Download ATA1226_light loads power_FINAL

Technical
Article
Using the latest boost converters in battery-powered
applications: how to mitigate the effects of EMI
Mark Shepherd
Technical
Article
Using the latest boost converters in battery-powered
applications: how to mitigate the effects of EMI
Mark Shepherd
A cursory glance at the operating voltage specifications in a typical microcontroller’s datasheet fails
to reveal the whole truth. In many cases, the datasheet will claim a wide operating voltage range,
often from 1.8V to 3.6V. This range appears to make the MCU ideal for battery-powered applications, since it matches the range of a battery’s output voltage from its peak when fresh or fully
charged, to its minimum output when nominally discharged.
Except that microcontrollers do not offer their full capability across this entire operating range. In
fact, the maximum operating frequency of the microcontroller is generally only sustained when the
operating voltage is near its peak – typically ≥3V (see Figure 1).
Frequency MHz
RL78/G13 Voltage-Frequency
Graph
40
32MHz
30
20
10
4 MHz
8 MHz
16 MHz
0
1,6
1,8
2,4
2,7
5,5
Voltage
Page 2 / 8
Technical
Article
Fig. 1: a microcontroller can often only maintain its maximum operating frequency at a supply voltage of around 3V or higher.
This is true of two of the world’s most popular microcontroller families, the MSP430 from Texas Instruments and the
RL78/G13 from Renesas.
The design engineer is then faced with a choice: either to sacrifice processing power by operating
directly from a battery such as a lithium coin cell or dual alkaline AA cells; or to use a boost regulator to guarantee the maximum processing speed by maintaining a higher voltage.
And as this article will show, the decision to use a boost regulator carries with it a number of potentially undesirable trade-offs. This article will show how to optimize the choice of boost regulator in
order to combine efficiency and integration while mitigating the effects of voltage ripple at the regulator’s output.
Using a boost regulator with noise-sensitive circuits
A typical power circuit for a device such as a battery-powered RF sensor is shown in Figure 2. The
sensor has a simple mode of operation: it wakes up, takes a reading, transmits the data to a wireless hub, and then goes to sleep.
It is important for the microcontroller to operate at maximum speed, so as to complete its task in the
shortest period of time and thus draw the smallest possible aggregate amount of battery power.
When the batteries providing the sensor’s power supply are freshly charged, the microcontroller can
operate without enabling the boost regulator, which is the reason for the pass-through diode D1 in
Figure 2.
As the batteries discharge, the MCU monitors their output voltage with an ADC, and a software
program in the MCU decides when to enable the boost regulator – in this case, an AS1310 from
ams (www.ams.com/as1310) – in order to provide a regulated 3V output.
This gives the designer the flexibility to boost power when the application needs the MCU to operate at high speed, or when other blocks in the system require a higher operating voltage than the
batteries can provide.
Page 3 / 8
Technical
Article
Fig. 2: typical power circuit for a battery-driven RF sensor
The benefit of using a device such as the AS1310 in this application is that it offers extremely good
efficiency (up to 92%) at light loads (60mA). At no load, it is capable of operating at a 1µA quiescent
current while boosting to a 3V output, making it the perfect complement to today’s generation of
ultra-low power MCUs. The AS1310 (see Figure 3) has an input range of 0.7V to 3.3V, and comes
in several fixed output voltage options up to 3.6V. It is capable of supplying load currents up to
100mA from a 1.8V input. A sister part, the AS1312, offers similar performance but provides for
higher output voltage options (up to 5V). [Note to editor: the AS1310 is in mass production. As of
February 2013, the AS1312 was not released, but samples were available on request.]
Fig. 3: block diagram of the AS1310 (1.8- 3.6V output) hysteretic boost converter from ams
Page 4 / 8
Technical
Article
Using a boost regulator such as the AS1310 can provide a regulated 3V output, however boost
regulators are sometimes shunned by designers because of their notoriously high switching ripple
components. The resulting EMI (electromagnetic interference) can play havoc with RF circuitry and
sensitive analog sensors. What can the system designer do to ensure that this design choice – to
enable the MCU to run at maximum speed – does not produce an undesirable side effect: analog or
RF outputs compromised by noise?
Good design practices for mitigating the effects of voltage ripple
A large part of the designer’s effort must now go in to board layout in order to minimize EMI and
ground bounce. Output switching ripple is directly related to the capacitor’s equivalent series resistance (ESR), the value of the inductor, switching frequency, duty cycle and the load current.
High-frequency noise spikes, on the other hand, are caused more by parasitic elements and other
phenomena attributable to the board layout. In fact, it is not uncommon to see spiking frequencies
of 1-10MHz.
So when designing with buck or boost regulators, good layout practices are of paramount importance. The specification of key filter components, such as inductors and input and output capacitors, has a smaller, but still noticeable, effect on system performance.
