| High power Flash LED control on a Camera |  |  |  |
| CONVERtronic - Power Electronics |
The quality of the movable telephones with camera improves continuously: greater resolution, better objectives, software of more powerful, stabilizing image processing of image, etc. The unique field in which the advance has not been so well-known is in the one of the feeding and the power of the flash to take photos in conditions of low light. Many mobile telephones resort to a commitment and use a flash LED of low current, that, nevertheless, does not provide light sufficient to take a photography in conditions of little illumination. So that a flash technology is useful actually, the light source must be able to offer an suitable illumination to an established distance (for example > 50 lux to 1 m). This objective can be obtained by means of the most recent LED white of great power and discharge luminosity, that they admit until 1500 mA by chip. The multifunctional mobile telephones put under the market to an increasing dynamism, with constant exigencies of miniaturization, versatility, form factor and exit time to the market. In order to satisfy these demands, TI presents the family of control loops of flash LED of optimized size TPS61050/2/4, that they simplify the phase of design to a great extent. With a occupied surface inferior to 25 mm2, these devices can provide up to 5 W of power to the LED.
In mobile applications with ion batteries lithium (Li-ion) of a single element, the sum of the voltage drop in the white LED and the difference of tension in the current regulator can be superior or inferior to the tension of the battery. Therefore, the topology of the control loop of the LED must allow the operation in reducing way and elevating way. The way simplest to implement the conversion by reduction is by means of a controlled linear regulator in current. The advantages of this method are the low cost and the high effectiveness, since the direct tension of the LED is generally slightly smaller than the nominal tension of the battery. This article deals with on the challenges about flash LED for cameras and the considerations on the architecture about control about the LED high power, the current of the battery and the voltage drop. Topology of control of flash LED for cameras Independent of the manufacturer, the size or the power, the optimal operation of the LED is obtained when a constant current is provided. The power of the light, measurement in lumina, is proportional to the current, for that reason the LED manufacturers specify the characteristics of their products (iluminancia, temperature of color, etc.) to determined current a direct IF. The LED of great power tends to show a very steep curve IV, reason why the control of the LED by means of a constant tension can give rise to remarkable and almost unpredictable variations of the direct current. The TPS6105x family uses a converter PWM in current way to a constant frequency of 2 MHz to generate the tension of exit necessary to control the LED of great power. The Integra device a stage of exit based on a commutator NMOS and a synchronous rectifier NMOS. The Integra device also a regulator of current of the LED when the tension of the battery is superior to the direct tension of the diode.
The control of current by reasons for simplicity and surface occupied in the silicon capsule has been adopted. The counter circuit of current is based on a mirror of current designed assets to work in the saturation region. Following the voltage drop in the current drain, the device automatically exchanges between the way of linear reduction and the way of inductive elevation presenting/displaying a mini tension of detection of 250 mV (nominal). The advantage of this architecture is the highest effectiveness with all the currents of LED and tension of battery, since the entrance tension can be lifted until the sum of the direct tension of the LED and the difference of tension in the current drain.
