Power supply systems for the latest digital signal processors (DSPs) and application specific integrated circuits (ASICs) are required to manage how multiple output voltages rise after input power is applied. These supplies must also be capable of starting up into pre-existing output voltages.
Another requirement gaining in popularity is supply voltage margining. This is the ability of the power system to momentarily adjust the output voltages to test the load circuit over its supply voltage tolerances.
Features built into the new PTH series of power modules from Texas Instruments make it easy to include these capabilities into the power system. This article describes these features, and provides examples of how these modules are able to meet the latest power system requirements with just a few external components.
Supply Voltage Sequencing
Coordinating the power-up of the I/O and core supply voltages, for either a DSP or an ASIC, can be accomplished using a number of techniques . The two most popular methods used are described as the "simultaneous," and "sequential," power-up sequence respectively. Figure 1 shows a simultaneous power-up sequence. Here both the I/O and core voltages must begin rising together, and then rise at the same rate until the lowest voltage reaches regulation. Figure 2 illustrates a sequential power-up sequence. In this example, the core voltage is allowed to rise after the I/O voltage has reached regulation.
The PTH series of power modules incorporate two features to help designers manage their power-up sequence requirements. The first is called Auto-Track. It significantly reduces the amount of circuitry required to produce a simultaneous power-up sequence. The second is called "Pre-bias Startup." This feature prevents the output of a power module from sinking current when a sequential power-up sequence is desired.
Figure 1 Simultaneous Power-up Sequence of the I/O & Core Supply Voltages
Figure 2 Sequential Power-up Sequence of the I/O & Core Supply Voltages
Simultaneous Power-up Characteristic Using Auto-Track
To produce a simultaneous power-up characteristic, at least one of the power modules (generating the supply voltages) must be precisely controlled during the power-up period. Auto-Track uses a control pin called "Track," which controls the output voltage of the module between 0V and its set-point voltage. Within this range the voltage at the module's output follows a signal applied to its Track pin on a volt-for-volt basis. Once the voltage at the Track pin is raised above the module's set-point voltage, the output voltage remains at its set-point.
By connecting the track pin of a number of these modules together, their output voltages are able to follow a common signal voltage during the power-up period. The Track control inputs also have an internal RC charge circuit. This operates off the module's input voltage to produce a suitable rising waveform. This allows the modules to be controlled from either an open-collector transistor or supervisor IC. Figure 3 shows an application circuit for providing a simultaneous power-up sequence.
Figure 3 Application Circuit for a Simultaneous Power-Up Sequence
U3 (TPS3838K33) is a nano-power voltage supervisor circuit with a programmable delay. U3 applies a ground signal to the common Track control as the input voltage starts to rise. This ground signal is maintained until the input voltage has risen above 4V, and then for 10ms thereafter. The 10 ms delay allows time for the power modules to complete their soft-start initialization. When U3's output allows the Track control to rise, the output voltage of each power module follows the Track voltage, to its respective set-point voltage.
Figure 4 shows the power-up waveforms from the circuit of Figure 2. Vo1 and Vo2 represent the output voltages from the two power modules, U1 (3.3V) and U2 (2.0V) respectively. The oscilloscope graph shows the voltages, Vo1 and Vo2, rising together to produce a simultaneous power-up sequence.
Figure 4 Simultaneous Power-Up Sequence with Auto-Track Control
Sequential Power-up Technique
When using a sequential power up technique with some ASICs, a leakage path within the IC causes some voltage from Vo1 to appear on Vo2. This is before the source for Vo2 has been enabled (see Figure 5 ). This is defined as a pre-bias start up condition. It occurs whenever an external voltage is applied to the output of a power module prior to its output being activated. A pre-bias startup can cause problems with power modules that incorporate synchronous rectifiers. This is because under most operating conditions, synchronous rectifiers can sink, as well as source, output current. The PTH series of power modules can also sink current under normal operating conditions, but the 3.3V and 5V input versions will not do so during the power-up period, or whenever the module is turned off via the inhibit pin.
In Figure 5 the module U1 (PTH05020W) produces 3.3V for the IC's I/O power, and U2 (PTH05010W) produces 2V for the IC's core power. The power supply voltage restrictions of the ASIC, requires that the output voltage from U1 (Vo1, 3.3V) rise before that from U2 (Vo2, 2V). The circuit uses the voltage detector, TPS3803-01 (U3), to monitor the output voltage of U1. U3 holds off U2, via its Inhibit control, until Vo1 has risen above 3V.
Figure 5 Application Circuit for a Sequential Power-up Sequence
Figure 6 shows the waveforms of the circuit after input power is applied. It shows Vo2 rising before its Inhibit control waveform (Vinh) goes high. This is caused by the leakage path within the ASIC. However, the waveform of U2's output current (Io2) is negligible until U2 raises Vo2 above the pre-bias voltage (point 'A'). Thereafter, the waveform of Io2 shows a positive output current.
Figure 6 Waveforms Showing a Sequential Power-up Sequence with Pre-bias Startup On Vo2
Power Supply Voltage Margining
Power supply margining is the process of dynamically testing the load circuit over its supply voltage range. This testing demonstrates the load circuit's ability to tolerate small changes in the power supply voltages that may occur over time and temperature. It can also verify the functionality of the supply voltage supervisors, and improve the long-term reliability of the end product.
The PTH series of power modules incorporate circuitry for performing output voltage margining. Two control inputs, 'Margin Up' and 'Margin Down', are provided. Connecting the Margin-Up control to ground increases the module's output voltage by up to 5%. Similarly, connecting the 'Margin-Dn' pin to ground decreases the output voltage by up to 5%. The circuit of Figure 7 shows an example of how the output margining feature is used. Only two low-leakage transistors are required. The resistors, RD, and RU are optional. They are included if the desired amount of adjustment is less than 5%.
Figure 7 Output Voltage Margining Circuit
 Power-Supply Sequencing for Low-Voltage Processors, by Brian Rush, Texas instruments. Published in EDN, September 1, 2000.
 Dual Output Power Supply Sequencing for High Performance Processors, by David Daniels, Tom Fowler, Texas Instruments. [SLVA117].
Chris Thornton is an Application Specialist with the Plug-in Power Solutions product line at Texas Instruments. He has more than 15 years of experience in the design and application of DC/DC converters. Chris has a Bachelor of
Science in Electrical Engineering from Coventry University, in the United Kingdom.