Auto-Track sequencing is a feature available on next-generation power modules (like TI's PTHxx series). The feature is designed to simplify the amount of external circuitry required to configure the modules for simultaneous power-up and power-down supply voltage sequencing.
The ability to sequence the power-up of multiple supply voltages in complex logic and mixed signal applications has become an important requirement for power system designers. This is because most VLSI logic devices, including DSPs, ASICs, FPGAs, and microprocessors, now require at least two supply voltages. This usually includes a low-voltage to power a high-speed logic core, and a standard logic voltage to power the input/output (I/O) interface, and supporting system devices such as memory, data converters and I/O ports.
The power-up sequencing techniques can be implemented using a number of techniques. The implementation of any one method depends largely on the time and/or voltage restrictions that are allowed between the two supply voltages during both power up and power down. The restrictions are imposed on the power system designer by the respective VLSI device manufacturers. Failure to meet the restrictions can result in undue voltage stress, and even "latch up" between the VLSI device's I/O port and a supporting peripheral. This can result in immediate if not latent damage to the VLSI device. In the case of the latter the long-term reliability of the affected device may be compromised.
Simultaneous Power Up
One of the most widely used power-up sequencing methods is the simultaneous power up of the circuit supply voltages. See Figure 1. The core and I/O power supply voltages must begin rising together and also rise at the same rate. The two voltages continue to rise until the core supply reaches its nominal regulation voltage. The higher I/O voltage then continues to rise until it too reaches its regulation voltage.Of the many alternative techniques, the simultaneous method is the more accepted for most dual-supply voltage applications.
This is because it significantly reduces the voltage difference that can occur between the two voltage rails, throughout the power-up sequence. However, it is not a universal answer for every application. As a rule, VLSI device manufacturers do not specify which sequencing method should be used; only the voltage and time restrictions that must be adhered to during power up.
Although most widely accepted, the simultaneous power-up method is more difficult to implement. It requires that one or more of the power supply circuits (generating the supply voltages) be precisely controlled during the power-up period. This level of control not only adds components but also requires that the power designer have intimate knowledge of the power supply regulation circuitry. While this may not be a problem for the designer of a discrete power supply, it adds an unwelcome level of complexity for designer's who prefer to use off-the-shelf power supply modules.
Figure 1. Simultaneous Power-Up of the Core & I/O Voltages.
Auto-Track is included all versions of the new series of plug-in power modules. Products with this feature incorporate additional circuitry that allows the module to be temporarily controlled from an external signal source. Using Auto-Track, the module's output can be directly controlled during power-up and power-down transitions. The control voltage can be a system-wide master ramp voltage, the output of another power supply circuit, or the module's own internal RC ramp waveform.
How Auto-Track Works
Auto-Track uses a control pin called "Track" to control the output voltage of the module over the range, 0V up to up to the nominal set-point voltage. Within this range the voltage at the module's output will follow the voltage applied to the Track pin on a volt-for-volt basis. However, once the voltage at the Track pin is raised above the module's set-point voltage, the module's output remains at its set-point voltage. As an example, if the Track pin of a 2.5-V regulator is at 1 V, the regulated output will be 1 V. But if the voltage at the Track pin rises to 3 V, the regulated output will not go higher than 2.5 V.
As the output from the module simply follows that at the Track pin, it is able to 'track' virtually any voltage source during the power-up sequence. For the designer's convenience, each Track pin is also provided with its own RC charge circuit that can produce a compatible rising voltage from the input source voltage.
The basic implementation of Auto-Track allows for simultaneous voltage sequencing of any number of Auto-Track compliant modules. The Track control pins of two or more modules are merely connected together (See Figure 2). This does two things. It forces the Track control of the modules to follow the same collective RC ramp waveform, and also allows them to be controlled through a single transistor or switch, Q1.
To initiate the power-up sequence, the Track control must be first pulled to ground potential. This must be done at or before input power is applied to the modules, and for 20 ms thereafter. This brief period gives the modules time to complete their respective internal power-up sequence so that they are ready to produce an output voltage. A logic-level high signal at the on/off control input turns Q1 on, and holds the Track control at ground potential. It should be noted that after the input voltage has stabilized, the output of all modules will remain at zero volts until Q1 is turned off.
After 20 ms, the Q1 may be turned off by applying a logic-level low drive voltage to the circuits on/off control. This allows the track control voltage to automatically rise toward to the modules' input voltage. During this period, the output voltage of each respective power module follows the common track control voltage, rising in unison with other modules to its set-point voltage.
Figure 3 shows the output voltage waveforms from the circuit of Figure 2 after the on/off control voltage to the circuit is set from a high to a low level. The waveforms, Vo1 and Vo2 represent the output voltages from the two power modules, U1 (3.3 V) and U2 (2.0 V) respectively. The oscilloscope graph shows the output voltages, Vo1 and Vo2, rising together to produce the desired simultaneous power-up characteristic.
Figure 2. Simplest Implementation of Auto-Track for Power-Up & Power- Down Voltage Sequencing.
Figure 3. Simultaneous Power Up of Two Modules Under Auto-Track Control.
