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Capacitor split technique halves mcu energy consumption in low power mode

Despite their sophistication, microcontrollers are increasingly found in products where they are used to accomplish relatively simple tasks.

Many of these products are handheld, battery powered devices that feature lcds, including kitchen weighing scales and air conditioning remote controls.

While most mcus offer various low power modes – and some include lcd controllers on chip – the challenge, until recently, has been to keep energy consumption low while the lcd is powered up. The capacitor split method of generating the lcd bias voltages can halve the power consumption of these mcu based products compared to previous solutions.

Driving and powering an lcd
The types of application being considered here are essentially those that provide a human machine interface. Hence the display requirements are often quite specific to the application and fall between the most basic alphanumeric displays and a full graphic (pixel matrix) type panel. Such displays may well include alphanumeric characters, but will typically also feature symbols, indicators or even predefined words that are relevant to the application.

Consequently, while there are some general purpose modules that combine alphanumeric characters with symbols like '°C' or words like 'Batt' or 'Mode', a custom lcd design provides the best solution for the majority of products. These can be cost effective, even at quite modest production volumes, especially when used with an mcu that has an lcd controller on chip.

LCDs require an ac drive voltage as dc operation causes electrochemical reactions that result in reduced life. Any element of an lcd that needs to be controlled individually is regarded as a segment and it is the magnitude of the ac voltage across a segment that determines whether it is on or off. In simple 'static drive' lcds, one side of each segment is tied to a common pin, often referred to as a backplane, with the other side having its own segment pin.

The frequency for directly driving an lcd is typically between 30Hz to 100Hz; lower frequencies result in display flicker and higher frequencies will increase power consumption since an lcd is a capacitive load whose impedance decreases with increasing frequency.

For displays with larger numbers of segments a 'multiplex drive' approach, using more than one backplane (or segment common), helps reduce the pin count for both the lcd glass and the microcontroller. While this drive method is essentially a time division multiplex, the number of time divisions needs to be twice the number of common planes to ensure the voltage at all segment locations is reversed periodically to avoid any dc voltage. LCD multiplex ratios are expressed as fractions so 1/2, 1/3 and ¼ relate to 2, 3 and 4 commons, but ratios from 1/8 to 1/64 are possible.

With static displays, a square wave ac drive signal is connected to the common plane and to any segments that are off while an inverted signal is applied to segments that need to be on. Hence there is zero voltage across the off segments and a double magnitude square wave across the on segments; this clearly requires only two voltage (or bias) levels.

However, the situation becomes more complex with multiplex displays since, in order to ensure all segments and commons see an ac drive (with no dc content), the common drive requires a multilevel waveform. Generating these intermediate levels requires additional bias voltages. Figure 1 shows the example of a 1/3 bias waveform as applied to a 1/3 ratio display, although other combinations of bias levels and multiplex ratios are possible e.g. 1/5 bias for 1/16 ratio.

Fortunately, generating these waveforms isn't something the equipment designer has to worry about as this is all taken care of by the lcd controller. However it is necessary to generate the required number of intermediate bias voltages and there are several means for achieving this. The most usual method for doing this is externally, with a simple resistive divider chain between supply and ground.

The values of the resistors will be determined by the required bias voltage levels, but will need to be low enough to not be affected by the capacitive load presented by the lcd; it may even be necessary to add smoothing capacitors between the bias points and ground to stabilise the levels.

An alternative approach, provided internally by some controllers, is a voltage boost circuit. This typically uses a reference voltage equal to the lowest bias level and then multiplies this up by as many times as necessary to provide the higher bias levels. One advantage of this approach is that the bias levels can be independent of the controller's supply voltage.

Renesas has pioneered another method for powering the lcd and providing these bias voltages in its RL78/L12 microcontroller series. The RL78 series has been designed specifically for use in ultra low power applications, featuring an innovative snooze mode that allows serial communication and a/d converter operation while in standby, suiting it to application in battery powered designs.

The recently introduced RL78/L12 is the first member of the RL78 family to offer an integrated lcd drive that provides flexible control, not only supporting the voltage boost and external resistive split bias methods, but also supporting a capacitor split method.

The capacitor split approach is similar in principle to resistive splitting in that the required bias voltages are defined by the values of the external capacitors, as shown in fig 2 (these capacitors should not be confused with the smoothing capacitors that may be used with the resistive split or voltage boost methods to stabilise the bias voltages from any load fluctuations).

The attraction of the capacitor split method is that there is no wasted current flowing continually through the resistive split bias chain or consumed by the capacitive charge pump circuit used for the voltage boost method. In the case of the RL78/L12, the capacitor split method enables an ultra low power mode (sub halt) that draws 0.62µA at 3V with the lcd active and real time clock running. This is less than half of the typical 1.5µA current taken by the voltage boost or resistive split methods. Even these figures disguise the true power saving – the capacitive split circuitry only draws 0.12µA, with the other 0.5µA being consumed by the real time clock.

Reducing the energy consumed in handheld or portable electronic equipment is key to ensuring these devices achieve acceptable battery life. Modern microcontrollers try to offset high performance with various low power modes. Driving lcds is a common requirement in these applications, so any technique that can save energy while the display remains active will always be welcome. The capacitive split biasing method offered by the Renesas RL78/L12 MCU provides just such a solution.

Steve Patrick is a field applications engineer with MSC Gleichman, while Mark Cullum is a senior mcu marketing engineer with Renesas Electronics.

Steve Patrick and Mark Cullum

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