Switching Voltage Regulator ICs | FAQ

What is the function of a switching voltage regulator IC?

A switching voltage regulator accepts a dc input voltage and converts it to a switched voltage that can extend into the MHz region. Then, it converts the switched voltage to a dc output using a rectifier and low pass filter. Because the switched voltage is on only part of the time, the switching operation results in higher efficiency than is possible with a linear regulator that is always on. Using output voltage feedback, this regulator generates a fixed output voltage that remains constant regardless of changes to its input voltage or load conditions. There are two main types of control for switching regulators: pulse width modulation (PWM) and hysteretic.

What is a PWM switching regulator?

In a PWM switching regulator (Figure 1), the input voltage is fed back to a PWM controller that varies the length of time the power switch is on (duty cycle) to keep the output voltage constant. Output MOSFETs for switching regulators may be either integrated (on-chip) or external to the IC.

How does switching frequency impact PWM regulator designs?

Higher switching frequencies mean the voltage regulator can use smaller inductors and capacitors. Higher switching frequency can also mean higher switching losses and higher noise in the circuit.

What is a hysteretic switching regulator?

The basic hysteretic regulator shown in Figure 2 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.

What are the characteristics of the hysteretic switching regulator?

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.

What is the constant on-time Hysteretic converter?

The constant on-time (COT) converter is a variation of the basic hysteretic converter. In the COT control scheme the converter applies the feedback signal to a comparator rather than an error amplifier. It requires no loop compensation, resulting in very fast load transient response.

What are some of the switching regulator topologies?

There are three topologies: buck (step-down), boost (step-up), and buck-boost (step-up/step-down). Other topologies include flyback, SEPIC, Cuk, push-pull, forward, full-bridge, and half-bridge.

What losses occur with the switching regulator?

Power losses occur as a result of the power needed to turn on and off the MOSFET. The MOSFET driver must bear this loss. Also, the MOSFET takes a finite time to switch to/from a conduction state to non-conduction state. Hence, power loss will be associated with this activity in the MOSFET itself. These losses are dominated by the MOSFET gate charge and the capability of the drive, in effect the energy needed to charge and discharge the capacitance of the MOSFET gate between the threshold voltage and gate voltage.

What design specifications are important for a voltage regulator IC?

The basic parameters include input voltage, output voltage, and output current. Depending on the application, other parameters may be important, such as output ripple voltage, load transient response, output noise, and efficiency. Important parameters for the linear regulator are dropout voltage, power-supply rejection ratio (PSRR), and output noise.

Are there additional voltage regulator IC features?

Some voltage regulator ICs have a margining feature that changes the output voltage of the associated power supply up or down, which provides a way to see the effect of power-supply output variations on a system. Another voltage regulator IC feature is output voltage tracking, which forces the output voltage of one power supply to track the output of another power supply after startup.

Can a voltage regulator IC control its input current on startup?

The soft-start feature reduces input current transients and prevents output voltage overshoot when powering up or resuming operation after shutdown, overload, short-circuit, or overload conditions.

What are the usual applications for linear and switching regulators?

The linear regulator’s power dissipation is directly proportional to its output current for a given input and output voltage, so typical efficiencies can be 50% or even lower. Using the optimum components, a switching regulator can achieve efficiencies in the 90% range. However, the noise output from a linear regulator is much lower than a switching regulator with the same output voltage and current requirements. Typically, the switching regulator can drive higher current loads than a linear regulator.

What are some typical commercially available switching regulator ICs?

Enpirion’s EN5395QI is a synchronous, pin-programmable power supply IC with integrated power MOSFET switches and integrated inductor. The nominal input voltage range is 2.375-5.5V. The output can be set to common pre-set voltages by connecting appropriate combinations of three voltage selection pins to ground. The feedback control loop is a type 3 voltage-mode and the part uses a low-noise PWM topology. Up to 9A of output can be drawn from this converter. The 5MHz operating frequency enables the use of small-size output capacitors.

The EN5395QI has a soft-start circuit that limits the in-rush current when the converter is powered up. A Power Good flag indicates whether the output voltage is within 90% to 120% of the programmed voltage.

The IC has the following protection features: programmable over-current protection (to protect the IC from excessive load current), thermal shutdown with hysteresis, overvoltage protection, and undervoltage lockout (UVLO) that disables the converter output when the input voltage is less than approximately 2.2V.

