Author:
Andy Fewster, Engineering Manager, austriamicrosystems AG
Date
04/04/2012
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Figure 1: Typical application circuit, using 2 AA batteries and a regulator IC, the AS1337, to produce a 3.3V output
Small, battery-powered medical analysis equipment presents a challenge to the power circuit designer. In meeting this challenge an understanding of the way boost conversion is implemented in today's power ICs will be extremely helpful. Medical devices can gain marketing benefits from the promise of long battery life. But this article is aimed at clarifying the designer's choice by illustrating the particular characteristics of two different approaches to boost conversion in low-load applications such as personal medical equipment: a hysteretic converter, and a Pulse Width Modulated (PWM) converter; their two different sets of characteristics are well suited to two different kinds of load profile. Designers can streamline the component specification process by matching the load profile of their application to the right converter type. The operating environment for the converter circuit Battery power in the types of medical devices in question usually comes from one of: • Lithium coin cells (2-3V per cell) • Alkaline primary cells (0.9-1.5V per cell) in AA or AAA outlines • Lithium primary cells (0.9-1.7V per cell) in AA or AAA outlines • NiMH rechargeable cells (0.9-1.25V per cell) in AA and AAA outlines Single and dual series connections are common with AAA and AA batteries. In this article, it is assumed that the power management circuit must provide at least 3.3V from a single or dual alkaline cell (see Figure 1), and that long battery life and low bill-of-materials cost are equally important objectives for the design.
Important additional features The basic requirement of the converter IC is to boost the voltage accurately and efficiently to the required level. But other capabilities can also enhance the operation of the power circuit. • Output supervisor: the output supervisor can produce an output reset pulse for the device's controller (normally an MCU) after a time period measured from the point at which the output voltage approaches the desired output value. • Input supervisor: usually an uncommitted comparator input used to monitor the battery voltage. Useful for providing a ‘low battery' warning just before the battery is fully exhausted. Sometimes this is a shared function with an output ‘power OK' flag. • Quasi step-down mode: this is useful when the terminal voltage of a fresh battery is slightly higher than the output regulated voltage. It allows the battery voltage to decay until boost-only operation commences. Quasi-step down mode, either switched or 100% duty cycle, entails some loss of efficiency. But it is convenient, because it eliminates the requirement for a more complex buck-boost converter arrangement - and the period in which Vin is greater than Vout is normally short. Efficiency and long battery life Achieving long battery life depends on a number of factors, but two dominate: 1) Conversion efficiency 2) Load characteristics, such as duty cycle and peak current. A meticulous approach to power management, powering down components such as the display and the controller whenever they do not need to be active, will have a marked effect on battery life. The load characteristics will also affect the designer's approach to optimising conversion efficiency. There is no such thing as a perfect boost converter, and each type has its strengths and weaknesses. Of particular importance are: • The value of the average load - some converter types supply light loads more efficiently, while others are better suited to heavy loads • The duty cycle - again, some converter types are better suited than others to applications that have long periods of no activity followed by short bursts of active operation. To define the load characteristic, the designer should measure the load current over time (assuming a constant supply voltage) under a variety of conditions, including different usage modes and different temperatures. The magnitude-versus-time envelope can then be converted to an RMS (root mean squared) value, from which the time-averaged power dissipation is calculated. Common load current profiles are shown in Figures 2 and 3, calculating the RMS power dissipation. T is the overall time between repetitive current profiles, and D is the duty cycle.
The shorter the duty cycle, the lower will be the RMS current of the load. For these applications, the designer must take into account not only the typical load profile, but the periods of non-activity: here, the contribution of the converter IC to quiescent current becomes crucial. Of two conversion techniques implemented in the latest boost converters from austriamicrosystems, one - hysteretic, implemented in the AS1310 - has a far lower quiescent current than the other, Pulse Width Modulated (PWM) technique implemented in the AS1337. Hysteretic control, also known as ripple control, achieves a lower quiescent current because it eliminates the other converter type's clock-based PWM core, employing a simple comparator instead. In Shutdown mode, the AS1310 draws less than 100nA. Other advantages of this operating scheme include low operating current, simplicity - since there is no need for the PWM scheme's closed-loop frequency compensation - and fast transient response. The AS1310 is optimised for light loads (60mA), at which it achieves efficiency of up to 92%. On the other hand, the designer must be prepared to accept the hysteretic technique's downsides: load-dependent variable operating frequency, and output ripple. In fact, output ripple is fundamental to the operation of the hysteretic converter, and is broadly equal to the hysteresis value set by the comparator. In addition, discontinuous current operation in the inductor produces higher peak input currents than the equivalent fixed-frequency current-mode PWM converter operating with a continuous inductor current. The alternative current-mode PWM control scheme, as implemented for instance in the latest AS1337 IC, provides exceptional line and load regulation. The continuous inductor current reduces peak input currents compared to a hysteretic control scheme. PWM conversion also offers higher efficiency at moderate and heavy loads than a hysteretic scheme. In the case of the AS1337, an automatic power-save mode, initiated if the output load current falls below a factory-programmed threshold, also improves efficiency at light loads, by removing power from all circuitry not required to monitor the output voltage, and operating in an intermittent PWM mode. Additional design guidelines for efficient conversion Conversion efficiency depends in part on constraining resistive losses in the main storage inductor, at least to a value below the resistive losses of the integrated power switch and must be able to handle the peak current of the dc-dc converter without saturating - and in a hysteretic mode converter this peak current will be higher than in the equivalent PWM converter. High-frequency ferrite core inductor materials produce lower frequency-dependent power losses than cheaper powdered iron types, and this results in improved converter efficiency. Another factor is the equivalent series resistance of both the input and output capacitors (see Figure 4): this should be minimised in order to reduce ripple at the output, and to reduce peak current and conducted noise at the input. Conclusion The load demand data will inform the choice of converter IC. Advanced new converter ICs from austriamicrosystems implement hysteretic and PWM voltage regulation schemes: understanding the different characteristics of these two schemes will help the designer ensure the chosen converter type best suits the power demands of the application. www.austriamicrosystems.com
