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Constant current output produces best pictures from your phone

Posted: 01 Nov 2005 ?? ?Print Version ?Bookmark and Share

Keywords:flash? led? power? battery? voltage?

In just a few short years since their introduction to the market, camera phones are now found almost everywhere. With this quick adoption comes the consumer's request for more!more pixels, more memory, better quality. As handset designs improve the quality of their imaging sensors, they are also turning to new technology to improve picture quality. Increases in white LED brightness have resulted in a new component type!the camera flash LED. These LEDs are physically larger than the typical LEDs used for display and keypad backlighting, and they subsequently consume more current from the handset's battery. When driven with the correct voltage and current, the light from these devices provides noticeable improvement to the resulting images taken in low-light conditions.

One dilemma handset designers face when using flash LEDs is determining how to generate the desired amount of light and keep it constant throughout the image capture process. Flash LEDs require much higher current than typical backlighting LEDs!typically about 500mA!and, hence, their forward voltages (Vf) must be driven higher than backlighting LEDs. Most mobile handsets are powered using Lithium batteries that operate in the 3.2V to 4.2V range and forward voltages for flash LEDs can reach 4V or higher at 500mA. This means that some type of boost converter is needed to drive the LED to the desired output level. Two options have emerged in the market!inductor-based boost converters and switched-capacitor charge pumps. Both systems are capable of generating the necessary voltage and current, but there are clear advantages to avoiding inductors.

Boost converters are commonly used to generate higher voltages than a battery can supply. Current is switched in and out of the inductor and voltage is built up on the output capacitor during the process to drive the load. The output voltage or current is typically monitored and fed back to the switching system to help the control logic keep the output at a constant level. Efficiency for these devices is generally good, but when used with flash LEDs in handsets, some obvious concerns arise.

First is size. Inductors capable of handling an average current of 500mA tend to be quite large. Most camera-phone systems are squeezed into the smallest PCB area possible so they don't adversely affect phone performance. Second is noise. Inductor-based boost converters produce EMI that can interfere with radio performance. Proper shielding is required to avoid EMI problems, adding another component to total system area and cost. A third issue is the mode of operation. Since most flash LEDs have Vf in the same range as a Lithium battery's voltage, there are periods when the converter will need to act as a step-down (buck) regulator and those when it will need to act as a step-up (boost) regulator.

This operation is much more complicated than simple boost conversion, adding to the cost and complexity of the system. An alternative is to operate the system as a boost-only circuit by boosting to a higher voltage than necessary and using series resistance to balance the voltage, but this is inherently inefficient, thus costing the user more battery life.

Charge-pump drivers tend to be simpler than inductor-based solutions. These devices use a switched-capacitor architecture that is much quieter than a boost converter. They also operate with very small ceramic capacitors, saving more PCB area than boost converters. These advantages have made charge pumps the preferred drive system for flash LED camera phones in the market today.

There are as many variations in charge-pump flash LED drivers as there are IC companies. Some devices provide a constant boosted voltage supply high enough to exceed the flash LED's Vf. This approach produces the same inefficiency as described with the boost converter. Other designs have taken the same approach as backlight LED drivers that change switching modes depending on the input supply level, output voltage requirement and current load. These devices are more efficient than the fixed-voltage devices, but they also can produce output glitches during the switch from LDO mode (step-down) to charge-pump mode (step-up).

Another charge-pump architecture simply performs current regulation using feedback techniques to set the output current to the desired level. These devices can also exhibit mode-switching problems unless they are kept in the same mode for the duration of the flash.

Adequate output current for the LED is clearly the determining factor for brightness and the output voltage must be capable of supplying the Vf of the LED. These characteristics, however, can be found in most of the drivers on the market today. The key parameter that most devices don't address is current regulation!the ability to maintain a constant current for the duration of the flash. The Semtech SC615 charge pump has been designed to address this concern.

CMOS imagers are now commonly used in camera phones because of their low cost and improved quality. Unlike more expensive CCD imagers, these devices have individual amplifiers for each pixel. A scanning "window" reads blocks of these amplifiers in succession to determine their output. During this scan, some pixels are read while others are still collecting light. If the light source varies during this time, the resulting picture may exhibit uneven lighting. It is critical to maintain constant lighting during this period to ensure quality images.

The SC615 uses an external power resistor to set the output current level (Figure 1). The charge pump increases current output until the voltage across the sense resistor reaches the programmed level. Once the device is activated in flash mode, the SC615 reaches its steady-state output in less than 1ms and maintains a constant current output until the flash control signal is released. These result in a stable, constant light output during image capture. This architecture also provides two side benefits. First, heat dissipation is transferred from the IC to the sense resistor, increasing the amount of current that can be generated by the charge pump and lengthening the maximum flash duration achievable before thermal shutdown occurs. Second, the internal reference voltage can be switched to a lower value by toggling a control input. This results in a lower output current that can be enabled indefinitely without causing the chip to overheat. This mode is commonly referred to as torch mode.

The output current plots in Figure 2 show how constant the current really is. Figure 2a shows output stability for a flash duration of 500ms. This stability guarantees that the LED will output constant light throughout image capture. Figure 2b shows the performance as the input battery voltage decays. This ensures consistent lighting for pictures taken, regardless of the battery's charge level.

If you want your flash LED to do more than just blink when a picture is taken, you have to not only select a high-output LED, but also choose a driver IC that can maintain the high current output during image capture. The Semtech SC615 meets all these needs, making it a suitable choice for next-generation camera phones.

- Tom Karpus

Handheld Systems Manager, Handheld Portable Power Products

Semtech Corp.

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