High-Voltage Charge Pump Simplifies Power Conversion Compared to Inductor-Based Switching Regulators

Background Information

Charge pumps (or switched capacitor voltage converters) fill the performance gap between linear regulators and inductor-based switching regulators, providing an alternative design option for engineers who don't like inductors. Compared to LDOs, charge pumps require an additional capacitor (the "floating" capacitor) to operate, but generally cost only slightly more, have higher output noise levels, and generally have lower output current capabilities. However, charge pumps also have some advantages that linear regulators do not have, such as higher efficiency, better thermal management due to higher efficiency operation, and the ability to step up and down or generate negative voltages. When compared to conventional switching regulators, charge pumps have lower output current capabilities and lower efficiency. However, charge pumps are simpler, easier to design, and do not require an inductor. Recent advances in process technology have enabled the input voltage range of charge pumps to be extended relative to previous generations. Table 1 compares the key performance parameters of the various topologies mentioned above.

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

Charge pump ICs use capacitors as energy storage elements to generate output voltages. For example, consider the basic “voltage doubler” charge pump circuit shown in Figure 1. This circuit uses a single floating capacitor (CFLY in the figure) and four internal switches (circles with “x” inside) driven by a two-phase clock to generate an output voltage that is twice the input voltage. During the first phase of the clock (θ1 in the figure), a pair of switches charges the floating capacitor to the input voltage (VIN). During the second phase of the clock (θ2 in the figure), the third switch connects the negative end of the capacitor to VIN, effectively creating 2 * VIN at the positive end of the capacitor. The fourth switch connects the positive end of the floating capacitor to the output capacitor. In the absence of load, charge is transferred to the output capacitor in each cycle until the output is charged to 2 * VIN, creating a voltage equal to twice the input voltage. When there is an output load, the output capacitor (COUT in the figure) provides the load current in the first phase, while in the second phase, the floating capacitor provides the load current and charges the output capacitor. To transfer the charge, the output will stabilize at a voltage slightly below 2 * VIN. The charging and discharging of the output capacitor on both clock phases creates an output ripple that is a function of the output capacitor value, clock frequency, and output load current.

Figure 1: Basic charge pump voltage doubler circuit

All other charge pump circuit topologies are based on this basic circuit, just adding/changing switches and capacitors and the number of clock phases. Depending on the controller and circuit topology, the charge pump can produce any output voltage, such as 2 times the input voltage, 3 times the input voltage, half the input voltage, negative output voltage, and output voltage proportional to the input voltage, such as 3/2, 4/3, 2/3 of the input voltage. The efficiency of the charge pump can be very high when it is close to the ideal charge ratio. In the voltage doubler example above, the input supply current is equal to twice the output load current, and the input power is equal to the output power. In reality, the efficiency is slightly lower than the ideal case due to quiescent operating current and other losses. Charge pumps are very versatile and can be used in many applications and market segments. Charge pumps are more rugged due to innovative design methods, creating opportunities for use in harsh industrial and automotive environments.

Challenges facing automotive and industrial design

Designing electronic systems for automotive applications is challenging for many reasons, including wide operating temperature ranges, stringent EMI (electromagnetic interference) and transient requirements, and the high quality required by automotive OEMs (original equipment manufacturers). Car dashboards are crowded and packed with electronics. Adding to the complexity are wireless systems ranging from Bluetooth to cell phone-based network connections. Therefore, it is imperative that any component added to this thermally constrained environment is not too hot or too EMI-intensive. There are stringent electromagnetic compatibility (EMC) requirements for radiated and conducted EMI, radiated and conducted immunity or radiated and conducted susceptibility, and electrostatic discharge (ESD). Meeting these requirements affects many aspects of IC design. The low EMI and low noise output of charge pumps (no magnetics, no inductors) make them an ideal choice. Charge pumps generally have lower EMI than inductive switches because the connecting wires to the floating capacitors can be minimized to reduce capacitive coupling and antenna effects. Inductors tend to be larger than capacitors and act like antennas, especially when unshielded. In reality, floating capacitor outputs do not generate higher EMI at all compared to typical digital outputs. In fact, they generate lower EMI because board traces are kept as short as possible. Looking first at the issue of wide operating temperature range, power ICs are challenged on two fronts. The first is power conversion, which, even at moderate to high efficiencies, consumes a certain amount of power as heat. Add to this the challenge of a wide ambient operating temperature range, and the maximum junction temperature of these ICs can often exceed 125ºC. Even when the electronics in the body of a car are not under the hood of the car, the ambient temperature inside the sealed plastic packaged electronic control module can reach 95ºC. Because of these temperature challenges, many ICs rated for 85ºC or even 125ºC are not adequate for sustained operation at high temperatures. Therefore, in many of these applications, the IC is required to operate properly at temperatures as high as +150ºC.

However, there are further challenges in the automotive environment (such as colder ambient temperatures), which require the ability to boost to 5V or survive a low voltage cold crank (~3V) to 5V conversion, where the input may be lower than the desired output. This is usually where a device that can both step down and step up is needed. In addition, the DC/DC converter connected to the vehicle battery input must withstand wide voltage swings caused by accidental excursions in the alternator voltage or a sudden start of the car. Therefore, a device that provides input voltage transient protection is needed here. The industrial market has similar requirements to the automotive market, especially in terms of extreme temperatures and wide supply voltage ranges.

