Power Management 101: Converter & Controller ICs

April 28, 2009
Converter ICs include an integrated power MOSFET, whereas controller ICs employ external power MOSFETs. Both configurations are classified as regulators because they regulate the output voltage.

Converter ICs include an integrated power MOSFET, whereas controller ICs employ external power MOSFETs. Both configurations are classified as regulators because they regulate the output voltage.

What are the power management converter & controller ICs?

LDO Regulator ICs

A low-dropout (LDO) voltage regulator operates in the linear region with the topology shown in Fig. 3-1. As a voltage regulator its main components are a pass transistor, error amplifier, voltage reference, and output power MOSFET. One input to the error amplifier, set by resistors R1 and R2, monitors a percentage of the output. The other input is a stable voltage reference (VREF). If the output voltage increases relative to VREF, the error amplifier changes the pass-transistor’s output to maintain a constant output voltage (VOUT).

Low dropout refers to the difference between the input and output voltages that allows the IC to regulate the output voltage. That is, the LDO device regulates the output voltage until its input and output approach each other at the dropout voltage. Ideally, the dropout voltage should be as low as possible to minimize power dissipation and maximize efficiency.

The major advantage of a LDO IC is its relatively “quiet” operation because it does not involve switching. In contrast, a switch-mode regulator typically operates between 50 kHz and 1 MHz, which can produce EMI that affects analog or RF circuits. LDOs with an internal power MOSFET or bipolar transistor can provide outputs in the 50 to 500mA range. The LDO’s low dropout voltage and low quiescent current make it a good fit for portable and wireless applications.

A regulator’s dropout voltage determines the lowest usable input supply voltage. That is, although specs may show a broad input voltage range, the input voltage must be greater than the dropout voltage plus the output voltage. For a 200mV dropout LDO, the input voltage must be above 3.5V to produce a 3.3V output.

With an LDO, the difference between input voltage and output voltage may be small, and the output voltage must be tightly regulated. Plus, transient response must be fast enough to handle loads that can go from zero to tens of amperes in nanoseconds. Further, output voltage can vary due to changes in input voltage, output load current, and temperature. Primarily, these output variations are caused by the effects of temperature on LDO voltage reference, error amplifier, and its sampling resistors (R1 and R2).

Multiphase Controller ICs

The trend toward higher current lower voltage microprocessors has created a need to supply up to 100A at voltages in the neighborhood of 1V. The multiphase converter answers this need. Multiphase converters employ two or more identical, interleaved converters connected so that their output is a summation of the outputs of the cells, as shown in Fig. 3-2. Multiphase cells operate at a common frequency, but are phase shifted so that conversion switching occurs at regular intervals controlled by a common control chip. The control chip staggers the switching time of each converter so that the phase angle between each converter switching is 360º/n, where n is the number of converter elements. The outputs of the converters are paralleled so that the effective output ripple frequency is n ´ f, where f is the operating frequency of each converter. This provides better dynamic performance and significantly less decoupling capacitance than a single phase system.

Current sharing among the cells is necessary so that one does not hog too much current. Ideally, each multiphase cell should consume the same amount of current. To achieve equal current sharing the output current for each cell must be monitored and controlled.

The multiphase approach also offers packaging advantages. Each converter delivers 1/n of the total output power, reducing the physical size and value of the magnetics employed in each phase. Also, the power semiconductors in each phase only need to handle 1/n of the total power. This spreads the internal power dissipation over multiple power devices, eliminating the concentrated heat sources and possibly the need for a heat sink. Even though this uses more components, its cost tradeoffs can be favorable.

PWM (Pulse Width Modulation) Controller ICs

Switch-mode dc-dc converters require a means to vary their output voltage in response to changes in their load. One approach is to use pulse width modulation (PWM) that controls the input to the associated power switch. The PWM signal consists of two values, ON and OFF. A low-pass filter connected to the output of the power switch provides a voltage proportional to the ON and OFF times of the PWM controller.

In operation, a small amount of the output voltage is fed back to the PWM controller, which varies its ON time in response to the feedback voltage. If the filtered output of the power switch tends to change, the negative feedback applied to the PWM controller regulates the output voltage.

For example, a fixed frequency voltage mode PWM controller IC targets off-line SMPS (switch-mode power supply) and dc-dc converter applications requiring minimal external components. It features a trimmed oscillator for precise duty cycle control, a temperature compensated reference, an on/off control, a high gain error amplifier, a current sensing comparator, and a high current totem-pole output.

It incorporates an on/off control and a soft-start circuit. Used in conjunction with complementary power MOSFETs and high performance power factor ICs, this PWM controller IC enables implementation of SMPS designs that provide high efficiency and allow for regulatory compliance with relevant standards for harmonic emission.

Among its features are pulse-by-pulse current limiting, undervoltage lockout ( UVLO), 7mA operating current (typ.), soft-start, on/off control, overload protection (OLP), overcurrent protection (OCP), and overvoltage protection (OVP).