Although not always a strict requirement, the same circuit also provides a power-down sequence. Power down is the reverse of power up, and is accomplished by lowering the track control voltage back to zero volts. An important constraint is that the input voltage must be present until the sequence is complete. It also requires that Q1 be turned off relatively slowly. This is so that the Track control voltage does not fall faster than Auto-Track's slew rate capability, which is 10 V/ms. The components R1 and C1 in Figure 2 limit the rate at which Q1 can pull down the Track control voltage. The values of 100 k-ohm and 0.047 uF correlate to a decay rate of about 0.6 V/ms.
The power-down sequence is initiated with a low-to-high transition at the on/off control input to the circuit. Figure 4 shows the waveforms of Vo1 and Vo2 after the on/off control voltage goes high. Although the Track control voltage begins its downward slope immediately, there's a short time delay before it reaches the voltage of the highest output. As the Track control voltage falls below the nominal set-point voltage of each power module, then the respective output decays with all the other modules under Auto-Track control.
Figure 4. Simultaneous Power Down of Two Modules under Auto-Track Control.
Simultaneous Power Up & Power Down from Another Power Module
One of the most powerful attributes of the Auto-Track feature is its flexibility. The Track pin of any Auto-Track compliant power module will follow almost any voltage up to a slew rate of 10 V/ms. This includes the output voltage of another power module; even a module that is not Auto-Track compliant. Figure 5 shows a schematic that illustrates this arrangement. It requires that the Track pins of the lower voltage modules be connected directly to the output of the module with the highest output voltage. In this case it a PT5801 (U1). In this configuration, U1 must be controlled from its on/off Inhibit control pin. This is opposed to its Track pin; even if it were available.
To initiate the power-up sequence the U1 Inhibit control must be held to ground as input power is applied, and then held there for another 20 ms. This allows time for the auto-tracking module, U2 (PTH05010EA), to complete its internal power-up. In this circuit, the TPS3838K33, a nanopower supervisor IC (U3), is used to both detect the input voltage and provide the required 20 ms time delay.
Figure 6 shows the power-up waveforms for the circuit in Figure 5. The combination of the capacitor C1, and the nanopower supply supervisor U3, delays the release of the ground signal to U1 until about 20 ms after the input source, Vin, has been applied. Soon after its Inhibit control input rises, U1's output voltage rises to its set-point voltage. The rate at which the outputs rise is limited only by U1's internal soft-start circuit. This is about 0.65 V/ms, and slow enough for the Auto-Track units to follow.
Figure 5. Sequenced Power Up & Power Down with Other Modules.
Figure 6. Simultaneous Power-Up Waveforms (from the Circuit of Figure 5).
As mentioned, power-down sequencing with Auto-Track is subject to the same constraint as power up. That is a valid input voltage must be available to all modules under the control of the Track control, throughout the power-down sequence. This constraint makes it necessary for the power system to conduct a coordinated power shutdown for all circumstances. This is irrespective of whether a shutdown is initiated by a human operator, or the result of a line voltage failure. In the case of the latter, there must be sufficient hold-up charge in the power system, to allow time for a power-down sequence to be completed prior to any drop in the input voltage to the circuit. The nanopower supervisor (U3) will only turn off U1 (via the Inhibit pin) after the input voltage has already begun to decay. Therefore, it cannot be used to initiate power-down. This requires the use of a separate transistor. Q1 in Figure5, is in parallel with U3 and can turn off U1 prior to any drop in the input voltage. When U1 (PT5801) is turned off, its output is tri-stated. This means it will neither source nor sink current from the load. This allows the output voltage to fall only as fast as the load discharges the output capacitors. Once the output voltage from U1 decays below U2's set-point voltage, it pulls down U2's output via its Track pin.
Figure 7 shows the output waveforms to the circuit of Figure 5 during power down. To ensure Auto-Track can follow the output of another module, the voltage being followed must not change faster than Auto-Track's slew rate capability. This is 10 V/ms. During power down, a decay rate faster than this will result in a delay before the lower voltage outputs begin to follow the higher voltage. This could produce an excessive voltage differential. The decay rate limitation correlates to a minimum of 100 uF of capacitance per ampere of load current at the output of U1. Also, in addition to having the highest output voltage, the module selected for U1 should be carefully chosen to ensure that it does not sink current when turned off via its on/off Inhibit control.
Figure 7. Simultaneous Power Down Waveforms (from the Circuit of Figure 5).
Auto-Track is a feature incorporated into all of the new PTHxx family of plug-in power modules from Texas Instruments. The feature makes it possible for the output voltage of these modules to be directly controlled (volt-for-volt) below their respective set-point voltages. This added flexibility allows such modules with different output voltages to be easily configured for simultaneous power-up and power-down voltage sequencing. The circuits discussed showed two examples of how Auto-Track can be configured. The first showed how the Track control input of a number of modules can be connected so that their output voltages rise in unison to their own internally generated RC ramp voltage. The second showed how the Track control of lower voltage modules can be connected to directly follow the output of another higher-voltage module, during both power-up and power-down transitions.
The Auto-Track feature is not limited to just two modules or output voltages. It can be used to configure a number of modules, with different output voltages, for supply voltage sequencing. For example, 3.3V, 2.5V, and 1.5V.
1 Power-Supply Sequencing for Low-Voltage Processors, by Brian Rush, Texas instruments. Published in EDN, September 1, 2000.
2 Dual Output Power Supply Sequencing for High Performance Processors, by David Daniels, Tom Fowler, Texas Instruments. SLVA117.