Adding over-voltage protection to your mobile/portable embedded design

Bernard Remaury

July 1, 2008

Bernard Remaury

This "Product How-To" article focuses how to use a certain product in an embedded system and is written by a company representative.

Portable electronic devices use internal batteries that need to be charged from an external power supply, most usually an AC/DC wall adapter. Li-ion battery technology has decreased the total weight of portable devices.

However, a bad charge sequence can induce Li+ temperature increase, thermal runaway and burst, and endanger people's lives. One of the primary safety measures is to protect the internal charger, which manages the battery pack charge, from the outside.

Causes of overvoltage
Portable electronic devices come with a wall adapter that is compatible with its maximum input rating. In this case, the output AC/DC voltage is well regulated to limit output ripple. But, despite recommendations from suppliers to always use original adapters, an after-market exists: second or third wall adapters are used for travel or just to replace the original one after a failure.

Depending on the complexity of the adapter, its output voltage can have output transients that far exceed the ratings of the sensitive electronic components required to make today's small portable products.

Figure 1: Using an overvoltage protection device protects systems from dangerous wall adapter transient voltage increases.

Another possible cause of wall adapter output voltage increase is the loss of optocoupler feedback in a switch-mode power supply charger. This failure can occur even in the high end AC/DC market. In this case, the output voltage could increase up to 20V. Using an overvoltage protection device (OVP) protects systems from this dangerous voltage increase as shown in Figure 1 above.

Overvoltage can also occur if an AC/DC is hot plugged. This behavior is due to the serial inductance in the adapter cabling. The maximum ripple voltage depends on the mobile system's input capacitance and parasitic inductance in the cable.

Protection by integration
Using an overvoltage protection device (OVP) can protect a mobile device against these causes of overvoltage. Compared with previous generations of OVPs, the pass element (N MOSFET or P MOSFET) has now been integrated to save PCB area.

The PCB area calculation of a dual die solution must take into account the package size and the layout between the two devices. A comparison between the new generation of OVP's and old driver + MOSFET solutions, gives up to 60 percent of saved space.

The PCB layout must be carefully designed to improve thermal dissipation. Extra copper surface must be added to reduce junction temperature connected to the background pad.

As this pad is connected to NMOS drain, the extra copper surface shall be connected to IN pins or to an isolated plane. In all cases, this area must not be connected to ground.

Another important point is overvoltage threshold definition. Overvoltage lockout (OVLO) and undervoltage lockout (UVLO) thresholds are determined by internal comparators which switch off the pass element when an under- voltage or over-voltage event is encountered (Figure 2 below).

Figure 2: OVLO and UVLO thresholds are determined by internal comparators which switch off the pass element when an under-voltage or over-voltage event is encountered.

The OVLO level must be higher than the AC/DC maximum operating output voltage and lower than the maximum rating of the first component of the system. Figure 3 below shows the typical portable device architecture based on a fully integrated OVP such as the ON Semiconductor NCP347MTAE.

To guarantee stability, an input capacitor must be placed in front of the OVP, as close as possible to the IN pins. The characteristics of the capacitor must be in line with those of protection device.

Figure 3: The typical portable device architecture based on a fully integrated OVP such as the ON Semiconductor NCP347MTAE is shown.

The capacitor's DC bias curves should be checked to ensure that actual capacitor's value is high enough regarding the UVLO to OVLO voltage range. For example, let's assume that 1µF ceramic capacitor is necessary in front of the protection device:

Taking into account that the breakdown voltage of a ceramic capacitor (higher than 200V) is higher than the OVP's maximum rating (30 V); a 10- or 16V/1µF can be used for these products. The breakdown voltage depends on the quality of the ceramic material.

Figure 4: A capacitor's DC bias curves will be checked to ensure the actual capacitor's value is high enough regarding the UVLO and OVLO voltage range

Figure 4 above shows an example of DC bias and Figure 5 below shows breakdown DC voltage of 0603/ X5R/1µF/16 V.

Additional features
It is now possible to conciliate very low Rdson with a low profile package. For example, the NCP347's has an Rdson of only 110m? for a 2mm x 2.5mm WDFN package. Such devices are able to sustain up to 2A DC current.

The typical dropout between wall adapter and charger is only 52mV at 25°C. Thanks to the very low losses, these products are able to support wall adaptors having low output voltage.

The lower the dropout, the lower the thermal dissipation in the portable device and higher is the capability to stand the wall adapter having bad load regulation. Innovative architectures have enabled very fast internal switch turn-off time to be compatible with very low current consumption.

Figure 5: The breakdown voltage depends on the quality of the ceramic material used.

Downstream systems never encounter overvoltage transient in most cases. In the example above, the typical turn-off time is 1µs, with a maximum of 5µs. An "enable" pin may be available to turn on the device, or to pull up to the battery if we wish to isolate the system from the wall adapter.

A status pin may be used to supervise the voltage level. When this pin is an open drain input, it must be pulled up to the battery, through a minimum of 10k? resistor. Connecting the status pin to a microcontroller input and the "enable" pin to an output, the OVP device can be completely turned off in the event of a constant voltage fault present on input pins.

On the other hand, the microcontroller can take into account the status pin, to turn on the OVP.

New solutions, standards
IC manufacturers are providing innovative solutions to efficiently protect mobile devices against overvoltage.

The NCP347 and NCP348 from ON Semiconductor are perfect illustrations of this trend. These fully integrated solutions are able to meet most application needs thanks to their 2A charge current capability and up to 28V protection, with very fast turn off transient.

To be compliant with the various AC/DC output voltages, several versions are proposed with different OVLO thresholds. Rdson, turn off time, current consumption are the most stringent requirements.

In particular, one of these versions is compatible with USB charge and is suitable for the new Chinese charging standard. Indeed, more and more portable devices are equipped with USB connectors and can be supplied with USB host or wall adapter with USB connectors.

Bernard Remaury is an Application Engineer in the Low Voltage Power Management Business Unit at ON Semiconductor Corp.