Designing supply voltage supervision for multi-rail boards
Circuit boards with FPGAs, microprocessors, ASICs, and DSPs have particular design criteria, resulting in multiple power supply rails. Pinkesh Sachdev, Linear Technology, looks at some voltage supervision challenges.
Circuit boards with FPGAs, microprocessors, ASICs, and DSPs have multiple power supply rails, ranging from sub-1V PoL to 12V intermediate bus, requiring voltage supervision to ensure reliable and error-free system operation. As silicon process technology has scaled to tens of nanometers, not only has the lowest PoL output voltage (usually powering the core) been below 1V but the processor core’s accuracy specification has also tightened to 3% and better. These accuracy specifications translate to tight tolerances both on the power supply voltage and the voltage supervisor monitoring such a supply.
Over the same period, the number of supply rails has proliferated as they are needed to power the core and I/O, memory, PLLs and other analogue circuits, making 10 or more rails common. Frequently, it is also the case that the exact supply voltage levels are not known until late in the design, or even after the board is fabricated and assembled. Optimising supply voltages to lower board power consumption entails customised trimming of the supply voltage and a corresponding adjustment to the supervision threshold based on each individual board’s performance. The voltage levels may also change with a revision of the FPGA, microprocessor, ASIC or DSP. Traditionally, supervisor threshold changes are implemented by reworking resistive dividers or setting jumpers, but the adjustment granularity is limited and the procedure is cumbersome, time consuming, and prone to errors.
Importance of voltage supervision
Supervisor accuracy plays an important role in determining the tolerance and cost for the required power supply.
Figure 1. Setting supervisor threshold
Assume a microprocessor specifies a 1V±3% voltage for its core power supply input, implying the valid operational range to be from 0.97 to 1.03V. For enhanced reliability, an external voltage supervisor is employed to monitor this supply instead of just relying on the processor’s internal power-on-reset. In an ideal world, the under-voltage supervisor threshold is set at 0.97V, signaling a reset as soon as the supply voltage drops below 0.97V (Figure 1). In reality, voltage supervisors are built out of analogue reference voltages and comparators, each of which has a tolerance band contributing to the supervision threshold variation. For a 0.97V supervisor threshold with ±1% accuracy, the threshold varies from 0.96 to 0.98V. When the threshold is at the low end, the supply could be outside the core’s valid voltage range but the supervisor will not be signaling a reset, leading to a malfunction. To remedy this, the nominal supervisor threshold is set 1% above the 0.97V end of the valid range. The downside is that when the supply voltage is below 0.99V, a reset could be signaled due to a high supervisor threshold. Therefore, the supply voltage needs to stay above 0.99V. As a result, the supervisor threshold accuracy eats into the power supply operating range.
Figure 2: LTC2933 programmable hex voltage supervisor with EEPROM and I2C/SMBus
The same analysis applies to the over-voltage threshold which will be nominally set to 1.02V, restricting the upper end of the supply voltage range to 1.01V or 1V+1%. Therefore, a ±3% specification on the microprocessor coupled with a ±1% supervisor threshold accuracy, yields a power supply tolerance requirement of ±1%. Note that a ±1.5% supervisor accuracy yields an impossible power supply tolerance of 0%. If over-voltage protection is not needed, the supply voltage can range from 0.99 to 1.03V; a 1.01V ±2% power supply works for this case. Supervisor accuracy plays an important role in determining the tolerance and cost for the required power supply.
Linear Technology offers programmable, six-supply voltage supervisors: LTC2933 (Figure 2) and LTC2936, with integrated EEPROM, 0.2 to 13.9V threshold adjustment range, and 8bit (256 choices) threshold registers set through an I2C/SMBus digital interface. Both devices offer ±1% accuracy for thresholds in the 0.6 to 5.8V range, with two adjustable-polarity thresholds per monitor input. For example, one of the thresholds can be configured as an under-voltage detector for reset generation while the other threshold can be employed either for over-voltage detection, protecting expensive board electronics against damage, or as a higher under-voltage threshold for an early power-fail warning, providing valuable time for the processor to back-up data. Threshold adjustment through the I2C/SMBus interface gets rid of external resistive dividers, freeing up board space and eliminating accuracy degradation due to resistor tolerances. Last-minute threshold changes are quickly achieved by writing to configuration registers instead of reworking boards, reducing time-to-market. Volatile memory holds instantaneous fault status while internal EEPROM stores register configuration and backs up fault history, speeding debug and reducing development time. To minimise nuisance resets from supply noise, the supervisors respond to input glitches based on comparator overdrive (Figure 3).
Figure 3: LTC2933 response time versus over-drive on V2 to V6 monitor inputs
Two general purpose inputs can be configured as a manual reset input, under-voltage or under/over-voltage fault disable input (e.g. during board margin testing), write protect input (LTC2936 only), or auxiliary comparator inputs. The auxiliary comparator’s fixed 0.5V threshold is ±2% accurate, extending monitoring to a total of eight supplies with external resistive dividers. Three general purpose I/Os (GPIOs) can be configured either as inputs or as reset, fault, or SMBus alert outputs. Any general purpose inputs, GPIO, or under-voltage/ove-voltage fault inputs can be mapped to any of the GPIO outputs. The GPIO pins are programmable for a delay-on-release time (1µs to 1.64s), output type (open-drain or weak pull-up), and polarity (active high or low). No software coding is needed as the LTpowerPlay development environment (Figure 4) configures devices through a GUI. The LTC2933 and LTC2936 also monitor negative supplies, such as those powering analogue circuits, with a resistive divider to the negative supply from a 2% accurate 3.3V linear regulator output.
The differences between the LTC2933 and LTC2936 are shown in Table 1. One of the LTC2933 inputs directly monitors a 12V intermediate bus while the other five inputs monitor supplies in the 0.2 to 5.8V range with thresholds adjustable in 4mV steps on the 0.2 to 1.2V precision range setting. The LTC2936 brings out each monitor’s comparator outputs to a pin, enabling a cascade sequencing application in which a supply is started up after the previous supply in sequence has reached its valid operating range.
Table 1: Differences between LTC2933 and LTC2936, hex voltage supervisors with EEPROM
One of the challenges presented by modern digital boards with multiple power supply rails is the need to precisely monitor a variety of supply voltages, some with levels not known until the last minute, in order to generate a reset for the processor system when the supplies power-up or brown-out. Being able to monitor and supervise six rails with programmable, accurate thresholds, speeds time-to-market, and meets the accuracy demands of modern processors without the need to procure and maintain inventory on multiple supervisor devices.