EDNE December 2012 - (Page 23)

VSMP IS VBAT VSW VBOOST 10 μF VBAL VBAT + − VBOOST + − IND VBOOST VDDA VDDD PSoC VSSA VSSC 22 μF 0.1 μF 22 μF 0.1 μF VSSB (a) (b) Figure 2 A boost converter circuit (a) is shown in comparison with the switched-mode pump in a microcontroller (b). VBAT is the input battery voltage; VSW is the switching waveform, which is a PWM with a 50% duty cycle. converter circuit. The boost converter shown in Figure 2a has two phases: a storage phase, during which the switch is on, and a discharge phase, during which the switch is off. When the switch is conducting, the inductor stores energy from the battery in the form of a magnetic field. When the switch is opened, the inductor current continues to flow in the same direction, causing the voltage at node VSMP to “fly back” to a voltage higher than the capacitor voltage. This action triggers the diode to begin conducting, which in turn allows the charge stored in the inductor to be transferred into the filter capacitor. A PWM, V SW, turns the switch on and off. In a microcontroller (Figure 2b), an on-chip generation unit makes this switching waveform available. The protection diode can be internal to the microcontroller chip or can be connected externally. The only components a developer has to connect are the inductor coil and filter capacitors. In the SOC shown in Figure 2b, V DDA and V DDD are the chip supply voltage. DeSign tipS High efficiency is desirable in lowpower, low-input-voltage SMPs used in embedded solutions, which impose space and cost constraints, but switch and passive-component losses can limit efficiency. The MOSFET switch, which is inter- nal to the controller, contributes to ohmic losses as well as switching losses; the higher the switching frequency, the greater the switching losses. The impedance of this switch is pretty much determined at the design stage of the chip. The inductor losses are similar to those of the switch. The designer must choose the switching frequency appropriately to optimize power and must choose an inductor based on the switching frequency. T h e o u t p u t c a p a c i t o r ’s E S R (equivalent series resistance) can cause significant ripple. If aluminum capacitors are chosen to reduce cost, a ceramic capacitor should also be connected in parallel to minimize ripple. The size of the capacitor used determines the hold time of the output. Schottky diodes are recommended because they have a low forward voltage and fast switching speed, though the Schottky diode’s forward drop and its own impedance account for some loss. The current rating of the diode should be greater than twice the peak load current. The SMP shown in Figure 2b has an internal diode. In microcontrollers, however, this diode is mimicked using a MOSFET switch, which is operated in synchronization with the SMP. Having an external Schottky diode results in higher power loss, attributed to the diode forward drop, which typically would be around 0.4V. The internal syn- chronous FET has lower drop (0.1V) and thus minimizes losses, resulting in better efficiency. The nature of the load also affects the efficiency of the SMP; efficiency is reduced if the load is not a constant load. high efficiency is desirable in low-power, lowinput-voltage sMps used in eMbedded solutions, which iMpose space and cost constraints, but switch and passive-coMponent losses can liMit efficiency. The layout design of a low-inputvoltage SMP circuit must be done with extreme care. Consider a boost converter that starts up at 0.5V, as is the case in Cypress Semiconductor’s PSoC3 programmable system on chip. Let us assume that the boost output is expected to be 3V with 50-mA current capability. With 100% efficiency, the input current is expected to be ((3×50)/0.5)=300 mA. With 300-mA current being pumped in, a PCB trace of 1Ω can easily produce a voltage drop of www.edn-europe.com december 2012 | eDN europe 23 http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE December 2012

Cover
Contents
Texas Instruments Europe
Microchip
Digi-Key
Masthead
EDN comment
Pulse
Digi-Key
Baker’s Best
Embedded World 2013
Test & Measurement World
Rohde & Schwarz
Digi-Key
Squeezing the most from battery cells with a switched-mode pump
Signal integrity
Processor architectures : one to rule them all ?
Digi-Key
Mechatronics in Design
Advances in wireless speaker performance and technology
Design Ideas
Teardown; the ultimate Consumer Product ?
Product roundup
Tales from the Cube

EDNE December 2012