introduction
The quality of camera phones is constantly improving, including higher resolution, better focal length, enhanced image processing software, and anti-shake. However, one of the more backward aspects is the flash power when taking pictures in low-light environments. Many mobile phones have a performance that is greatly compromised by providing a low-current LED photo flash or flash, which is not enough for photos with acceptable image quality in low light conditions.
To be a practical flash technology, the light source must be able to provide sufficient brightness over a range of targets (eg >50 lux @ 1 m). Industry-leading high-power, high-brightness white LED technology achieves this goal—with a drive current of up to 1500mA per chip.
Functionally integrated mobile devices will become more and more popular in the market, thus presenting an increasing demand for miniaturization, general flexibility, form factor and time to market. To meet these needs, TI has introduced a range of high-power LED flash drivers (TPS61050/2/4) that are easy to design and optimize. These devices have a solution size of less than 25mm2 and are capable of delivering up to 5W of power to LEDs.
Figure 1 TPS61050 Application Overview
In portable applications using a single-cell lithium-ion (Li-Ion) battery, the sum of the voltage drop across the white LED and the headroom voltage across the current regulator can be lower or higher than the battery voltage. This means that the LED driver topology should be able to handle the buck and boost modes of operation.
The easiest way to implement a buck conversion is to use a linear low-side current regulator. The advantage of this approach is the low cost and high efficiency because the LED forward voltage is usually slightly lower than the rated battery voltage.
This article addresses LED camera flash applications and related challenges, including: high power LED driver architecture, battery current, and voltage drop.
LED camera flash drive topology
All LEDs perform best at constant current drive regardless of manufacturer, model, size or power. The light output in lumens is proportional to the current, so the LED manufacturer specifies many of the characteristics of the device at the specified forward current IF (eg, luminosity, color temperature, etc.). High-power LEDs exhibit a steep IV curve, so driving the LEDs at a constant voltage can result in significant and almost unpredictable changes in forward current.
The TPS6105x products use a 2-MHz constant frequency, current mode pulse width modulation (PWM) converter to generate the output voltage required to drive high power LEDs. The device integrates an NMOS switch-based power stage and a synchronous NMOS rectifier. In addition, the device implements a linear low-side current regulator to control the LED current when the battery voltage is above the diode forward voltage.
Figure 2 TPS61050 functional structure diagram
For the purpose of simplifying and reducing chip area occupation, we used a low-side current sensing circuit based on an active current mirror designed to operate in a saturated region. The device automatically switches between a linear buck mode and an inductive boost mode with a minimum 250mV sense voltage based on the voltage drop across the current sink.
The advantage of this architecture is that it is very efficient at all LED currents and battery voltages because the input voltage can be boosted to the sum of the LED forward voltage and the current sink headroom voltage.
The challenge of current sensing is accuracy and efficiency, which are two conflicting aspects. The lower the headroom voltage across the current sense/regulation circuit, the more power is saved, but at the expense of noise sensitivity.
Figure 3 Typical efficiency
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