Posted on 23 May 2019

Simple Light Load Mode Control Optimizes Power Efficiency

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The target is in Point-of-Load Switching Regulators

Efficiency ratings for power supplies at full load do not adequately reflect the unit’s power consumption under actual operation conditions. Improved efficiency at all operating points including more typical situations such as very light loads is an important concern in the design of next generation “green” products. The 80 PLUS Program, for example, requires products to achieve 80% or better efficiency at 20%, 50% and 100% loading.

By Kevin Turchin, FAE Manager,ROHM Semiconductor USA


Common/popular consumer and portable product applications with variable point-ofload (POL) requirements include set-top boxes, liquid crystal displays, portable digital players, GPS devices and cell phones. These products and other devices which don’t operate at high power levels under all circumstances could benefit from POL control with improved efficiency over the entire range.

To optimize the overall efficiency in realworld applications, power supplies need a new approach to specifically address the requirements at light loads. In addition, the control strategy applied at light loads must be designed to seamlessly transfer to and from traditional PWM control as the load level changes. This article will describe just such a control technology given the name: Simple Light Load Mode™ (SLLM) Control.

Before discussing SLLM™, it will be helpful to first review the basic operation of current mode, step-down switching regulators designed for point-of-load applications.

Basics of Current Mode POL Switching Regulators

To efficiently convert the 5V power source in electronic products to other common voltages such as 3.3V, 2.0V, 1.8V or lower required for high-performance microprocessors and their peripherals, system designers typically utilize high-efficiency POL switching regulators. Today, many POL switching regulators employ current-mode (CM) control to provide both efficiency and fast response to load changes.

The CM pulse-width modulation (PWM) control system shown in Figure 1 shows the key elements of this design. In contrast to a “controller IC,” these switching regulator ICs integrate the power MOSFETs as well as the control circuitry into a single integrated circuit. The current mode switching regulator in Figure 1 combines a current feedback loop with voltage feedback for improved PWM control.

A current feedback loop added to the voltage feedback loop in a POL switching regulator

The operation of the PWM regulator is shown in Figure 2. A fixed PWM oscillation frequency of 0.5 to 2 MHz, a SET signal from OSC turns ON the high-side MOSFET with the low-side MOSFET OFF and the inductor current, IL, increases.

The timing diagram of a current mode PWM regulator shows the response to various input signals

The current comparator, Current Comp, receives both a current feedback control signal (SENSE) that is IL converted to a voltage and a voltage feedback control signal (FB). If both input signals are identical, the comparator generates a RESET signal, turning OFF the high-side MOSFET and turning ON the low-side MOSFET for the rest of the fixed period. The PWM control repeats this operation. This process provides improved response time for lower voltage swings under varying load conditions.

In the design of POL devices, the amount of voltage drop when the load on the POL regulator increases and how fast the regulator responds are key factors in a well-designed system. In fact, tight load regulation can be critical to avoid operational performance problems and even failure in voltage-sensitive devices, especially in circuits with supply voltages of 1.8V or less.

Principals of SLLM - Advanced Control for Light Loads

To keep pace with the rapidly improving capability and performance of high-speed ICs in the system, conventional PWM regulators cannot provide a completely acceptable solution. While CM control “solves” the load regulation problem, the demand for efficiency and extended battery life calls for a two-tiered control strategy. The advanced capability to respond quickly to load changes and provide improved efficiency at light loads is the basis of a new switching regulator series.

To improve the efficiency of products in the previously ignored low-power operating regime, ROHM Semiconductor has adapted the technique commonly know as “pulse skipping” as the basis for an implementation called Simple Light Load Mode Control. Figure 3 shows the comparison of SLLM to traditional PWM control.

In contrast to normal PWM timing switching does not occur during the portion of PWM

Under higher load levels, normal current mode PWM control is utilized. When a light load is detected, the switching pulse (SW) turns OFF the normal PWM control loop allowing linear operation without an excessive voltage drop or deterioration in transient response during the switching from light load to heavy load or vice versa.

The PWM control loop continues to operate with a SET signal from OSC and a RESET signal from Current Comp (refer to Figure 1). However, the RESET signal is not issued in the light-load mode so switching does not occur. This intermittent activation or discontinuous mode of operation reduces the switching dissipation and improves the efficiency. Rather than dissipating unnecessary power, the SLLM technique allows the regulator to function normally when required and to shut off to save power at low load conditions. While this sounds rather straightforward, the circuitry to do this requires precise timing and an understanding of the application requirements.

Figure 4 shows the improvements in the lower power range through the use of the SLLM control. This pulse-skip control technology is an integral part of a new series of highly efficient switching regulators that are now available from ROHM Semiconductor.

SLLM improves efficiency in the lower power range by 100 to 200 percent over standard PWM.

Higher Overall Efficiency and Performance

While the operation is simple and easy to describe, the circuitry behind SLLM requires a level of sophistication to avoid problems during the transitions between SLLM and standard PWM control. This light-load control method ensures low power consumption and high efficiency under varying load conditions even in the “not switching” or suspension mode.

Consider Efficiency under All Operating Conditions

SLLM technology with synchronous rectifier design and other circuitry techniques provides exceptional load regulation and high efficiency at both low and high power operating modes. Switching regulators incorporating this approach address the emerging needs for “green” product designs. With SLLM technology, system designers can provide longer, problem-free operation of portable devices as well as reduced energy consumption under all operating conditions. ROHM Semiconductor’s new BD91x Series of Switching Regulators utilizes SLLM to provide high efficiency over a wide range of load conditions.



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