Port Expanders for Flip-Cell Phones Save Cost and Space

Category: Telephone, cellular phone and intercom
Manufacture: Maxim Integrated Products
Datasheet: Download this application note

Keywords: port expanders, GPIO, I2C, push-pull outputs, open-drain outputs, LED intensity control, constant-current drive, MAX6966, level translation, transition detection,
Port Expanders for Flip-Cell Phones Save Cost and Space
Abstract: This article discusses the features available in recent-design port expanders, which are particularly relevant for the design of space-limited, cost-sensitive flip phones. The MAX6966 and MAX6965 drivers illustrate the GPIO port-expander technology discussed in the note.
The goal for general-purpose input output (GPIO) port expanders has always been to provide a modest number of I/O ports in a small, low-cost IC. Parts offering 8 or 16 ports have been around almost as long as the IC and SPI serial busses to which they connect. Features offered by these early parts included simple open-drain or push-pull outputs with limited drive current, and logic inputs with non-latching transition detection. The smallest available package was a TSSOP. This application notes discusses the features available in recent-design port expanders, which are particularly relevant for the design of space-limited, cost-sensitive flip phones.
The Flip-Phone Interconnection Problem
The case of a flip phone comprises two halves that fold together like a clamshell (Figure 1). Baseband and radio circuitry reside in the main section, usually the thicker half, together with the keyboard, battery, and antenna. A common layout for compact phones places a large display on the inside of the flip and a smaller one on the outside, as shown. The outside display, often a trans-reflective LCD that is readable without a backlight, operates at all times to show when the phone is idle and other status information. The flip enclosure holds the phone earpiece, and in some cases other audio and ring-tone circuitry. Many designs also include a camera module in the flip.
Figure 1. A flexible circuit connects the two cell-phone halves. A flip-phone's hinge is, however, an interconnection bottleneck. In most flip enclosures, the display(s) and camera include separate, moderately fast (Mbytes/second) parallel-interface busses for updating the displays and downloading the camera pictures. Passing data from the flip to the main body through the hinge, however, creates a bottleneck. That hinge connection is typically a flexible circuit made of Mylar with copper traces. Thus, to ensure a reliable circuit after repeated flexes, the trace density (and consequently the number of traces) must be limited. To exacerbate the problem, phone designers are always pressured to reduce the number of connections between the flip and the main body.
Advantage of GPIO Port Expanders in Flip-Phones
In general, a flip-phone design should minimize the number of copper traces on the flexible circuit (flexi) joining the flip to the main body. An industry trend serializes the high-speed parallel connections to the flip display(s) and camera. An easy way to reduce the other connections is to identify signals and controls that can be synthesized directly on the flip, rather than imported across the flexi. A small, low-cost port expander can control logic-input signals, outputs, LED drivers, or powercontrol switches. The port expander connects to the main board through an IC or SPI interface, which may be already available on the flip. Port expanders are also low-power devices. To be useful in a cell-phone architecture, a port expander must:
Have a physically small package (2mm x 2mm or 3mm x 3mm thin QFN) to allow placement wherever needed. Have a standard serial-protocol interface such as IC or SPI. Be interrupt driven to avoid power-wasting CPU polling. Operate main functions (PWM, input monitoring) without CPU intervention. Operate from a low supply voltage of 1.8V to 3V, and eventually below 1V. Draw supply currents in the 1A range.
LEDs in cell phones are used for display and keypad backlights (2 to 6 LEDs), function and status indicators, RGB fun lights, and battery and signal-strength indicators. Port expanders save space and power while reducing unnecessary interaction within the system in several ways: PWM intensity control for individual LEDs; high-voltage and high-current drive without the need for space-consuming discrete transistors; and direct-from-battery LED drive, which saves the cost and EMI of charge-pump or inductive-boost power supplies.
Open-Drain Ports Provide High-Current Drive
An open-drain output port can easily drive an LED. The port operates as a hard output switch to ground, and a series resistor (often called the ballast resistor) sets the LED current. Port expanders suitable for driving LEDs have high-current ports rated for voltages higher than the supply voltage, and pulse-width modulation (PWM) to adjust the LED brightness. The MAX6965 LED driver, for example, is a 9-output device with intensity control and hot-insertion protection in a 3mm x 3mm QFN package. It provides nine 7V-rated open-drain GPIOs with 50mA current-sink capability and individual output PWM control.
Constant-Current Ports Drive LEDs Directly from the Battery
A better approach to LED driving is a constant-current sink, which replaces the more conventional hard output switch and current-limiting ballast resistor. Constant-current LED drivers offer two key advantages:
LED current is independent of variations in the LED forward voltage or the LED supply voltage. LED supply voltage can be lower (closer to the LED forward voltage), which improves efficiency.
A constant-current driver allows a lower LED supply voltage because the voltage across a ballast resistor must be high enough to offset variations in the LED's supply voltage and forward voltage drop. As an example, consider a white LED driven from a 5V 5% supply, with forward voltage specified at 3.1V 0.25V. Voltage across the ballast resistor is 1.9V nominal, and varies from 1.4V to 2.4V. The worst-case variation in current is therefore 26%. If the supply voltage is reduced to 4V 3%, the ballast-resistor voltage is 0.9V nominal and varies from 0.53V to 1.27V. The worst-case current variation is now 41%, despite a tighter tolerance on the supply. A constant-current driver such as the MAX6966 (a 10-port - LED driver and I/O expander with PWM intensity control) correctly regulates its constant-current outputs, provided that the voltage drop across the port output is above the minimum specified (Figure 2). The port's output voltage is the difference between the load's supply voltage (typically for an LED) and the voltage across the load (the LED forward voltage). If a drop in the LED supply causes the port output voltage to fall below the minimum, the result is a brownout in the driver-output stages, which causes the load current to fall. Minimum port voltage for the MAX6966 is 0.5V at 10mA sink current, and 1V at 20mA sink current.

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