Space-Grade, 100-krad, –2.5-V, Discrete Negative LDO Linear Regulator Circuit
Design Goals
| Parameter | Design Goal |
|---|---|
| Input voltage range | –4 V to –3 V |
| Output voltage | –2.5 V |
| Output current range | 0 mA to 100 mA |
| Total ionizing dose | 100 krad (Si) |
| Single-event latch-up immunity | 75 MeV-cm2/mg |
Design Description
The vast majority of electronic circuits operate from one or more positive supply voltages, which is why there are many more positive voltage regulators on the market than negative ones. The dearth of negative voltage regulators is even more pronounced in the space market, where there is a very limited number of such devices. There are times, however, when a particular device or application circuit requires a negative voltage regulator.
The following figure shows a 100-mA negative low-dropout (LDO) voltage regulator circuit that that was developed to provide the negative supply voltage for a LMH5401-SP fully-differential amplifier. The circuit uses a conventional LDO architecture but uses the positive supply as its reference instead of a separate bandgap device. For applications like powering an amplifier, the absolute accuracy of the supply voltage is not critical, and using the positive supply is a simple and cost-effective way to achieve the required performance.Note that the feedback network is connected to the non-inverting input of the op-amp because of the signal inversion caused by Q1.
Design Notes
The power dissipation and junction temperature in pass transistor Q1 must not exceed the maximum values allowed by the device and by your application. In space applications, the maximum allowed power dissipation in a transistor may be significantly less than the rating of the device. For example, the European Space Agency (ESA) recommends a derating factor of 65%. Similarly, although Q1 is specified with a maximum junction temperature of 200°C, ESA recommends a junction temperature not greater than 110°C. In applications with a large voltage drop across the negative regulator, the power dissipation in Q1 might exceed the maximum allowed. However, in such applications there no need for an LDO regulator and a standard type with larger dropout such as the LM137QML-SP can be used.
The op-amp used in this circuit must have an output stage capable of operating close to the negative supply rail of the op amp. The LM158QML-SP was chosen for this circuit because it meets this criterion, has a long space heritage, and withstands a total ionizing dose up to 100 krad (Si). Other op amps can be used, but the feedback stabilization may need to be adjusted to suit the frequency response of the op amp.
