No magnetics required – the charge pump handles the voltage anyway!

background

A charge pump (or switched capacitor voltage converter) uses a capacitor as an energy storage element to generate an output voltage. A basic charge pump circuit, the voltage doubler, uses a single flying capacitor and four internal switches driven by a two-phase clock to double the input voltage. 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, the third switch connects the negative terminal of the capacitor to VIN, which effectively creates a voltage of 2 * VIN on 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 is transferred to the output capacitor every cycle until the output is charged to 2 * VIN, thus doubling the input voltage. When an output load is connected, the output capacitor provides the load current in the first clock phase, while the flying capacitor provides the load current and charges the output capacitor during the second phase. To enable charge transfer, the output is regulated at a voltage slightly below 2*VIN. The charging and discharging of the output capacitor during the two phases of the clock will produce an output ripple that is a function of the output capacitor value, clock frequency, and output load current.

All other charge pump circuit implementations are derived from this basic scheme by adding/changing the number of switches and capacitors and clock phases. Charge pumps can double, triple, halve, invert, multiply or adjust voltage by fractions (e.g. x3/2, x4/3, x2/3, etc.) and generate any voltage (depending on the controller and circuit topology). Charge pump efficiency can be quite good when approaching its ideal charge ratio. In the voltage doubler example above, the input supply will be equal to twice the output load current, so that in an ideal case the input power is equal to the output power. In reality, the efficiency will be slightly lower than ideal due to quiescent operating current and other losses. The versatility of charge pumps allows them to be used in a wide range of applications and market segments.

Charge pumps fill a small gap in the performance spectrum between LDOs and switching regulators and can provide an excellent alternative for designs that might object to the use of an inductor. Compared to LDOs, charge pumps require an additional capacitor (the "flying" capacitor) to operate, but do not require an inductor, are generally slightly more expensive, have higher output noise levels, and tend to have lower output current capabilities. However, they offer many advantages over LDOs, such as higher efficiency, good thermal management (due to the switching architecture), and greater flexibility for stepping up and down or generating negative voltages. When compared to traditional switching regulators, charge pumps have lower output current capabilities and efficiency. However, charge pump designs are simpler and do not require an inductor. In addition, advances in process technology have expanded the input voltage range of charge pumps compared to previous product families. Table 1 compares the key performance parameters of the different topologies.

Table 1: Performance comparison between LDO, charge pump and switching regulator

Design and application challenges of charge pumps

Some industrial environments have single-ended, higher voltage supplies readily available. However, these supplies are not suitable for driving op amps and other circuits that require bipolar supplies, such as the ±15V rails required to power a dual-rail, low-noise, high-voltage op amp from a single +24V supply. An op amp driven close to its negative rail has very poor distortion. Therefore, it is desirable to have a negative rail below the lowest signal level to provide the lowest distortion at the op amp's output. The right kind of charge pump can meet this requirement and generate a negative output supply locally to drive the voltage rails of an op amp or other noise-sensitive circuit using a low-noise post regulator.

Many new communications devices use sensitive RF receivers, but the combination of noise generators (switching power supplies) and noise-sensitive circuits can cause potential interference. The traditional solution is to keep the noise-generating circuits as far away from the noise-sensitive circuits as possible. However, in today's handheld products, the design is very compact, so this approach is no longer feasible. And solving the problem by adding shielding is not feasible in terms of both cost and size. The noise energy of traditional switching power supplies is mainly manifested in the form of concentrated narrowband harmonics. However, if one of these harmonics happens to coincide with a sensitive frequency (for example: the intermediate frequency [IF] passband of the receiver), it is likely to cause interference. Charge pumps provide a low enough noise threshold to fill this gap.

All solutions designed to meet the above charge pump IC design constraints will integrate an efficient high voltage charge pump with regulated output and low output noise.

A novel and simple solution

Linear Technology has developed simple, yet sophisticated, high voltage negative output monolithic charge pump ICs for such applications. The LTC3260 and LTC3261 are general purpose charge pumps. The LTC3261 is a high voltage negative output charge pump that can provide up to 100mA of output current. The LTC3260 has a built-in negative output charge pump and positive and negative LDO regulators, each of which can provide up to 50mA of output current using low dropout voltage operation. The negative LDO post regulator is powered by the output of the negative output charge pump. The positive and negative LDO output voltages can be adjusted to 1.2V and –1.2V, respectively, using external resistor dividers. Both devices operate over a wide input voltage range of 4.5V to 32V. See Figures 1 and 2 for details.

Figure 1: LTC3260 Application Circuit

Figure 2: LTC3261 Application Circuit

Both the LTC3260 and LTC3261’s internal charge pumps can operate in either Burst Mode® for low quiescent current or constant frequency mode for low noise (efficiency up to 88%). In Burst Mode operation, the charge pump output regulates to –0.94 • VIN. Additionally, in Burst Mode operation, if both LDOs are enabled, the LTC3261 draws only 60μA of quiescent current, while the LTC3260 draws only 100μA. Constant frequency operation provides low input and output ripple; in this mode, the charge pump produces an output equal to –VIN and operates at a fixed 500kHz frequency or set from 50kHz to 500kHz with an external resistor. Other IC features include few external components, stability with ceramic capacitors, soft-start circuitry to prevent excessive current at startup, and short-circuit and overtemperature protection. The LTC3260 and LTC3261 are ideal for a variety of applications such as low noise bipolar/inverting supplies from high voltage inputs, low noise bias voltage generators for industrial/instrumentation, portable medical devices and automotive infotainment systems.

The LTC3260 is available in a low-profile (0.75mm height) 3mm x 4mm 14-pin DFN package and a 16-pin MSOP package, both with bottom-side thermal pads. The LTC3261 is available in a 12-pin MSOP package, also with a bottom-side thermal pad. Both devices operate from a –40°C to +125°C junction temperature.

Low output ripple

The LTC3260 is designed with inherent characteristics to provide low noise performance. The device’s high operating frequency enables low output ripple. As shown in Figure 3, the LTC3260’s LDO further suppresses this ripple, providing a very low noise output (<1mVp-p), making it ideal for noise-sensitive applications such as op amps and ADC drivers.

Figure 3: LTC3260 low output ripple performance

Protection Circuit

The LTC3260 has built-in short-circuit current limiting and thermal protection circuits. In a short-circuit condition, the device automatically limits its output current to approximately 160mA. If the junction temperature exceeds approximately 175°C, the thermal shutdown circuit will prohibit current from being supplied to the output. When the junction temperature drops back to approximately 165°C, current is resumed to the output. When the thermal protection circuit is in operation, it indicates that the junction temperature is outside the specified operating range. The thermal protection function is for short-term overload conditions outside the normal operating range. Continuous operation at above the specified maximum operating junction temperature may impair the reliability of the device.

Table 2 briefly summarizes the features and benefits of Linear Technology's new charge pumps, the LTC3260 and LTC3261.

Table 2: Features and Benefits of the LTC3260 and LTC3261 Charge Pumps

in conclusion

Charge pumps have been almost forgotten in some ways due to their limited voltage range and performance that has traditionally been between LDOs and switching regulators. Fortunately, Linear Technology has addressed this need with the LTC3260 and LTC3261 high voltage charge pumps. The LTC3260, which can deliver 150mA, packs many useful features into a small footprint, reducing the overall solution size, which in turn allows for a more compact and simpler design. The LTC3261 is a subset of the LTC3260 and provides a 100mA high voltage negative output. Therefore, for designers who don’t like using inductors, they can use a simple high voltage charge pump instead.