Voltage Regulator Protection (EE Tip #103)

In many cases, the load connected to a voltage regulator is not returned to ground. It goes to an even lower voltage or perhaps even the negative power supply voltage. (Here we make the assumption of using positive voltages, when using voltage regulators with negative output voltages the reverse is true.)

Op-amps and level-shifters come to mind. In such cases, a diode (1N4001 or equivalent) connected across the output of the regulator IC usually provides sufficient protection (see Figure 1).

Source:Ton Giesberts, Elektor, 080943-I, 4/2009

Source:Ton Giesberts, Elektor, 080943-I, 4/2009

Polarity inversions which could occur, for example, during power on or during a short circuit could prove fatal for the regulator IC, but such a diode prevents the output of the IC going lower than ground (well, minus 0.7 V, to be accurate).

A short-circuit proof voltage regulator (such as the 78xx series) will survive such a situation without any problems. It is also possible for the input voltage of a voltage regulator to drop quicker than the output voltage—for example, when there is a protection circuit that shorts the input power supply voltage as a result of an overvoltage at the output.

If the output voltage of the regulator is more than 7 V higher than the input voltage, the emitter-base junction of the internal power transistor can break down and cause the transistor to fail.

You can use a shunt diode to prevent this condition (see Figure 2). This ensures that any higher voltage at the output of the regulator is shorted to the input.

—Ton Giesberts, Elektor, 080943-I, 4/2009

Linear Regulator with Current and Temperature Monitor Outputs

Linear Technology Corp

Linear Technology Corp

The LT3081 is a rugged 1.5-A wide input voltage range linear regulator with key usability, monitoring, and protection features. The device has an extended safe operating area (SOA) compared to existing regulators, making it well suited for high input-to-output voltage and high output current applications where older regulators limit the output.

The LT3081 uses a current source reference for single-resistor output voltage settings and output adjustability down to ”0.” A single resistor can be used to set the output current limit. This regulator architecture, combined with low-millivolt regulation, enables multiple ICs to be easily paralleled for heat spreading and higher output current. The current from the device’s current monitor can be summed with the set current for line-drop compensation, where the LT3081’s output increases with current to compensate for line drops.

The LT3081 achieves line and load regulation below 2 mV independent of output voltage and features a 1.2-to-40-V input voltage range. The device is well suited for applications requiring multiple rails. The output voltage is programmable with a single resistor from 0 to 38.5 V with a 1.2-V dropout. The on-chip trimmed 50-µA current reference is ±1% accurate. The regulation, transient response, and output noise (30 µVRMS) are independent of output voltage due to the device’s voltage follower architecture.

Two resistors are used to configure the LT3081 as a two-terminal current source. Input or output capacitors for stability are optional in either linear regulator or current-source operation mode. The LT3081 provides several monitoring and protection functions. A single resistor is used to program the current limit, which is accurate to ±10%. Monitor outputs provide a current output proportional to temperature (1 µA/°C) and output current (200 µA/A), enabling easy ground-based measurement. The current monitor can compensate for cable drops. The LT3081’s internal protection circuitry includes reverse-input protection, reverse-current protection, internal current limiting, and thermal shutdown.

A variety of grades/temperature ranges are offered including: the E and  I grades (–40°C to 125°C), the H grade (–40°C to 150°C), and the high-reliability MP grade (–55°C to 50°C). Pricing for the E-grade starts at $2.60 each in 1,000-piece quantities.

Linear Technology Corp.

DC Motor for Fine Rotary Motions

The RE 30 EB precious metal brushed motor features a low start-up voltage, even after a long period in standstill. With a 53-mNm rated torque, the powerful motor provides twice the power of an Maxon RE 25 EB. In addition, the RE 30 EB features minimal high-frequency interference.

The RE 30 EB motor is specifically designed for haptic applications (e.g., surgical robots). Therefore, the motor can also be used as a highly sensitive sensor, acting as the sense of touch to register mechanical resistance.

Contact Maxon for pricing.

Maxon Precision Motors

Dual-Display Digital Multimeter

The DM3058E digital multimeter (DMM) is designed with 5.5-digit resolution and dual display. The DMM can enable system integration and is suitable for high-precision, multifunction, and automatic measurement applications.

The DM3058E is capable of measuring up to 123 readings per second. It can quickly save or recall up to 10 preset configurations, including built-in cold terminal compensation for thermocouples.

The DMM provides a convenient and flexible platform with an easy-to-use design and a built-in help system for information acquisition. In addition, it supports 10 different measurement types including DC voltage (200 mV to approximately 1,000 V), AC voltage (200 mV to approximately 750 V), DC current (200 µA to approximately
10 A), AC current (20 mA to approximately 10 A), frequency measurement (20 Hz to approximately 1 MHz), 2-Wire and 4-Wire resistance (200 O to approximately 100 MO), and diode, continuity, and capacitance.

The DM3058 is ideal for research and development labs and educational applications, as well as low-end detection, maintenance, and quality tests where automation combined with capability and value are needed.

The DM3058E digital multimeter costs $449.

Rigol Technologies, Inc.

Two-Channel CW Laser Diode Driver with an MCU Interface

The iC-HT laser diode driver enables microcontroller-based activation of laser diodes in Continuous Wave mode. With this device, laser diodes can be driven by the optical output power (using APC), the laser diode current (using ACC), or a full controller-based power control unit.

The maximum laser diode current per channel is 750 mA. Both channels can be switched in parallel for high laser diode currents of up to 1.5 A. A current limit can also be configured for each channel.

Internal operating points and voltages can be output through ADCs. The integrated temperature sensor enables the system temperature to be monitored and can also be used to analyze control circuit feedback. Logarithmic DACs enable optimum power regulation across a large dynamic range. Therefore, a variety of laser diodes can be used.

The relevant configuration is stored in two equivalent memory areas. Internal current limits, a supply-voltage monitor, channel-specific interrupt-switching inputs, and a watchdog safeguard the laser diodes’ operation through iC-HT.

The device can be also operated by pin configuration in place of the SPI or I2C interface, where external resistors define the APC performance targets. An external supply voltage can be controlled through current output device configuration overlay (DCO) to reduce the system power dissipation (e.g., in battery-operated devices or systems).

The iC-HT operates on 2.8 to 8 V and can drive both blue and green laser diodes. The diode driver has a –40°C-to-125°C operating temperature range and is housed in a 5-mm × 5-mm, 28-pin QFN package.

The iC-HT costs $13.20 in 1,000-unit quantities.

iC-Haus GmbH