In summary, the main challenges facing automotive and industrial system designers are as follows:

Balancing power consumption and high temperature operation

Immunity to radiated and conducted noise, with low radiation

Handling large voltage transient swings

Regulates 5V or (3.3V) during low voltage cold crank conditions

Minimize solution size and footprint

The traditional approach to solving these design problems is to integrate high-voltage buck and boost switches, or a true 4-switch buck-boost DC-DC converter. However, such solutions can be large and costly, and often require additional measures to avoid EMI issues. Another solution that meets the above constraints uses high-efficiency, high-voltage buck charge pumps or buck-boost charge pumps, which have extensive protection features, high temperature operation, and high efficiency. Moreover, they only require 3 small capacitors.

Simple high voltage solution

Linear Technology Corporation has developed simple, innovative high voltage monolithic buck-boost and step-down charge pump ICs specifically for automotive and industrial applications.

The first of these devices is the LTC3245, a general-purpose 250mA high voltage buck-boost charge pump. It uses a switched capacitor fractional conversion method to maintain regulation over a wide input voltage range of 2.7V to 38V and produces a regulated output that is adjustable to 3.3V, 5V, or 2.5V to 5V. Internal circuitry automatically selects the conversion ratio (2:1, 1:1, or 1:2) to optimize efficiency over varying input voltage and load conditions. Low operating current (18μA at no load, 4μA in shutdown) and few external components (3 small ceramic capacitors, no inductor) make the LTC3245 ideal for low power, space-constrained applications such as automotive ECU/CAN transceiver supplies, industrial housekeeping supplies, and high efficiency low power 12V to 5V conversion. See Figure 2 below for a typical application circuit.

Figure 2: LTC3245 Typical Application Circuit

The LTC3245's unique constant frequency architecture provides lower conducted and radiated noise than traditional switching regulators. The device can operate in a pin-selectable Burst Mode®, which allows the user to choose to trade slightly increased output ripple for higher efficiency/lower quiescent current. Other features of the IC include few external components and stability with ceramic capacitors, soft-start circuitry to prevent excessive current at startup, and short-circuit and overtemperature protection.

The LTC3245 is available in a low profile (0.75mm) 3mm x 4mm 12-lead DFN package with bottom thermal pad and a 12-lead MSOP package with bottom thermal pad. The operating junction temperature is -40ºC to +125ºC for the E- and I-grades, -40ºC to +150ºC for the H-grade, and -55ºC to +150ºC for the MP-grade.

Table 2 summarizes the features and benefits of the LTC3245.

Table 2: LTC3245 Features and Benefits 

LTC3255 Step-Down Charge Pump

The LTC3255 is as rugged as an LDO but simpler than a switching regulator. It is a general-purpose high voltage step-down switched capacitor converter that provides up to 50mA of output current. In applications where the input voltage exceeds twice the output voltage, the charge pump is nearly twice as efficient as an equivalent linear regulator, providing a space-saving, inductorless solution that can replace switching DC/DC regulators. The LTC3255 produces a regulated output of 2.4V to 12.5V and operates over a wide input range of 4V to 48V with a +60V/-52V input tolerance. At no load, Burst Mode operation reduces VIN quiescent current to only 16µA, and a 2:1 capacitive charge pump enhances output current capability to approximately twice the input current. The LTC3255 is suitable for a variety of applications such as industrial control, factory automation, sensors and supervisory control and data acquisition (SCADA) systems, housekeeping power supplies, and current step-up regulators for 4mA to 20mA industrial current loops. See Figure 3.

Figure 3: LTC3255 Application Circuit – 4mA to 20mA Current Loop

The LTC3255 can be used as a general purpose step-down charge pump with a 2:1 or 1:1 conversion ratio, or as a current doubling shunt regulator. When operating in normal mode, the conversion ratio is selected based on VIN, VOUT and load conditions, and switching between conversion modes is automatic. When operating in shunt mode, the device is forced into 2:1 mode, providing a regulated output voltage from a current source input that can deliver nearly twice the input current to the load. For example, this feature enables a 4mA current loop to continuously deliver 7.4mA load current at a regulated output voltage of 3.3V. The LTC3255 can withstand reverse input supplies as low as -52V and output shorts without being damaged. Safety features include output current limiting and overvoltage protection, which further enhances ruggedness.

The LTC3255 is available in low profile (0.75mm), 3mm x 3mm 10-lead DFN and 10-lead MSOP packages, both with bottom metal pads for enhanced thermal conductivity. The E- and I-grade versions operate from -40ºC to +125ºC. The H-grade version operates from -40ºC to +150ºC, while the high reliability MP-grade is specified from -55ºC to +150ºC.

Table 3 summarizes the features and benefits of the LTC3255.

Table 3: LTC3255 Features and Benefits

Table 4 summarizes Linear Technology's latest high voltage charge pump family.

Table 4: Linear Technology's next generation high voltage charge pump family

in conclusion

Charge pumps have now come of age. In some ways, they have been almost forgotten due to their limited voltage range and historically performance that was somewhere between LDOs and switching regulators. However, innovative design approaches have improved charge pump performance and functionality, including buck-boost architectures, extensive input transient protection, and the ability to double current in 4mA to 20mA loop applications. Depending on operating conditions, charge pumps have achieved efficiencies approaching those of switching regulators. Therefore, there is really no reason not to use charge pumps in high-voltage designs.