PASSIVE SOLUTIONS

Current Monitor with Thermal Device Enhances Overcurrent Handling

Many of today’s systems that require circuit protection also risk data corruption or loss if an overcurrent event occurs. Examples include USB port protection, where the designer must ensure that end users are unable to overload the host system’s power supply. One versatile approach to this and many other applications combines a PolySwitch™ Polymeric Positive Temperature Coefficient (PPTC) device from Raychem Circuit Protection with a Zetex current monitor IC to protect the load and warn of an impending overcurrent state.

The key to designing with PolySwitch devices is to understand their normal and fault-mode operating characteristics. The device has a relatively constant on-resistance from no-load through to its maximum holding value, I_H (see Figure 1).



Figure 1 - Typical Resistance Change of the nanoSMDC075F PolySwitch Device Versus Load Current

At certain currents beyond the maximum holding current, the resistance of the PolySwitch device starts increasing rapidly, moving towards the tripped condition. When the load current reaches the device’s definite trip current I_T, the PolySwitch device is definitely ‘tripped’ and is in a high-resistance state. The definite trip current value I_T is generally twice the holding current I_H. Once the fault condition is removed and the temperature of the device drops, the PolySwitch device returns to a low resistance value and normal circuit operation can resume.

Importantly for applications that have to accommodate inrush currents, the device’s thermal trigger characteristic ignores short-duration events, thereby avoiding nuisance tripping.

By inserting a PolySwitch device between the V_SENSE+ and V_SENSE- terminals of a ZXCT1021 current monitor IC (Figure 2), the PolySwitch device can both protect the load and generate the sense voltage that the current monitor requires. Assuming constant ambient temperature, the voltage across the device rises linearly with increasing load current until reaching its maximum holding value. Beyond the hold current, as the PolySwitch device starts tripping, its impedance increases rapidly, as does the voltage across it.


Figure 2 - A Current Monitor Uses the PolySwitch Device to Measure Load Current

As a result, the current monitor provides an output that tracks load current through the PolySwitch device. The voltage V_OUT that the monitor generates is directly proportional to the sense voltage across the PolySwitch device, and is ground-referenced for easy connection to an analogue-to-digital converter or other control circuitry.

If the PolySwitch device trips, the full supply voltage will appear across the sense inputs of the current monitor IC. The 10kΩ resistor in series with the V_SENSE- input protects the ZXCT chip from overvoltages if its maximum V_SENSE voltage (V_SENSE+ - V_SENSE-) rating is exceeded.

By substituting a ZXCT1030 current monitor chip, a PolySwitch device can also help to provide an early warning that an overload current or a current surge is occurring (Figure 3). The ZXCT1030 features an internal comparator and voltage reference, while the PolySwitch device appears in series with the supply rail and load as before.



Figure 3 - A PolySwitch Device and a Current Monitor Provide Early Warning of an Overload

Circuit design parameters and device types are selected as follows: firstly, select an appropriate PolySwitch device by considering the holding current (I_H) specification. The device must remain in its low impedance state and sustain the load current necessary for normal circuit operation. The device’s datasheet shows that the maximum operating resistance value of a PolySwitch device before tripping is defined by R_1MAX. At 20°C, the value will be less than R_1MAX unless the device has tripped, or the load current is beyond the maximum holding current I_H.

The overcurrent start V_SENSE (trip start), point can be calculated using R_1MAX and the maximum holding current I_H, as follows:

V_SENSE (trip start) = R_1MAX x I_H

To find the level for an overcurrent ‘tripped’ condition, also approximate V_SENSE (trip) using I_T and R_1MAX:

V_SENSE (trip) ≈ R_1MAX x I_T

Clearly, the choice of sense voltage will depend on the system’s requirements. The output voltage of the ZXCT1030 or ZXCT1021 is V_SENSE (trip) x 10, where 10 is the typical signal gain that these current monitors offer. Other family members offer a gain factor as high as 100, as well as alternative input voltage ranges, low offset-voltage operation, and bidirectional output voltage indication in 5-pin SOT23 packages. Current output versions are also available, where the output voltage can be set with a resistor.

Available in an eight-pin MSOP package, the ZXCT1030 allows designers to set a reference trigger level at the comparator’s non-inverting input that switches the comparator’s open-collector output. Internally, the device’s output voltage connects to the comparator’s inverting input, so if V_OUT exceeds the threshold value, the comparator’s output pulls low. This output typically connects to the system microcontroller via a 10kΩ pull-up resistor to inform the system that excessive current is flowing into the load. This facility allows the system to make an intelligent decision, for instance to save data into non-volatile memory. It can also dispense with the need for an analogue-to-digital converter in some applications.

USB port protection is a typical application that can take advantage of the PolySwitch device and current-sense amplifier/comparator configuration. In this scenario, a PolySwitch nanoSMDC075F or nanoSMDC150F device will help to protect single or dual USB ports. Depending on the trigger level set at its comparator’s input, the ZXCT1030 provides an error flag to signal that an overcurrent event is imminent and/or has occurred.

Features - Raychem PolySwitch Resettable Devices

• Polymeric Positive Temperature Coefficient (PPTC) resettable devices
• Voltage ratings from 6V to 240V
• Interrupt voltage up to 600V
• Operating currents up to 15A
• Temperature ranges reaching as low as -40°C and as high as +125°C
• Wide range of time-to-trip specifications
• Choice of surface mount and through-hole packages
• Choice of grade including automotive, industrial/consumer and telecoms

Features - Zetex Current Monitors

• High-side current sense monitor
• Internal voltage reference
• Optional integral comparator
• Trip point settable via potential divider
• Range of input voltage operating ranges
• Optional x 10 and x 100 current gain factor (voltage output versions)
• Current output versions also available
• Choice of package and pin-out including eight-pin MSOP (ZXCT1030) and five-pin SOT23 (ZXCT1021)

For further information, including Raychem and Zetex FREE Samples, Datasheets and the AN46 Application Note, please click here.