In general, the inductor, converter IC and filter capacitors should be placed as close together as
possible, and only on one side of the board. Their traces should be kept as short and wide as possible to reduce parasitic inductance and impedance. The ground pads of the input capacitor, output
capacitor and the IC’s ground should ideally be connected in a copper pour on the component level
of the board, and only then terminated to the ground plane below by stitching in many vias to provide a single-point low-resistance connection to ground.
To simplify layout, the designer should start by placing the inductor pads as close to the AS1310 as
possible, while making sure the pad layout for the inductor is reasonably large to keep the impedance of the high-current loop small. One example of a good shielded inductor to consider is Murata’s LQH2HPN6R8MG0, a 6.8µH inductor in a 2520 package.
Next, choose input and output filter capacitors that will service the input and output peak currents
with acceptable voltage ripple. Choose input capacitors with a working voltage higher than the maximum input voltage. The input filter capacitor caps the peak currents drawn from the input source,
while limiting noise at the input source. The input voltage source impedance determines the required value of the input capacitor. When operating from two NiMH cells placed close to the
AS1310, a single 22µF ceramic capacitor followed by a 0.1µF ceramic capacitor should be sufficient. With higher impedance batteries, such as alkaline and lithium types, input capacitors with
higher values can improve efficiency.
Page 5 / 8
Technical
Article
Place the input capacitors as close to the input of the AS1310 as possible; the smaller capacitor
should be closer to the IC than the larger. The smaller capacitor cleans up the high-frequency hash,
while the larger bulk capacitor is used for overall filtering of the switching regulator.
The same applies to the output capacitor: the smaller capacitor should be placed closer than the
larger to the output of the AS1310. (If this were a non-synchronous boost regulator, for which an
external blocking diode was required, then the two output capacitors would be placed as close as
possible to the cathode of the blocking diode.)
Output voltage ripple occurs when the peak inductor current dumps energy into the output capacitor
(positive ripple), and when the current from the load causes negative ripple as the output capacitor
is discharged. The AS1310 is a hysteretic boost converter, which means one switching cycle will not
begin until the inductor current from the previous cycle is very near zero. Some ringing can be observed on the LX pin during the wait state of the regulator as a result of the empty inductor and
parasitic capacitance. This ringing can be ignored due to its very low energy signature.
The output voltage is set internally in the AS1310; no feedback lines are necessary from the Vout
pin. This greatly simplifies the layout constraints. If, instead, the design used a boost regulator that
required external feedback resistors, these resistors would need to be placed as close to the feedback pin of the IC as possible to prevent noise injection.
The reference capacitor (CREF), connected to the reference pin, should be placed as close as possible to the AS1310 since this device can influence the accuracy of the output voltage, and any
ripple in this reference voltage will impair the operation of the device.
These good layout practices do much to mitigate the effect of switching noise on the performance of
the surrounding circuitry. But more can still be done.
Page 6 / 8
Technical
Article
Fig. 4: a ground ring around the boost converter and its associated inductor and capacitors help to prevent switching noise
escaping into noise-sensitive parts of the circuit
First, a ground guard ring or split plane can be employed on the component layer to encircle the
entire IC, together with its inductor and the input and output capacitors (see Figure 4). An opening
the size of the sum of the distances of the input and output trace layouts is provided in order to preserve the continuity of ground at the component layer. The opening should be pointed towards the
area least likely to be affected by noise. Care should be taken to chamfer the edges of the guard
ring to eliminate the high-frequency EMI signals that would otherwise be produced by pinpoint corner edges.
Small ferrite beads (L2 & L3) should bridge over the split plane and be immediately followed by two
additional ceramic capacitors; the smaller capacitor should be closer to the guard ring. This creates
a second-order Pi filter, which helps to reduce the switching ripple on both the output and input
sides.
If the ferrite bead is a 0603 size, then make the gap in the split plane suitable for bridging across the
width of the component. When using ferrite beads it is also important to bear in mind the size of the
current to be carried. This is more than just the RMS current at the input pin; it must in fact withstand the peak current. In the case of the AS1310, this means specifying the bead for the maximum
current limit of 480mA. Also, as the current in the bead increases, its effective inductance decreases – an effect called offset current. It is advisable to use twice the rated current of the bead to minimize the effect.
Page 7 / 8
Technical
Article
Murata has a good family of ferrite beads for power-supply applications. For example, the
BLM18KG221SN1 ferrite bead, offered in a 0603 package, works well in circuits with the AS1310.
Conclusion
The use of a boost regulator in a battery-powered device allows the designer to ensure that the
microcontroller’s power supply always supports operation at maximum frequency. But in devices
which experience long periods of low or no activity, the efficiency of the boost regulator at light
loads is of crucial importance. The AS1310 from ams maintains high efficiency at light as well as full
load, drawing just 1µA at zero load. Designers specifying a boost regulator such as the AS1310 in
noise-sensitive applications must take care to manage the voltage ripple noise in the system, and
this article describes the layout techniques that can be used to reduce noise and mitigate its effects.
For further information
ams AG
Tel: +43 (0) 3136 500
[email protected]
www.ams.com
Page 8 / 8