Power Electronics

Charge Pump-LDO Chip Offers Option for Generating Low-Noise Supplies

Low-dropout regulators (LDOs) are a natural choice for generating supply voltages in noise-sensitive applications. However, in many cases, the input voltage isn't high enough relative to the desired output voltage, either because the input is nominally too low, or because it varies over a range that doesn't satisfy the regulator's dropout requirements. In these cases, where the LDO alone is inadequate, the regulator may be combined with a charge pump that boosts the input voltage before feeding it to the LDO. The resulting dc-dc converter offers a boost circuit that benefits from the linear regulator's low-noise output and the charge pump's high efficiency. Some chip makers have recognized the need for this type of circuit in specific applications, and integrated the charge pump and LDO functions in a single chip.

Aimed primarily at space-limited portable applications, these charge pump plus regulator ICs operate off of an existing low-voltage supply rail or directly off a battery. Beyond these basic similarities, these chips feature widely varying electrical specifications, reflecting their application-specific nature. Although those differences make it difficult to make apples-to-apples comparisons, a look at some of the existing devices reveals the performance options and tradeoffs available to designers.

One recently introduced charge pump plus LDO circuit is the CM3702 from California Micro Devices. Optimized to power audio codecs in notebook applications, this chip converts a 3-V to 5.5-V input to a regulated 5-V output (see figure) at currents up to 200 mA. In the typical target application, this device typically takes power from a 5-V supply generated by a switching regulator.

With its ability to boost the input, the CM3702 offers an alternative in existing designs that run an LDO of a 5-V supply and regulate down a lower value such as 4.75 V. In doing so, concerns about the LDO's performance in dropout, particularly PSRR, are alleviated. At the same time, the input-voltage range permits operation off a 3.3-V rail or a single Li-ion cell. This input range leads to some secondary applications, such as 3-V to 5-V conversion in PCMCIA cards, white LED drivers, flash memory supplies, 3-V to 5-V SIMM cards and backup battery-boost converters.

The CM3702 is optimized to limit noise in the audio spectrum. With a 5-V input, PSRR is typically 42 dB from about 60 Hz to above 10 KHz. When the input supply is 3.3 V, that performance increases to better than 45 dB of ripple rejection from 20 Hz to 20 kHz. These measurements assume an output current of 100 mA, an output capacitance (CO) of 10 µF and a bypass capacitance (Cbyp) of 0.1 µF (see part b of the figure).

The charge pump in the CM3702 switches at a nominal 250 kHz. However, this is the free running frequency of the charge pump's internal oscillator, and this value is typically seen only during start up. The output of the charge pump is a sawtooth ripple waveform that's usually at a much lower frequency. This mode of operation reflects a design trade-off in which the lower ripple frequency is the price for reduced power dissipation, which in turn allows use of a small package without an external heatsink.

The ripple frequency is a function of the input voltage and the choice of output capacitors, Cp and Cs in the figure. In addition, the ratio of these capacitance values affects the amplitude of the ripple. The datasheet gives guidelines regarding the selection of Cp and Cs as well as the other external capacitors. It also gives guidelines on PCB grounding, which can be critical to dealing with charge pump noise in the application. In this regard, separate analog and digital grounds on the CM3702 can alleviate ground-noise feedthrough from the charge pump to the regulator. Nevertheless, the noise performance of the CM3702 ultimately must be characterized in the system. In particular, noise performance under light loads should be verified because the charge pump ripple frequency will be lowest at the minimum load current.

