Title: Designing Effective Power Management Circuits in ...



Designing Effective Power Management Circuits in Battery Operated Environments

Class #249

By Bonnie C. Baker Microchip Technology Inc.

One of the primary challenges a designer faces when configuring a battery-powered system is determining the power-management strategy. Beyond the selection of the battery, where chemistry, charging methodologies, protection and fuel gauges are a major concern, the selection of the power-conversion strategy has a significant effect on the dynamic performance and efficiency of the system. This class covers the theories and practices for good power-supply design, while providing good efficiency guidelines to optimize the design solution.

Selection of the Right Battery Chemistry for Your Application

Batteries are categorized into two fundamental groups: primary cells and secondary cells. One of the fundamental differences between these two types of cells is found in their ability to recharge after use. The primary cell battery is not rechargeable and consequently is thrown away after use. The more common chemistries for these types of batteries are Zinc, Carbon, Alkaline, and Lithium.

The most popular chemistry used for the primary cell is Alkaline. This type of battery can be found in electronic calculators, cameras, electric shavers, tape recorders, and remote controllers. The attributes that these applications have in common are their requirements for larger current/battery capacity, lower self-discharge, low internal resistance and low cost/ease of replacement capability. An example of some of the typical performance specifications for Alkaline batteries is given in Table 1.

|Battery Size |Nominal Voltage |Rated Capacity |Rated Cut-off Voltage |Energy Density by Weight |

| | | |(V) |(mWh/g) |

| |(V) |(mAh) | | |

|D |1.5 |17,000 |0.8 |180 |

|C |1.5 |7,800 |0.8 |167 |

|AA |1.5 |2,780 |0.8 |179 |

|AAA |1.5 |1,150 |0.8 |143 |

|9V |9.0 |570 |4.8 |114 |

Table 1: Shows the typical Alkaline battery specification. The definitions of specifications are: Nominal Voltage – typical operating voltage for the cell. Rated Capacity – amount of energy available until the cell reaches the cut-off voltage. Rated Cut-off Voltage – the minimum operating voltage. Energy Density by Weight – amount of energy available in the battery based on weight.

The Alkaline battery is a good “workhorse” for everyday flashlights, radios and toys. The nominal voltage of 1.5V is often doubled or tripled by putting batteries in series. The rated capacity is significantly larger than secondary cells such as Nickel-Cadmium (NiCd), Nickel Metal Hydride (NiMH), or Lithium Ion (Li-Ion) batteries.

Secondary-battery cells are rechargeable. The more typical secondary cells include sealed lead acid, NiCd, NiMH, Li-Ion and Lithium-Polymer (Li-Poly). The sealed lead acid battery is usually found in automobiles or applications where weight is a secondary consideration. The NiCd, NiMH, and Li-Ion batteries are usually used in portable applications, where weight is a consideration. The Li-Poly and fuel cell batteries are new to the market and will not be discussed in this session.

The general applications and characteristics of the secondary batteries used in portable applications are summarized in Table 2.

|Battery |Some Typical Applications |Energy Density by |Operating |Primary Charge Termination|

|Chemistry | |Weight |Voltage |Method |

| | |(Whr/kg) |(V) | |

|NiCd |Power tools, electronic tools |40-80 |1.2 |-ΔV/dt |

|NiMH |Shavers, digital cordless phones, toys. May |60-100 |1.3 |-ΔV/dt or -ΔT/dt |

| |replace NiCd. More environmentally friendly than | | | |

| |NiCd. | | | |

|Li-Ion |Cellular phones, notebook PC |110-130 |3.6 |IMIN + Timer |

Table 2: Shows the typical secondary-battery cell applications and specifications. The definitions of the specifications are: Energy Density by Weight – the ratio of available energy to its weight. Operating Voltage – a typical operating voltage for a fully charged cell. Primary Charge Termination Method – is used to identify a fully charged battery. -ΔV/dt uses a drop in battery cell voltage over a predetermined time. -ΔT/dt uses a drop in battery cell temperature over a predetermined time. IMIN uses a measurement of the current into the battery during charge.