Offline AC-DC Converter ICs

Offline ac-dc converters operate from the rectified ac powerline voltage, so these ICs are optimized for a high voltage input. Isolated topologies must be used because these systems require galvanic isolation of the secondary circuit voltages from the powerline. These self-contained converter ICs are usually oriented toward use as ac adapters in battery-based systems, and computer peripherals, such as printers and scanners. The ICs are usually rated at 100W and below.

Switch-mode Regulator/Converter ICs

Fig. 3-3 shows a simplified diagram of a switch-mode dc-dc converter. In a typical dc-dc converter the power switch accepts a dc input, converts it to the switching frequency and then rectifies it to produce the dc output. A portion of its dc output is compared with a voltage reference (VREF) and fed back to the power switch oscillator circuit to regulate the dc output voltage. If the output voltage tends to increase, the voltage fed back to the power switch reduces its duty cycle, causing its output to reduce and maintain the proper regulated voltage. Conversely, if the output voltage tends to go down, the feedback causes the power switch duty cycle to increase, keeping the regulated output at its proper voltage.

This type of converter can be found in a single IC package, including the power semiconductor switch. The only required external components are usually just passive devices. Internal power switches may be either a bipolar or MOSFET device capable of handling the required current and power. Typically, the power semiconductor switch turns on and off at a frequency that may range from 100kHz to 1MHz, depending on the IC type. Most power switches employ pulse width modulation to control the output voltage, so the duty cycle varies according to the desired output voltage. The ability of the converter to regulate a specific output voltage is expressed as a percentage; most single-IC converters can regulate the output within ±5% or less.

Switching frequency determines the physical size and value of filter inductors, capacitors, and transformers. The higher the switching frequency, the smaller the physical size and component value. To optimize efficiency, magnetic core material for the inductor and transformer should be consistent with the switching frequency. That is, the transformer/inductor core material should be chosen to operate efficiently at the switching frequency.

There are two types of dc-dc converters: isolated and non-isolated, which depends on whether there is a direct dc path from the input to the output. An isolated converter employs a transformer to provide isolation between the input and output voltage. The non-isolated converter usually employs an inductor and there is no voltage isolation between the input and output. For the vast majority of applications, non-isolated converters are appropriate. However, some applications require isolation between the input and output voltages. An advantage of the transformer-based converter is that it has the ability to easily produce multiple output voltages, whereas the inductor-based converter provides only one output.

Are there any other switching regulator topologies?

The basic hysteretic regulator shown in Figure 3-4 is another type of switching regulator. It consists of a comparator with input hysteresis that compares the output feedback voltage with a reference voltage. When the feedback voltage exceeds the reference voltage, the comparator output goes low, turning off the buck switch MOSFET. The switch remains off until the feedback voltage falls below the reference hysteresis voltage. Then, the comparator output goes high, turning on the switch and allowing the output voltage to rise again.

There is no voltage-error amplifier in the hysteretic regulator, so its response to any change in the load current or the input voltage is virtually instantaneous. Therefore, the hysteretic regulator represents the fastest possible dc-dc converter control technique. A disadvantage of the conventional hysteretic regulator is that its frequency varies proportionally with the output capacitor’s ESR. Since the initial value is often poorly controlled, and the ESR of electrolytic capacitors also changes with temperature and age, practical ESR variations can easily lead to frequency variations.

Charge Pump ICs

Charge pumps (switched-capacitor) ICs provide dc-dc voltage conversion using a switch network to charge and discharge one or more capacitors. The switch network toggles between charge and discharge states of the capacitors. As shown in Fig. 3-5, the "flying capacitor " (C1) shuttles charge, and the "reservoir capacitor " (C2) holds charge and filters the output voltage.

The basic charge pump lacks regulation, which is generally added using either linear regulation or charge-pump modulation. Linear regulation offers the lowest output noise, and therefore provides better performance. Charge-pump modulation (which controls the switch resistance) offers more output current for a given die size (or cost), because the regulator IC need not include a series pass transistor.

A major advantage of the charge pump is elimination of the magnetic fields and EMI that comes with an inductor or transformer. There is one possible EMI source - the high charging current that flows to a "flying capacitor" when it connects to an input source or another capacitor with a different voltage

Multiple Output Controller or Converter/Regulator with Similar ICs

Multiple output controller ICs consist of two or more converters/regulators or controllers in a single package. They can be two switch-mode converters or two LDO regulators. Controllers employ external power switches whereas converters/regulators have an internal power switch.

An example of a dual switch-mode regulator is a dual current mode PWM step-down dc-dc converter with internal 2A power switches, this IC operates from a 3.6V to 25V input, enabling it to regulate a wide variety of power sources such as four-cell batteries, 5V logic rails, unregulated wall transformers, lead acid batteries and distributed-power supplies. The two regulators share common circuitry including input source, voltage reference and oscillator, but are otherwise independent. Their feedback loop controls the peak current in the switch during each cycle. This current mode control improves loop dynamics and provides cycle-by-cycle current limit.

