Chipworks Analysis: Atmel’s MXT 540E Touch-Screen Controller

July 17, 2012

Chipworks explains via their chip analysis techniques some of the differences between Atmel's very popular MXT224 capacitive touch screen controller and the next generation MXT540E.

Roger Allan

Atmel’s MXT224 capacitive touch-screen controller has racked up most of the design wins in smart phones and tablets—except for Apple’s iPhone and iPad. Now, it faces stiffer competition from other devices. Atmel’s response is the MXT540E.

Given the extremely competitive nature of touch-screen controller ICs, Atmel offers no data sheets or major performance parameter data on the E-series, and it only provides a brief product summary on the MXT224. But a Chipworks analysis shows that about 41% of the MXT540E’s die consists of analog circuitry. The rest comprises digital circuitry for the processor, memory, and I/O circuitry.

The E-series is part of the company’s maXTouch controller family. According to Atmel, the maXTouch line represents one of the highest growth areas in its microcontroller business. The MXT540E supports 540 nodes, but its die can address up to 768 nodes.

System noise poses the greatest challenge to touch-screen performance. The maXTouch E-series was designed with enhanced analog sensing improvements and industry-leading noise immunity levels, Atmel says.

Compared to the MXT224, the MXT540E includes more filtering, more gain stages, greater charge-handling capacity, and more noise-handling capability. It assigns two acquisition y-pins per analog front-end (AFE) channel (Fig. 1), while the MXT224 offers one-to-one correspondence (Fig. 2).

1. Two acquisition pins used as inputs in the AFE section of the Atmel MXT540E touch-screen controller (a) are then fed as inputs to a pre-amplifier stage (b). (courtesy of Chipworks)

2. A single acquisition pin used by the Atmel MXT224 touch-screen controller in the AFE section (a) is then fed an input to a pre-amplifier stage (b). (courtesy of Chipworks)

Also, the MXT540E has eliminated precision current mirror stages that use amplifiers to equalize drain-to-source voltages in favor of simpler cascaded current sources supported by an increased 4-bit slew-rate controller to reduce die block area for this circuit. There was a notable process migration from 0.25 µm to 0.18 µm as well. These improvements are part of the story that leads to twice the resolution and a higher output impedance level than the MXT224.

The MXT540E targets new smart phones with diagonal sizes larger than 5 in. (6 to 12 in.) like Samsung’s latest Galaxy models. It uses 18 X by 30 Y electrode lines (versus 16 by 14 for the MXT224) and features 16 channels and five aluminum-bonded metal layers (versus 14 channels and four metal layers for the MXT224).

The E-series is considered the “enhanced” version of the maXTouch family, since it improves the performance of the previous series. But despite greater efficiency in the die utilization of some features, the increased capability and performance of the MXT540E means that the whole die is larger than that of the MXT224. The MXT540E’s die size is 4.17 by 4.06 mm, compared with the MXT224’s 3.73 by 3.58 mm. The MXT540E yields a die area of 16.93 mm² within the die seals (Fig. 3).

3. With a die area of 16.93 mm² within the die seals, Atmel’s MXT540E touch-screen controller chip is larger than its predecessor, the MXT224. (courtesy of Chipworks)

Coupled with the additional metal layer, two polysilicon/polysilicon layer capacitors, and embedded flash memory, the MXT540E is a more complex chip that is likely costlier to manufacture than the MXT224 but provides the greater performance needed for leading-edge smart phones and tablets. It’s housed in an 80-ball ultra-fine ball-grid array (BGA) package. There are 112 total bond pads arranged around the die’s periphery with 88 of the pads bonded.