Depending on the application, these three types of secondary-battery cells each have their own set of advantages and disadvantages. The NiCd battery was the first major rechargeable battery on the market and is capable of supplying larger current spikes. This is particularly useful in applications such as power tools. If the charging is implemented with a slow approach, the electronics to support this strategy is relatively inexpensive. On the other hand, the energy density of this type of battery is significantly lower than the other two types shown in Table 2. In addition, it has some environmental issues in terms of storage and shipping.

Another secondary-battery type is the NiMH battery. This battery followed the NiCd battery to the marketplace with improved energy density. It is also environmentally friendly. As compared to the NiCd battery, the NiMH battery does not have the ability to effectively handle large current rushes as the NiCd does, and the charging electronics are a little more sophisticated.

The more popular secondary-battery cell for portable applications is the Li-Ion cell. This battery type followed the NiCd and NiMH to the marketplace. The energy density of the Li-Ion cell today is best in class.

Taking the Battery Voltage to a Useful System Voltage

When selecting the application’s battery power-management strategy, efficiency, in addition to performance, cost and size, are some of the considerations in the final decision. The choices for this application problem are Switched Power Converters (SPC), charge pumps, and Low Dropout Regulators (LDO). As far as efficiency is concerned, the SPC is overall the best of class for almost all applications. If the application can tolerate the switching EMI (Electro-Magnetic Interference) noise, its efficiency is relatively independent of line voltage and output current. Ideally, the SPC efficiency is 100 %, the charge pump is nearly comparable to the SPC solution with power efficiency in terms of line voltage, but is more efficient for a small range of load current. However, typically charge pumps have unregulated outputs. If the output of a charge pump needs to be regulated, a LDO is required at the output of the charge pump and the efficiencies of the charge pump and LDO are multiplied together. The result is a system with a lower efficiency than either the LDO or the charge pump alone. The unregulated linear regulator efficiency can be approximated by the ratio of VOUT over VIN. Given this scenario, the efficiency is dynamic and decreases linearly with the increase in line voltage. If the source voltage has wide changes, the linear regulator is the poorer choice for your power-supply design. But the good news is that a linear regulator’s efficiency is relatively independent of output-current load if the application can tolerate the power dissipation.

With all of these options, it would seem that choosing the right power system for your application would be difficult. But if time is taken, the careful selection of the power-supply strategy can provide a competitive edge by providing the most efficient, compact, low-cost solution for your circuit.

Defining Power Supply Efficiency

The remaining portion of the power supply that is discussed in the seminar is illustrated in Figure 1. The power-supply management circuit block diagram can be implemented with discrete components or a combination of integrated circuits and discrete components. The purpose of this power-supply management circuit is to match the change of the source voltage to an output voltage while complementing the output drive requirements. In the best of cases, this task is achieved with optimum efficiency. The integrated circuits that are typically used in this type of application are the SPCs, charge pumps or LDOs. In all cases, the integrated circuit conditions the source voltage to a different output voltage.

[pic]

Figure 1: Shows that the control and reconfiguration of a power supply, such as a battery, can be implemented with the three fundamental devices: SPC, charge pump or LDO. These three devices use a variety of building blocks to achieve the power-supply voltage and current transformations.

The Efficiency of the Buck SPC Circuit

A simplified example of a Buck-SPC circuit is shown in Figure 2. With this style of converter, a simple chopping network in combination with a low-pass LC (inductor-capacitor) filter is used. In this discussion, the Buck Converter is operating in a continuous Inductor Current Mode. The input is “chopped” using a Pulse Width Modulator (PWM) signal to the switch with resulting pulses that are averaged to create a DC-output voltage. This converter is only capable of stepping down the input voltage (from a high value to a lower value).

[pic]

Figure 2: Shows a Buck-SPC with the dynamic evaluation calculated for the device in the Continuous Mode. This power-supply solution is the most robust between a charge pump and LDO circuit; however, it does require an external inductor making manufacturing more difficult and is also capable of higher emitted noise.

The following assumptions are made with the evaluation of the circuit shown in Figure 2:

1. The input voltage is always greater than the output voltage as is required.

2. The output voltage is essentially DC, which implies that the output filter is large enough to average this voltage, (typically ................
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