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 ±15V rails from a single +24V 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.
Figure 1: LTC3260 application circuit.
Figure 2: LTC3261 application circuit.
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 solution
Linear Technology has developed 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 50mA 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 1.2V and -1.2V, respectively, using external resistor dividers. Both devices operate over a wide 4.5V to 32V input voltage range. See figures 1
for details. 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-percent efficiency. In Burst Mode operation, the charge pump output regulates to –0.94 • VIN
. Also, in Burst Mode operation, the LTC3261 draws only 60μA of quiescent current, while the LTC3260 draws only 100uA 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 500kHz or to a programmed value between 50kHz to 500kHz, 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 suitable for 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 (0.75mm) 3x4mm 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.
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 160mA. 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.
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. The high voltage LTC3260 and LTC3261 charge pumps address these needs. The 150mA 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 100mA 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.
Figure 3: LTC3260 low output ripple performance.
About the author:
Steve Knoth is Senior Product Marketing Engineer in the Power Products Group at Linear Technology Corporation. Visit www.linear.com
Courtesy of EETimes Europe
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