016-017-018_EETE-VF

EETE JULAUG 2012

DESIGN & PRODUCTS POwER SUPPlIES No magnetics required - charge pumps can handle the Volts! By Steve Knoth A chArge pump, or switched capacitor voltage converter, uses capacitors as energy stor- age elements to generate an output voltage. For example, one basic charge pump circuit, the “doubler,” doubles the input voltage, using a single fly- ing capacitor and four internal switches driven from a two- phase clock. In the first phase of the clock, a pair of switches charges the flying capacitor to Table 1: LDOs vs. charge pumps vs. switching regulator performance the input voltage (VIN). In the second phase of the clock, a third and usually have lower output current capability. however, switch connects the negative terminal of the capacitor to VIN ef- they have many benefits over LDOs such as higher efficiency, fectively generating 2*VIN at the positive terminal of the capaci- good thermal management due to switching architecture and tor. The fourth switch connects the positive terminal of the flying have more flexibility to step a voltage up as well as down, or capacitor to the output capacitor. under no load conditions, generate negative voltages. When compared to conventional charge will transfer to the output capacitor on each cycle until switching regulators, a charge pumps’ output current capability the output charges to 2*VIN thus doubling the input voltage. and efficiency are lower. However, they are simpler to design When an output load is present, the output capacitor pro- and do not require an inductor. Furthermore, advancements vides the load current during the first phase, while the flying in process technology have enabled an expansion of charge capacitor provides the load current and charges the output pump input voltage range compared to previous generations. capacitor during the second phase. For charge transfer to oc- Table 1 provides a comparison of key performance parameters cur, the output will regulate at a voltage slightly lower than 2*VIN. between topologies. The charging and discharging of the output capacitor in the two phases of the clock generates an output ripple that is a function Design & application challenges for charge of the output capacitor value, the clock frequency and output pumps load current. There are several industrial environments that have single-end- All other charge pump circuit implementations follow from ed, higher voltage power supplies readily available. however, this basic scheme by adding/changing switches and capacitors these supplies are not suitable for driving op amps and other as well as the number of phases of the clock. charge pumps circuits that require bipolar power supplies such as power- can double voltages, triple voltages, halve voltages, invert ing dual-rail, low noise high voltage op amps, requiring ±15V voltages, fractionally multiply or scale voltages such as x3/2, rails from a single +24V supply. Op amps driven close to their x4/3, x2/3, etcetera, and generate arbitrary voltages, depending negative rail have very poor distortion. Therefore, it is desirable on the controller and circuit topology. The efficiency of charge to have a negative rail that is lower than the lowest signal level pumps can be quite good when near their ideal charge ratio. in order to provide the lowest distortion at the op amp output. In the doubler example above, the input supply will be equal to The right type of charge pump may service this need and lo- two times the output load current such that input power equals cally generate an inverting power supply to drive the rails of the output power in the ideal case. In reality the efficiency will be op amp or other noise sensitive circuitry using low noise post slightly lower than ideal due to quiescent operating current and regulators. other losses. In reality, the efficiency will be slightly lower than many modern communication devices use sensitive rF ideal due to operating current and other losses. The versatility receivers, but the combination of noise generators (switch- of charge pumps enables their usage in a wide variety of ap- ing power supplies) and noise-sensitive circuitry can create plications and market segments. the potential for interference. The traditional solution has been Charge pumps fill a niche in the performance spectrum to keep noise generating circuits away from noise sensitive between LDOs and switching regulators and offer a nice circuitry. however, in modern handheld products, everything is alternative to designs that may be inductor-averse. compared so tightly packaged that this is no longer possible. Shielding is to LDOs, charge pumps require an additional capacitor (a not practical for both cost and size reasons. Traditional switch- “fly” cap) to operate but do not require inductors, which are ing power supplies concentrate noise energy into narrow-band generally slightly more costly, have higher output noise levels harmonics. Yet, if one of these harmonics happens to coincide with a sensitive frequency (a receiver’s intermediate frequency Steve Knoth is Senior product marketing engineer in the power or IF passband, for instance), interference is likely to result. products group at Linear Technology corporation - Charge pumps offer sufficiently low noise thresholds and can fill www.linear.com this void. 16 Electronic Engineering Times Europe July/August 2012 www.electronics-eetimes.com


EETE JULAUG 2012
To see the actual publication please follow the link above