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It’s no secret that DSPs are turning up in more and more applications these days. DSP suppliers have done a great job in providing the high performance, low power consumption and small size that engineers are looking for in their designs. However, as DSP speeds increase and geometries get smaller, the result is lower supply voltages and a need to pay more attention to the power requirements of the DSP and the overall system. One solution to this challenge comes in the form of newer dual low dropout (LDO) regulators.

The real issue arises from the fact that many, if not most, of today’s most popular DSPs require a split voltage power supply. While the processor core might use a 1.8-V rail, the device’s I/O will typically use 3.3 V. At the same time, the voltage requirement of the overall system needs to be considered. In all likelihood, a higher I/O voltage of 3.3 V or 5 V is required.

Until recently, designers have had few alternatives but to design multiple LDO regulators, as well as supply voltage supervisors (SVS), into their systems (see Fig. 1.)

Fig. 1 Two LDOs and Supervisors Split Voltage Needs of DSP

Obviously, in some applications, this type of design has its drawbacks. In portable and battery-powered applications, board real estate comes at a premium, and designers may require a more integrated and cost-effective solution. For example, single-DSP systems like wireless handsets, portable audio, digital hearing aids and other battery applications would be obvious beneficiaries if chip count and board space could be reduced.

Moving to LDOs

Many designers of DSP-based systems have looked at LDO regulators as a solution for split power supply architecture. Switch-mode voltage regulators could be used as alternatives to LDOs in applications where boosting the voltage or more efficiency is required, both increasing the system costs. For applications where the voltage level is only being dropped, LDOs are a more cost-effective solution and produce less electrical noise than switch-mode regulators.

Solution: Dual LDOs With Dual Voltage Outputs

Clearly, a dual LDO with output voltages tailored to meet the voltage requirements of these split-voltage DSPs would be the optimal solution. The first prerequisite for meeting the power supply requirements of some of today’s advanced DSPs (see Fig. 2) is to have two LDOs in the same package with output voltages meeting the two distinct voltage requirements. Beyond this, there are several additional factors for the designer to consider, such as transient response, integrated power supervisor, low quiescent current and thermal conditions.

 

DSP	Voltage Requirements
TMS320C206	Core: 3.3 V I/O:  5.0 V
TMS320C5402	Core: 1.8 V I/O:  3.3 V
TMS320C5410	Core: 2.5 V I/O:  3.3 V
TMS320C6201B	Core: 1.8 V I/O:  3.3 V
TMS320C6211	Core: 1.8 V I/O:  3.3 V

Fig. 2 Split Voltage Needs of TI’s DSPs

Transient Response

The current requirements of many DSP-based systems can vary greatly over a short period of time. The DSP can move from a state of relative inactivity to a heavy processing load in an instant. The parameter that indicates how effectively a regulator can respond to rapid load swings is its transient response. Circuit transient response is also affected by the characteristics of the regulator’s output capacitor, including its capacitance value and equivalent series resistance (ESR). Generally, higher capacitor values and smaller ESR values contribute to better output voltage transients.

Some of the new dual LDOs are designed to be stable and have fast transient responses with low-cost ceramic 10-m F low-ESR capacitors. LDOs, such as Texas instruments’ TPS767D3xx family, have a very fast transient response rate and can respond to a one ampere load demand change in just two microseconds (see Fig. 3.) With this kind of transient response, the designer can specify a smaller, less-costly capacitor, which also saves board space.

Fig. 3 Rising-Edge Response: 1 A/µs For Transient Input Pulse On a TPS767D333
Dual LDO With 100 µF Tantalum Cap.

The device accuracy at 2% over load/line and temperature is 2%, so the total accuracy of the device and transient is less than five percent.

Integrated Power Supervisor

In addition to having two low dropout regulators, some of the new dual LDOs have taken the integration process one step further and included supply-voltage supervisors (SVSs), which are being used extensively in DSP-based systems because they significantly augment the system’s reliability. Two of the most prominent features of SVSs are their ability to supervise the power-on reset process as well as watch for any under-voltage condition. During the power-on process, the power supplied to the DSP does not reach the required minimum voltage immediately, and if the DSP attempts to start up before the supply voltage stabilizes, malfunctions could occur in software execution and hardware. A SVS is able to hold the power-on process in a reset condition until the power has reached the predetermined minimum level to assure a safe start-up process.

In under-voltage conditions, a SVS serves a similar function, in as much as it supervises the shutdown process in an orderly fashion. The power supply often degrades over time for battery-powered applications, and as the useful life of the battery approaches its limit, the processor will shut down. Once the processor begins the shutdown process, the voltage across the battery may rise, which might cause the processor to begin the start-up process. To guard against this unstable process, an SVS has an under-voltage supervisory function that allows the processor to go through the shutdown process and holds it in a reset condition until the power supply has returned to a minimum level for a predetermined period of time.

By integrating two SVSs onto a dual LDO, designers can improve system reliability while saving board space, simplifying design and reducing component count.

Low Quiescent Current and Thermal Considerations

Other issues that designers should consider when selecting LDOs or other power supply components are the power consumption of the devices themselves and their thermal performance. If a dual LDO is being considered for a battery-operated system, the device should have a low quiescent current to enable a longer battery life.

Thermal performance is also critical in portable systems because PCBs are usually densely populated with very small system enclosures. Large or exotic heat dissipation techniques are often not feasible in these types of systems; to overcome this, some dual LDOs, such as this noted Texas Instruments family, are being placed in new thermally-enhanced packages which can efficiently dissipate heat without requiring a great deal of board space or large heat sinks.

More Effective DSP Designs

The emergence of dual LDOs is just another indication of how designing with DSPs has become simpler and more effective. And because of components that tightly complement their needs, we will continue to see DSPs infiltrate newer and more innovative applications.

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