The ENABLE pin provides a means to shut down the device, or enable normal operation. A logic low will disable the converter and cause it to shut down. A logic high will enable the converter into normal operation. When the ENABLE pin is asserted high, the device will undergo a normal soft start.

The EN5395 is internally compensated and is optimized for use with about 100µF of output capacitance and will provide excellent loop bandwidth and transient performance for most applications. Voltage mode operation provides high noise immunity at light load. In some cases modifications to the compensation may be required. The EN5395QI provides access to the internal compensation network to allow customization.

3-24V Input, 8A, High-Efficiency, Integrated Synchronous Buck Regulator

Fairchild Semiconductor’s FAN2108 TinyBuckTM is a highly efficient, small footprint, 8A, PWM-controlled synchronous buck regulator. It contains both synchronous MOSFETs and a controller/driver with optimized interconnects in one package, which enables designers to solve high-current requirements in a small area with minimal external components. External compensation, programmable switching frequency, and current limit features allow design optimization and flexibility. The summing current mode modulator uses lossless current sensing for current feedback and over-current protection. Voltage feedforward helps operation over a wide input voltage range.

Fairchild’s advanced BiCMOS power process, combined with low RDS(ON) internal MOSFETs and a thermally efficient MLP package, provide the ability to dissipate high power in a small package.

Output overvoltage, undervoltage, and thermal shutdown protections help protect the device from damage during fault conditions. FAN2108 prevents pre-­biased output discharge during startup in point-of-load applications.

Flyback Controller in ThinSOT Operates at 150°C

Shown in Figure 3 is Linear Technology’s H-Grade version of the LTC3803, a current mode flyback dc-dc controller in a 6-pin ThinSOT™ package. With its 40µA start-up and 240µA quiescent current, the device allows the use of a high-value input resistor and a low-value capacitor for low power dissipation and quick power supply start-up. The LTC3803H contains all the necessary features to design a high efficiency single-ended isolated or non-isolated flyback converter ideal for automotive, industrial, telecom, Power-over-Ethernet and datacom requirements.

With adjustable slope compensation, an LTC3803H-based power supply achieves fast transient response with minimum output capacitance. The constant 200 kHz operating frequency is maintained down to very light loads, resulting in less low frequency noise generation over a wide range of load currents. An integrated soft-start function reduces inrush current, reducing output voltage overshoot. The LTC3803H can be powered with inputs from 9 V to 75 V through a series resistor clamped with the IC's internal shunt regulator, making it ideal for 4:1 input range applications. The 100 mV current sense threshold enables the use of a very small current sense resistor even at high power levels, ensuring high efficiency.
The H-Grade version is production tested and guaranteed over an operating junction temperature range from -40°C to 150°C, making it ideal in wide temperature range applications. The E- and I-Grade versions are specified for 85°C and 125°C maximum junction temperature, respectively.

42V Constant On-Time Buck Switching Controller

National Semiconductor’s LM25085 is a PFET buck (step-down) DC-DC controller using the constant on-time (COT) control principle, which is a variation of the basic hysteretic converter. The input operating voltage range of the LM25085 is 4.5V to 42V. The use of an external PFET in this buck regulator greatly simplifies the gate drive requirements and allows for 100% duty cycle operation to extend the regulation range when operating at low input voltage. However, PFET transistors typically have higher on-resistance and gate charge when compared to similarly rated NFET transistors. Consideration of available PFETs, input voltage range, gate drive capability of the LM25085, and thermal resistances indicate an upper limit of 10A for the load current for LM25085 applications.

Constant on-time control is implemented using an on-time one-shot that is triggered by the feedback signal. During the off-time, when the PFET (Q1) is off, the load current is supplied by the inductor and the output capacitor. As the output voltage falls, the voltage at the feedback comparator input (FB) falls below the regulation threshold. When this occurs Q1 is turned on for the one-shot period which is determined by the input voltage (VIN) and the RT resistor. During the on-time the increasing inductor current increases the voltage at FB above the feedback comparator threshold.

The LM25085 is available in both an 8 pin MSOP package and an 8 pin LLP package with an exposed pad to aid in heat dissipation. An 8 pin MSOP package without an exposed pad is also available.