Monitoring and sequencing supply voltages - Part 2

For most electronic systems, monitoring system voltages with a power-on reset (POR) ensures proper initialisation at power-up.

Figure 4 shows a voltage detector connected to the gate of a MOSFET, which turns on and off VCC1. An n-channel MOSFET is appropriate for this application when there is a higher voltage available to provide a gate-to-source voltage that's large enough to fully enhance the MOSFET.

Figure 4: If a higher voltage is available, a voltage detector can sequence a low voltage by turning on an n-channel MOSFET.

A problem can occur, however, during power-up of this circuit if VCC2 is present before VCC1 reaching a level sufficiently high to turn on the voltage detector's output.

In this situation, VCC2 will enhance the MOSFET (ie, it will be on) until VCC1 rises sufficiently for the voltage detector's output to assert a low.

This same sort of circuit can be realised with a voltage detector and a p-channel MOSFET without the need for a second, higher voltage.

This circuit isn't suitable for low-voltage supplies, though. Also, the higher on-resistance of a p-channel MOSFET makes it impractical for high-power applications.

An easier and more reliable approach is to use a device such as the MAX6819 to perform both the monitoring and the sequencing functions (see Figure 5).

Figure 5: After the primary supply has powered up, the MAX6819 turns on the secondary supply. Its onboard charge pump enhances the MOSFET to minimise its on-resistance.

This type of IC monitors the first voltage with a reset circuit to determine whether it's within specification. When it is, the IC turns on the MOSFET via its MOSFET driver.

An internal charge pump adds a fixed voltage to the secondary supply and applies the resulting voltage to the gate of the MOSFET, which helps ensure that the gate-to-source voltage is sufficiently high to fully enhance the MOSFET.

During the manufacturing phase of many types of telecommunication, networking, storage and server equipment, a process called margining is often used to assess the robustness and future reliability of these systems.

This technique involves an evaluation of the system (or processor), which is performed by deviating the power supplies from their nominal levels. To change those levels, it's common to adjust a DC-DC converter power supply by altering its feedback loop with a digital pot or a current DAC.

Figure 6 depicts two of the many ways of margining a supply. Other common methods include programming the supply's output through a digital interface or trimming the power supply.

Figure 6: Two simple techniques for performing voltage margining include adding a digital pot or a current DAC to a DC-DC converter's feedback loop.

Different degrees of margining control include 'pass/fail' approaches where you increase or decrease all power supplies to some level (eg, +/-5% or +/-10%), or a finer-adjustment approach where you increment or decrement the supplies in smaller steps (eg, 10 or 100 mV). The latter approach allows you to evaluate the system performance in greater detail.

An ADC can be used to measure these values more accurately. It's tempting to use the ADC contained within a microcontroller for this function. However, when the supplies powering the microcontroller drop below specification, its internal reference can be out of tolerance, thus affecting the ADC's accuracy.

Also, it's necessary during margining to disconnect or disable the reset output so that the system can continue to operate, otherwise the system will reset, making it impossible to discover the supply-voltage levels at which the system fails.

The process of performing these margining functions can be quite tedious when working with large systems.

Although many processors require only two voltages - one to power the core and another to power the I/O - other devices such as DSPs, ASICs, network processors, and video processors can require up to five supply voltages.

Within a single system, supervisory circuits may need to monitor and sequence more than 10 voltages. As the number of supply voltages in these systems continues to increase, the number of ICs needed to monitor, sequence and margin them also increases.

Costs go up and more board space gets consumed. When changes to parameters such as a voltage threshold or a reset-timeout period are necessary, a new device may be required and changing the sequencing order becomes quite a difficult task.

One way to reduce the level of circuit complexity is to use a programmable system-management IC that combines the monitoring and sequencing functions. The programmability of these devices makes changes easy to handle.

They eliminate the need to swap parts in and out of a design during the prototyping and manufacturing stage.

For many of these parts, a serial interface allows you to program the internal registers that both configure these devices and set threshold levels and delays. Onboard EEPROM stores the contents of those registers.

Figure 7 shows a MAX6870 system-management device set up to monitor and sequence several system supplies. When the +12 V bus voltage powers up and exceeds its threshold (stored within the MAX6870), one of the device's outputs enables the +5 V voltage regulator, either immediately or after a delay period (also stored within the memory).

Figure 7: A programmable system-management device provides a flexible means for voltage monitoring and sequencing.

After the +5 V regulator comes up and its output crosses its corresponding threshold, the +3.3 V supply is taken out of shutdown.

The remaining supplies then power up in succession under this same scheme (except that the +5 V switched supply becomes available when the device enhances the n-channel pass element).

You can usually program this type of system-management device to provide additional supervisory functions such as reset circuits and watchdog timers. Also, these devices can monitor parameters other than supply voltage through their analog and digital inputs.

In the circuit of Figure 7, AUXIN_ (analog inputs) and GPI_ (digital inputs) monitor a temperature reading and a power-supply current-sense reading.

The 6870 includes a 10-bit ADC that digitises those readings; the microcontroller monitors the status of those digitised readings.

The temperature sensor and the current-sense monitor each include a comparator output that indicates that a fault has occurred (ie, the temperature or the current has exceeded a specific limit). Each comparator output connects to a 6870 general-purpose input (GPI). The device can be configured to turn off one or more power supplies when these fault conditions occur, thus lightening the load on the +12 V supply.

The internal ADC makes it easy to accurately margin a power supply. The voltage at each supply's output can be read from the ADC registers during margining. The margin input can disable the outputs or program them to a known state when the supplies are margined, thus preventing the system from resetting during this procedure.