Table. A sampling of options among regulated, low-noise charge pumps.
Model/part number Target applications Input voltage range (V) Regulated output voltage (V) Maximum output current (mA) Charge pump switching frequency Quiescent current Noise (typical)* Package type, dimensions (max)
California Micro Devices, CM3702 low-noise charge pump and LDO Analog supply for audio codec, PCMCIA cards 2.8 to 5.5 5 100 (200 with restricted input voltage range) 250 kHz 180 µA typ. at 100 mA (2 µA typ. in shutdown) 35 µVrms at 5-V and 100-mA output, BW from 22 Hz to 22 kHz 10-pin MSOP, 3.1 mm × 5 mm (max)
Linear Technology LTC1682 charge pump with low-noise LDO VCOs 1.8 to 4.4 3.3, 5, or an adjustable 2.5 to 5.5 50 550 kHz typ. 150 µA typ. (1 µA typ. in shutdown) 64 µVrms at 5-V and 10-mA output, BW from 10 Hz to 100 kHz 8-pin MSOP, 3 mm × 4.9 mm; 8-pin SOPs
Linear Technology LTC1928 charge pump with low-noise LDO VCOs 2.7 to 4.4 5 30 550 kHz typ. 190 µA typical (4 µA typ. in shutdown) 90 µVrms at 5 V and 10 mA output from 10 Hz to 100 kHz 6-pin SOT-23 (ThinSOT), 3.10 mm × 3 mm max.
National Semiconductor LM2750 low-noise switched-capacitor boost regulator White and colored LED display lighting, cell phone SIM cards 2.7 to 5.6 5 or adjustable voltage 120 mA with VIN = 2.9 to 5.6 V, 40 mA with VIN = 2.7 V 1.7 MHz typ., fixed to 2.9 V 5 mA typ. (2 µA max in shutdown) 15 mV p-p at 100 mA (bandwidth output ripple) 10-pin LLP with die attach pad, 3 mm × 3 mm
Texas Instruments TPS6024x switched-capacitor buck-boost converter VCOs, Smartcard readers 1.8 to 5.5 2.7, 3, 3.3 with full input range or 5 V with 2.7- to 5.5-V input 25 160 kHz typ. 250 µA typ. (0.1µA max in shutdown) 170 µ Vrms, VIN < 2.5 V and IOUT = 5 mA, ESR <0.1Ω, bandwidth from 20 Hz to 10 MHz 8-pin MSOP 3.05 mm × 4.98 mm max
*Output noise also depends on capacitor selection and other variables.

Compared with previous charge pump plus LDO chips, the 3702 stands out for its high output current. It delivers 100 mA across the full-specified input range or 200 mA with a restricted input range. (The latter reflects the part's power dissipation limits.) The different current ratings on the chips reflect varying current requirements in the target applications.

Thus, while the 3702 is meant primarily to power audio codecs, other devices have been developed to power lower-current applications, such as high-frequency VCOs. Also, there are design alternatives to the charge pump plus LDO devices. Some vendors offer regulated charge pumps, which can limit output noise by pre-regulating the input to the charge pump and by maintaining a constant switching frequency at a very high value. One example is National's LM2750. This approach reduces the switching noise reflected back to the source, which can be important when that source is a battery rather than another switching regulator.

According to the vendor, the LM2750 is a general-purpose device that may be used for driving white LEDs and other applications. Keep in mind, however, that many power management ICs are developed specifically for use as white LED drivers. Those ICs, which include inductor-based switching regulators and charge pumps, aren't necessarily optimized for low noise, as would be parts like the CM3702 and existing devices such as Linear Technology's LTC1682 and LTC1928, and TI's TPS6024x (see the table). The latter parts have been available for a few years or more. The CM3702 is currently sampling and is expected to be in production this quarter. Pricing on the part will be $0.57 each in lots of 10,000.

The components mentioned here provide a sampling of the options for generating low-noise on-board supplies in portable applications when step-up of voltages is required. The table gives some key specifications for these ICs. Note that selection of external capacitors — both the types and values — as well as line and load conditions and design layout will influence noise performance in the application. Although data on efficiency isn't included, that performance no doubt will be significant in many applications, and the different device options will present trade-offs in efficiency and other parameters.

National Semiconductor; www.national.com CIRCLE 350 on Reader Service Card
Linear Technology; www.linear.com CIRCLE 351 on Reader Service Card
California Micro Devices; www.calmicro.com CIRCLE 352 on Reader Service Card
Intersil Corp., Milpitas, Calif., www.intersil.com CIRCLE 353
Maxim Integrated Products, Sunnyvale, Calif., www.maxim-ic.cm CIRCLE 354
Texas Instruments Inc., Dallas, Tx., www.ti.com CIRCLE 355
Catalyst Semiconductor, Sunnyvale, Calif., www.catalyst.com CIRCLE 356

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