« Previously: How to shrink satellites, telemetry subsystems  

In Part 1, we discussed how the ISL72813SEH components are able to shrink footprint size to half, while at the same time double the number of telemetry data input and command outputs. In this article, we'll further analyse the advantage of adapting the new telemetry system design.

Benefits of the new design

One ISL72813SEH replaces four discrete current driver array ICs, 32 blocking diodes, address decoding and level shifting control circuits for more than a 2:1 footprint reduction.

Higher integration with the new Intersil devices enables a 2x increase in the number of telemetry inputs and command outputs. The reduced size of the command and telemetry subsystem improves the ratio of bus equipment to payload equipment on a spacecraft. This improves the satellite performance, since the payload is the source of revenue for a communication satellite.

The ISL72813SEH at 500mA has a VCE(SAT) voltage limit of 1.45V vs. the discrete IC Darlington driver’s limit of 2.1V @ 500mA. The lower VCE(SAT) rating of the ISL72813SEH reduces the overhead voltage to drive the coil load and minimises power dissipation by almost 30%.

The ISL72813SEH’s integrated decoder simplifies the logic control circuitry required by the old design built with 1-input to 1-output driver arrays. The ISL72813SEH can be controlled by 3.3V or 5V logic levels.

The ISL72813SEH has very low OFF channel leakage current of 40nA max vs. 100µA for the Darlington driver. This eliminates voltage glitches that can be present on lightly loaded Darlington outputs of the old design.

The ISL72813SEH drives the relay and switch coils that typically require -28Vdc commonly used in spacecraft and satellite applications. The device’s common emitter requires a negative voltage to operate properly and can operate with a negative supply voltage up to −42V terrestrial and −33V in the space environment. The discrete current drivers in the old design were primarily designed to operate with a positive voltage. Connecting its ground to −30Vdc required additional discrete components such as blocking diodes and a level shifting circuit at the driver inputs.

The ISL72813SEH enable function allows multiple devices to be easily cascaded together to achieve the desired number of command outputs.

Radiation hardened for mission assurance

Space radiation effects on electronic devices are an important system design consideration. They can cause problems ranging from operational malfunctions to severe physical damage to the devices and possibly a catastrophic mission failure.

ICs are susceptible to two types of space radiation effects caused by electrons and protons trapped in the terrestrial magnetic field: total ionising dose (TID) and single-event effects (SEE). TID effects are the result of accumulated exposure to ionising radiation. SEE are the result of a single high-energy particle that strikes the device.

The total dose radiation (TID) exposure is measured in rads. The term rad (radiation-absorbed dose) quantifies the total radiation exposure of a material. One rad(Si) is equal to 10×10-6 W of energy absorbed per gram of silicon. The total dose radiation threshold of a device is the minimum level of rad(Si) that will cause device failure. Typical commercial devices can survive around 5krads before functional failure occurs.

SEE events are significantly more hazardous to the satellite. The intense proton and heavy ion environment encountered in space applications can cause a variety of SEE in electronic circuitry, including single-event upset (SEU), single-event transient (SET), singe-event functional interrupt (SEFI), single-event gate rupture (SEGR) and single-event burnout (SEB). SEE can lead to system-level performance issues including disruption, degradation, and destruction. For predictable and reliable space system operation, individual electronic components should be characterised to determine their SEE response.

TID testing on the ISL72813SEH was done to a high dose rate of 100krad(Si) at 50rad(Si)/s to 300rad(Si)/s) and to a low dose rate of 50krad(Si) at 0.01rad(Si)/s under biased and grounded conditions. These tests were followed by a biased anneal at 100ºC for 168 hours as outlined in MIL-STD-883 Test Method 1019. No rejects to SMD parametric limits were encountered. All parameters showed excellent stability over irradiation, with no observed dose rate or bias sensitivity. The ISL72813SEH device has a high dose rate guarantee of 100krads and a low dose rate guarantee of 50krads.

The SEE testing of the ISL72813SEH part was intended to find the limits of the VCC and VEE supply voltages for avoiding destructive SEB at a linear energy transfer (LET) of 86 MeV•cm2/mg. In addition, testing was done to look for significant SETs impacting the driver channel activity at LET of 86 MeV•cm2/mg and 28 MeV•cm2/mg. Single-event latch-up (SEL) was not an explicit consideration since the part is manufactured in a dielectrically isolated process.

Summary of SEE testing results

  • No SEL: Manufactured on Intersil’s proprietary PR40 SOI process ensures single-event latch-up immunity
  • No SEB for ions with LET £ 86 MeV•cm2/mg with VCC = 6.5V and VEE = −33V with a case temperature of +125°C
  • SET of ≤ −2V VCx of an off switch detected for LET = 28 MeV•cm2/mg (Fluence 2 × 106 ion/cm2) to a cross section of 5.10x10-5 cm2
  • SET of ≤ −2V VCx of an off switch detected for LET = 86 MeV•cm2/mg (Fluence 1 × 106 ion/cm2) to a cross section of 1.22x10-4 cm2
  • SET error rate estimate for a GEO orbit of 7.093 × 10-5 events/device/day (approximately 1 event/device every 40 years)

Conclusion

The ISL72813SEH is the industry’s first radiation hardened, high voltage, high-current driver circuit to integrate the decoder, level shifter and 32-channel driver array in a single monolithic IC, allowing satellite manufacturers to reduce command and telemetry subsystem solution size by up to 50%. The device offers a 4× higher density channel count compared to the nearest competitor and the integration of the decoder and level shifter circuitry eliminates many peripheral discrete components.

The device is specifically designed to drive relays, waveguide switches and coaxial switches that are commonly used in communications spacecraft. The ISL72813SEH device allows satellite and spacecraft manufacturers to shrink the size of their command and telemetry subsystems by reducing mass, cost, and power (SWaP). This allows customers to accommodate increase payload size by reducing the footprint and mass of bus components. Since the payload generates the revenue from the satellite this is a win/win improvement for the satellite manufacturer and their customers.