SSZT176 april 2021 TPS7A4501-SP , TPS7H1101A-SP
Sree Alvarado
Radiation-hardened low-dropout regulators (LDOs) are vital power components of many space-grade subsystems, including field-programmable gate arrays (FPGAs), data converters and analog circuitry. LDOs help ensure a stable, low-noise and low-ripple supply of power for components whose performance depends on a clean input.
But with so many LDOs available on the market, how do you choose the right radiation-hardened device for your subsystem? Let’s look at some design specifications and device features to help you with this decision.
An LDO’s dropout voltage is the voltage differential between the input and output voltage, at which point the LDO ceases to regulate the output voltage. The smaller the dropout voltage specification, the lower the operating voltage differential one is able to operate with, which results in less power and thermal dissipation as well as inherently higher maximum efficiency. These benefits become more significant at higher currents, as expressed by Equation 1:
LDO power dissipation = (VIN-VOUT)xIOUT (1)
In the radiation-hardened market, it can be difficult to find truly low dropout regulators that offer strong performance over radiation, temperature and aging. TI’s radiation-hardened LDO, the TPS7H1101A-SP, is one example, offering a typical dropout voltage (Vdo) of 210 mV at 3 A – currently, the lowest on the market. If you have a standard 5-V, 3.3-V, 2.5-V or 1.8-V rail available, this LDO can regulate output voltages down to 0.8 V to supply any required voltage, as well as the current needed for one or more space-grade analog-to-digital converters (ADCs) or clocks.
With satellites in space for 10 or more years, getting the maximum performance out of the onboard integrated circuits helps ensure design longevity. In order to provide a clean, low-noise rail for high-performance clocks, data converters, digital signal processors or analog components, the internal noise generated by the LDO’s circuitry needs to be minimal. Since it is not easy to filter internally generated 1/f noise, look for LDOs with inherent low-noise characteristics. Lower-frequency noise is often the largest and most difficult to filter out. The TPS7H1101A-SP offers one of the lowest 1/f noise levels, with a peak around 1 µV/√Hz at 10 Hz. See Figure 1 below for RMS noise over frequency.
The power-supply rejection ratio (PSRR) is a measure of how well an LDO can clean up, or reject, incoming noise from other components upstream. For higher-end ADCs, the input supply noise requirements continue to get tighter to minimize bit errors. At higher frequencies, it is difficult to have high PSRR given the characteristics of the control loop. Often, designers need to use external components to filter the noise to reach an acceptable effective PSRR, which increases solution size – an obvious issue for space applications, where size and weight tie directly to satellite launch costs. The PSRR is most important at the switching frequency of the upstream supply (since there is a voltage ripple at this frequency). Additionally, PSRR is important above this frequency because of the switching harmonics. If you’re looking for good PSRR, the TPS7A4501-SP LDO offers a PSRR of over 45 dB at 100 kHz.
Outside of dropout voltage, PSRR and noise, let’s look at several smart features that can be integral to the performance of a radiation-hardened LDO.
With so many options available, it can be difficult to pick the right LDO. Consider which capabilities and features are the most important. For example, if your application is powering a high-end FPGA or high-speed data converter, features like output-voltage accuracy, reference accuracy, PSRR and noise might be the priorities. If, however, you are designing a low-performance analog circuit or working with an older FPGA where tolerance requirements are not so stringent, having the smallest-sized, lowest-cost solution while retaining good-enough capability might be the better option.
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