Power Electronics

Protecting Low-Voltage Systems or Sensors Against Short Circuits

Important specifications and concepts must be considered to protect low-voltage subsystems in consumer applications, including the use of input and output capacitors, thermal performance, and switching impedance.

In today's consumer applications, sensors are increasingly being used to validate the user prior to accessing the equipment. For example, the increased popularity of fingerprint authentication sensors in devices such as cell phones and notebooks offer the convenience and security advantages of biometrics to consumers and businesses.

A key challenge for sensor manufacturers is protecting equipment from any short circuits caused by an external strike, which could translate into high current through the sensor and eventually damage the equipment motherboard. With fingerprint silicon sensors, overheating the touchpad could lead to burns for the end user.

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To protect a system, designers are very careful about any overcurrent strikes coming from the external environment. As stated previously, the fingerprint sensor is a subsystem that needs additional protection to avoid damaging the system. The designer will want to protect any subsystem, such as USB connectors, SD, and other SIM cards, subject to external strikes. A large overcurrent created on those connectors/sensors can be harmful to the main system and needs to be monitored and controlled.

The easiest solution for overcoming this problem is a current-limited power switch or current-limited load switch. The switch monitors the subsystem's supply current (sensor, connector) and limits the output current if an overcurrent is detected. When the switch current reaches the predefined maximum limit, integrated solutions like the TPS2294x series automatically move to a constant-current mode to prohibit excessive currents from causing damage. A solution based on discrete semiconductors (e.g., FET, resistors, etc.) could be used as well, but an integrated solution is more desirable.

IMPORTANT LOAD-SWITCH PARAMETERS

Most power switches comprise either a P-Channel MOSFET biased by an N-channel MOSFET or an N-channel MOSFET with a charge pump. They include integrated protection features like overcurrent protection, reverse current blocking, thermal shutdown, or even undervoltage lock-out (UVLO).

The most important parameters to consider when designing a load-switch-based solution to protect a subsystem are:

  • VIN — Input voltage range
  • rON — On-state resistance from drain to source of the pass FET
  • ILIM — Minimum current limit
  • tRISE — Output rise time of the switch
  • VIH/VIL — Control thresholds of the switch
  • ICC and ISHUTDOWN — Quiescent and shutdown current
  • Automatic control loop: blanking time (tBLANK) and auto-restart (tRESTART)
  • Fault monitoring
  • Short-circuit current-limit response time
  • Thermal dissipation
  • Packaging
  • System Interoperability

Each parameter's importance depends on what the user wants to protect. In a high-current application like USB VBUS switching, it is important to select a switch that works at 5.25 V and features a high current limit (>500 mA), while having a fairly low ON resistance to avoid excess dropout voltage. Using an integrated product like the TPS2552 would be a good choice in this application.

For low-voltage sensors, the switched current is usually below 50 mA. So ON resistance isn't as critical, but the user needs to make sure that the switch is working with input voltages as low as 1.8V. A product like TPS22945 can support such a design.

On low-voltage subsystems like a fingerprint sensor, the designer is frequently asked to provide two power rails to supply the I/Os (3.3 V, for instance) and the core voltage (1.8 V or 1.5 V, for instance) of the subsystem. Many sensors are very sensitive to noise. Therefore, they require good ac performance as high power-supply ripple rejection (PSRR), or good load and line transient response.

SYSTEM INTEROPERABILITY: CHOOSE THE RIGHT COMPONENTS

The designer must carefully select the low drop-out (LDO) regulator to comply with the sensor's needed supply regulation performance. Putting a general-purpose current-limited load switch in front of the LDO as indicated in Fig. 1 is usually a good solution to protect the subsystem. Another solution is to use a fully integrated solution like the TPS22949.

Be very cautious about system interoperability. An incorrect LDO or capacitor choice can translate into system instability, which will result in unreliable system behavior or failure. The choice of different capacitors (load cap, storage cap, bypass cap) is crucial since it will help stabilize the design.

Although an input capacitor is usually not required to stabilize the current limiters available in the market, it is considered good analog design practice to connect a 0.1-µF to 1-µF low-ESR capacitor across the input supply. This capacitor counteracts reactive input sources and improves transient response, noise rejection, and ripple rejection.

To prevent parasitic board inductances from forcing VOUTCL below GND when the switch turns off, place a capacitor (COUTCL) between VOUTCL and GND. For devices with blanking time, keep the total output capacitance below a maximum value, COUTCL(max), to prevent the part from registering an overcurrent condition and turning off the switch. Using Eq. 1, you can easily determine the maximum output capacitance:

Due to the integral body diode in a MOSFET switch, a CIN greater than COUTCL is highly recommended. A COUTCL greater than CIN can cause VOUTCL to exceed VIN when the system supply is removed. This could result in current flow through the body diode from VOUTCL to VIN.

A storage capacitor (COUTCL) at the current limiter's output is recommended to provide enough current to the LDO during the start up sequence. The storage capacitor is needed to reduce the amount of inrush current supplied through the current-limited load switch to the LDO during the power-up sequence (Fig. 2).

If the COUTCL capacitor is too small, the inrush current required to start the LDO and charge COUTLDO could be interpreted by the current limiter as an overcurrent and trigger the current-limiting feature of the switch. The switch tries to limit the current to 100-mA, causing an undesired drop on the supply line (Fig. 3).

Also, be careful of the system's power dissipation. During normal operation as a switch, the power dissipation is small and has little effect on the part's operating temperature. Calculate the maximum power dissipated through the device by using Eq. 2 and the maximum current-limit specified for a particular switch:

For a switch like the TPS22945, the maximum current dissipated is:

If the part goes into current limit, maximum power dissipation occurs when the output is shorted to ground. For devices featuring auto-restart time, tRESTART, and the overcurrent blanking time, tBLANK, the maximum power dissipated is:

(See equation 4)

If you are not using a current-limited switch, power dissipation can become a major issue for system reliability. For instance, a 0.9-Ω short applied to a non-current-limited load switch with a 3.3-V input voltage (switch ON resistance being ~100 m)Ω translates into a dissipated power as shown in Eq. 5:

Usually, this power dissipation is too high for most packages in the market, which can result in failure and reliability issues.

For devices with a current limit, but not the blanking and auto-restart feature, an overcurrent condition is indicated by the error flag and the user can manually reset the part. If the switch is turned on/off at a fast duty cycle, the temperature can raise, causing the device to get into over-temperature mode and activate the thermal shutdown.

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