputs, six ISL72813SEH parts would replace 24 discrete driver array parts, 192 blocking diodes, two 3-to-8 decoders, and 16 level shifting circuits, which includes 48 resistors and 16 transistors. The new output design offers better than a 2:1 footprint reduction over the old design.
Figure 3: Components replaced by one ISL72813SEH part.
Figure 4 shows the ISL72813SEH functional block diagram. It is a radiation hardened 32-channel high-voltage, high current driver circuit with integrated level shifter and 5-bit to 32-bit logic decoder. The decoder selects one of the 32 available drivr channels, and its enable pin can disable all 32 channels. The device integrates 32 high current complementary Darlington drivers that feature high-voltage, common emitter, and open-collector outputs with a 42V breakdown voltage and peak current rating of 600mA. In addition to the 32 driver channels and the level shifting circuitry to reference the output base of a selected channel to a negative voltage, the ISL72813SEH’s six logic pins control the 32 drivers, which eliminates 4:16 decoder and 16 level shifters used in the old design.
Figure 4: ISL72813SEH functional block diagram.
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