An example of a dual-output, low-dropout voltage regulator IC has integrated reset, power on reset (POR) and power good (PG) functions. Quiescent current is typically 190µA at full load. Differentiated features, such as accuracy, fast transient response, supervisory circuit (power on reset), manual reset input, and independent enable functions provide a complete system solution. These voltage regulators have extremely low noise output performance without using any added filter bypass capacitors and are designed to have a fast transient response and be stable with 10µF low ESR capacitors.

Because the PMOS device behaves as a low-value resistor, the dropout voltage is very low (typically 170 mV) and is directly proportional to the output current. Additionally, since the PMOS pass element is a voltage-driven device, the quiescent current is very low and independent of output loading (maximum of 230µA over the full range of output current and full range of temperature).

This LDO family also features a sleep mode; applying a high signal to either enable input shuts down Regulator 1 or Regulator 2, respectively. Putting the regulators in the sleep mode reduces the input current to 2µA at TJ = 25°C. Each regulator, has an internal discharge transistor to discharge the output capacitor when the regulator is turned off (disabled).

Multiple output controller ICs can also consist of two or more charge pump converters in a single package. They can be controllers that employ external power switches or regulators with an internal power switch. One possibility is a 5V output and a 3.3V output for processor and logic applications.

For example, a typical multiple output charge pump controller ICs can step-down dc-dc converters that produce two adjustable regulated outputs from a single 2.7V to 5.5V input. The IC uses switched capacitor fractional conversion to achieve a typical efficiency increase of 50% over that of a linear regulator. No inductors are required.

The IC has two switched capacitor charge pumps to step down VIN to two regulated output voltages. The two charge pumps operate 180° out of phase to reduce input ripple. Regulation is achieved by sensing each output voltage through an external resistor divider and modulating the charge pump output current based on the error signal. A two-phase non-overlapping clock activates the two charge pumps running them out of phase from each other.

A constant frequency, spread spectrum architecture provides a very low noise regulated output as well as low noise at the input. The spread spectrum oscillator utilizes random switching frequencies between 1MHz and 1.6MHz, and sets the rate of charging and discharging of the flying capacitors. This architecture achieves extremely low output noise and input noise is significantly reduced compared to conventional charge pumps.

Multiple-Output LDO + Switchmode Regulator & Controller ICs

One type of multiple output controller IC consists of two or more of switchmode dc-dc converters in a single package or combinations of LDOs and dc-dc converters. They can be controllers that employs external power switches or regulators with an internal power switch. One possibility is a 5V output and a 3.3V output for processor and logic applications.

A typical example of combined LDO/switch-mode converter is an IC that combines a dual synchronous buck controller and a linear regulator controller, providing a cost-effective, high performance and flexible solution for multi-output applications. You can configure the dual synchronous controller as two-independent or two-phase controller. In the two-phase configuration, the IC provides a programmable current sharing that is ideal when the output power exceeds any single input power budget. It drives its two output stages 180°out of phase. In two-phase configuration, the two inductor ripple currents cancel each other, reducing the output current ripple and allowing a smaller output capacitor for the same ripple voltage requirement.

What is digital power conversion?

Generally, digital power conversion employs a digital processor to control system operation and peripheral support. This can take any one of four possible configurations. At the top, Level 4 provides full-loop control provided by on-chip firmware algorithms. At the bottom, Level 1 employs primarily hardware control of power conversion with some software support. Level 4 employs the highest degree of sophistication, particularly with the associated software (firmware).

This newest generation of Microchip Technology’s 16-bit digital signal controllers (DSCs) for multi-loop power conversion applications. You can configure these DSCs for several different topologies, giving designers the freedom to optimize performance for specific product applications.

The DSC’s 16-bit (data) modified Harvard RISC processor combines the control advantages of a high-performance 16-bit microcontroller with the high computation speed of a fully implemented DSP. Most instructions require only one clock cycle to execute, and the dsPIC DSC has a fixed, deterministic interrupt latency, allowing very predictive, real-time performance.

These dsPIC33F “GS” series DSCs include on-chip Analog-to-Digital Converters (ADCs) that provide low latency and high-resolution control. The on-chip1 ns duty-cycle resolution Pulse Width Modulators (PWMs) can easily handle the precise timing requirements of all switching power-supply topologies, including precise multiphase synchronous rectifier timing requirements. The ICs feature interactive peripherals that both minimize the intervention of the processor and can handle the real-time demands of high-speed current-mode control. These DSCs are suited for ac-dc converters, dc-dc power converters and other power conversion applications, such as embedded power-supply controllers, power inverters, Uninterruptible Power Supplies (UPSs) and digital lighting. Fig. 3-6 is the circuit of a single-phase synchronous buck converter using the dsPIC33FJ06GS202 and two external power MOSFETs.

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