No magnetics required - charge pumps can handle the volts
July 09, 2012 | Steve Knoth | 222904840
Steve Knoth Senior Product Marketing Engineer, Power Products Group Linear Technology Corporation examines the benefits of using charge pumps to provide versatile application solutions.Background
A charge pump, or switched capacitor voltage converter, uses capacitors as energy storage elements to generate an output voltage. For example, one basic charge pump circuit, the “doubler,” doubles the input voltage, using a single flying 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 the input voltage (VIN). In the second phase of the clock, a third switch connects the negative terminal of the capacitor to VIN effectively generating 2*VIN at the positive terminal of the capacitor.
The fourth switch connects the positive terminal of the flying capacitor to the output capacitor. Under no load conditions, charge will transfer to the output capacitor on each cycle until the output charges to 2*VIN thus doubling the input voltage. When an output load is present, the output capacitor provides the load current during the first phase, while the flying capacitor provides the load current and charges the output capacitor during the second phase.
For charge transfer to occur, the output will regulate at a voltage slightly lower than 2*VIN. The charging and discharging of the output capacitor in the two phases of the clock generates an output ripple that is a function of the output capacitor value, the clock frequency and output load current.
All other charge pump circuit implementations follow from this basic scheme by adding/changing switches and capacitors as well as the number of phases of the clock. Charge pumps can double voltages, triple voltages, halve voltages, invert voltages, fractionally multiply or scale voltages such as x3/2, x4/3, x2/3, etcetera, and generate arbitrary voltages, depending on the controller and circuit topology. The efficiency of charge pumps can be quite good when near their ideal charge ratio. In the doubler example above, the input supply will be equal to two times the output load current such that input power equals output power in the ideal case. In reality the efficiency will be slightly lower than ideal due to quiescent operating current and other losses. In reality, the efficiency will be slightly lower than ideal due to operating current and other losses. The versatility of charge pumps enables their usage in a wide variety of applications and market segments.
Charge pumps fill a niche in the performance spectrum between LDOs and switching regulators and offer a nice alternative to designs that may be inductor-averse. Compared to LDOs, charge pumps require an additional capacitor (a “fly” cap) to operate but do not require inductors, which are generally slightly more costly, have higher output noise levels and usually have lower output current capability. However, they have many benefits over LDOs such as higher efficiency, good thermal management due to switching architecture and have more flexibility to step a voltage up as well as down, or generate negative voltages. When compared to conventional switching regulators, a charge pumps' output current capability and efficiency are lower. However, they are simpler to design and do not require an inductor. Furthermore, advancements in process technology have enabled an expansion of charge pump input voltage range compared to previous generations. Table 1 provides a comparison of key performance parameters between topologies.
(Click on image to enlarge)
Table 1. LDOs vs. Charge Pumps vs. Switching Regulator Performance
Design & Application Challenges for Charge Pumps
There are several industrial environments that have single-ended, higher voltage power supplies readily available. However, these supplies are not suitable for driving op amps and other circuits that require bipolar power supplies such as powering dual-rail, low noise high voltage op amps, requiring ±15 V rails from a single +24 V supply. Op amps driven close to their negative rail have very poor distortion. Therefore, it is desirable to have a negative rail that is lower than the lowest signal level in order to provide the lowest distortion at the op amp output. The right type of charge pump may service this need and locally generate an inverting power supply to drive the rails of the op amp or other noise sensitive circuitry using low noise post regulators.
Many modern communication devices use sensitive RF receivers, but the combination of noise generators (switching power supplies) and noise-sensitive circuitry can create the potential for interference. The traditional solution has been to keep noise generating circuits away from noise sensitive circuitry. However, in modern handheld products, everything is so tightly packaged that this is no longer possible. Shielding is not practical for both cost and size reasons. Traditional switching power supplies concentrate noise energy into narrow-band harmonics. Yet, if one of these harmonics happens to coincide with a sensitive frequency (a receiver's intermediate frequency or IF passband, for instance), interference is likely to result. Charge pumps offer sufficiently low noise thresholds and can fill this void.
Any solution to satisfy the charge pump IC design constraints outlined above would combine an efficient high voltage charge pump with regulated outputs and low output noise.
A New and Simple Solution
Linear Technology has developed simple, yet sophisticated, high voltage inverting monolithic charge pump ICs for these applications. The LTC3260 and LTC3261 are versatile charge pumps. The LTC3261 is a high voltage inverting charge pump that can deliver up to 100mA of output current. Whereas the LTC3260 includes an inverting charge pump plus both positive and negative LDO regulators that can source up to 50 mA output current each with low dropout voltage operation. The negative LDO post regulator is powered from the inverting charge pump output. The positive and negative LDO output voltages can be adjusted down to 1.2 V and -1.2 V, respectively, using external resistor dividers. Both devices operate over a wide 4.5 V to 32 V input voltage range. See Figures 1 & 2 for details.
Figure 1. LTC3260 Application Circuit
Figure 2. LTC3261 Application Circuit
The internal charge pump of both the LTC3260 and LTC3261 functions in either low quiescent current Burst Mode operation or low noise constant frequency mode at up to 88% efficiency. In Burst Mode operation, the charge pump output regulates to –0.94 • VIN. Also, in Burst Mode operation, the LTC3261 draws only 60 μA of quiescent current, while the LTC3260 draws only 100 uA with both LDOs enabled. Constant frequency operation offers low input and output ripple; in this mode the charge pump produces an output equal to –VIN and operates at a fixed 500 kHz or to a programmed value between 50 kHz to 500 kHz, using an external resistor. Other IC features include low external parts count with ceramic capacitor stability, soft-start circuitry to prevent excessive current flow during startup, plus short circuit and thermal protection. The LTC3260 and LTC3261 are well-suited for a variety of applications such as low noise bipolar/inverting supplies from a high voltage input, industrial/instrumentation low noise bias generators, portable medical equipment and automotive infotainment systems.
The LTC3260 is available in a low-profile (0.75 mm) 3 mm x 4 mm 14-lead DFN package and a 16-lead MSOP package, both with a backside thermal pad. The LTC3261 is available in a 12-lead MSOP package with backside thermal pad. Operating junction temperature for either device is -40°C to +125°C.
Low Output Ripple
The LTC3260's design inherently provides low noise performance. The device's high operating frequency leads to a low output ripple. The LTC3260 LDOs further reject this ripple as shown in Figure 3 to deliver very low noise outputs <1mVp-p that are ideal for noise sensitive applications such as operational amplifiers and ADC drivers.
Figure 3. LTC3260 Low Output Ripple Performance
The LTC3260 has built-in short-circuit current limit as well as over temperature protection. During a short-circuit condition, the part automatically limits its output current to approximately 160 mA. If the junction temperature exceeds approximately 175°C the thermal shutdown circuitry disables current delivery to the output. Once the junction temperature drops back to approximately 165°C current delivery to the output is resumed. When thermal protection is active the junction temperature is beyond the specified operating range. Thermal protection is intended for momentary overload conditions outside normal operation. Continuous operation above the specified maximum operating junction temperature may impair device reliability.
Table 2 shows a summary of Features and Benefits of Linear Technology's new charge pumps the LTC3260 and LTC3261.
(Click on image to enlarge)
Table 2. Features & Benefits of the LTC3260 & LTC3261 Charge Pumps
The charge pump in some ways has been nearly forgotten due to limited voltage range and historical performance that placed it somewhere in between an LDO and a switching regulator. Fortunately, Linear Technology has addressed these needs by introducing the high voltage LTC3260 and LTC3261 charge pumps.
The 150 mA LTC3260 offers a number of useful features in a small footprint, reducing overall solution size and in turn enabling more compact, simpler designs. The LTC3261 is a subset of the LTC3260 and provides a 100 mA high voltage inverted output. So, for those designers who do not like to use inductors, there are simple, high voltage charge pumps to be used instead.
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