The difficulty of the detection of current is to achieve that he is exact and effective, two objectives directly opposed. Whichever minor is the difference of tension in the counter circuit/current regulation, minor will be the consumption, but at the expense of the immunity to the noise. Since flash LED usually is not used very frequently on the telephones with camera, the idea arose to use the stage of inductive elevator for an additional function. The TPS6105x not only works like a source of regulated current, but also like an elevating regulator of standard tension. The operation in way tension can be activated by means of software or with a signal of hardware (ENVM). This way of additional operation can turn out from utility to correctly synchronize the converter at the time of feeding other devices on great consumption on the system, like the control loops of the LED, the audio amplifier for free hands or any other component that require a tension of feeding superior to the tension of the battery. In order to as much admit the regulation of current of the LED like the regulation of the exit tension, the TPS6105x implements a new scheme of multipurpose regulation (it consults Figure 2), that provides a flexible transition enters control bows both. Power of the LED, current of the battery and voltage drop The relation of voltage drop that will be used in the calculation of the efficiency is PLED = VF x IF. The efficiency of the “controller” of the LED, that is to say, the relation between electric power of the LED and the power of the battery, are equivalent a: Image To the inverse one, Image For a given current of LED, the direct tension varies according to the process and the temperature. This means that the efficiency of the electrical energy conversion of the luminance production A battery can vary although a constant luminosity stays, since this last one depends solely on the current. Therefore, the efficiency is not an suitable parameter to evaluate the electrical consumption. What one is due to consider is the current of the battery with respect to the luminosity of the LED, that is to say, the current of the LED. The entrance power is the true measurement of the energy given by the battery for a certain luminosity of the LED. When a great load is applied to the battery, it changes the tension in open of the same by the voltage drop due to the internal impedance of the battery. The impedance of the batteries is in favor certain to a great extent of the following parameters:
- Internal Impedance of the elements. The new Li-ion elements show an impedance of the order of 50~70 mW. The impedance depends on the element and approximately varies a 15% within a same lot of production. - Effect of relaxation. The tension of the battery continues varying after the retired application or of the pulsating load. - Temperature. The impedance of the element shows a great dependency with respect to the temperature and can increase a 50% by each 10 ºC of reduction. - State of the load. The internal resistance depends on the state of the load and increases at the end of the unloading process. - Circuit of protection. The packages of batteries Li-ion in series have MOSFET of protection with the elements that offer a resistance of the order of 50~70 mW. - Connector. The package of batteries is connected to the system by means of a pair of contact wharves, each with a resistance of 25 mW in CC.
From the electrical point of view, the batteries imagine frequently like a source of tension or a source of tension with a connected resistance in series, that represents the internal resistance of the battery. In order to correctly represent the transitory behavior of the battery we will have to use an equivalent circuit instead of a simple resistance. When load or unloads a battery its abierto tension in changes, reason why it is possible to be represented like a condenser of variable capacity (CO). In Figure 6, equivalent Circuit of a battery, RA and RC represent the resistance of conduction, diffusion and charge transfer of the anode and the cathode, respectively. CA and CC are the superficial capacities. RSER is the resistance in series that it includes to the electrolyte, the receivers of current and the resistance of the wiring. Each stage is associate to its own time constants, which brings about a complex electrical behavior. As it is appraised in Figure 7, the answer of tension of the battery before a step of current shows a certain delay, to come near in series after a time to the behavior of a condenser with a resistance. After the conclusion of the current the tension of the battery does not return immediately to the state without current, but it increases slowly until finally reaching the value of the tension of equivalent condenser, that is the tension in open. Although the battery even contains the sufficient energy, the voltage drop in the high internal impedance can cause that the system reaches the tension of cuts and that the indicator of “low battery” activates. Consequently, the mbile device is reinitiated or stopped working. It is necessary to have this aspect in account at the time of calculating the tension of cuts of the processor of the camera and the maximum current of flash LED. In the systems based on TDMA like telephones GSM/GPRS, the radio-frequency power amplifier (RF) also it demands forts tips of current to the battery. The TPS61050 includes a sideburn of ES of general use (GPIO) that can be formed like a standard logical input/output or like an entrance of flash camouflage (Tx-MASK).
This function of camouflage makes work the LED like a center instead of a flash, which almost very instantaneously reduces the tip of demanded current to the battery. This function of the system prevents that the telephone goes out when avoiding that two great loads as the amplifier of RF and flash LED activate at the same time. Optimization of the current of flash LED In applications of movable telephony, the processor of the specific camera in a rank of temperatures that can lower until 0º C or -10º C. In order to obtain a trustworthy operation of the system, the current of flash LED is due to fit to the maximum permissible voltage drop of the battery, that is to say, principle impedance of the battery and minim room temperature.
In order to dynamically optimize the current of flash LED (luminance production) with respect to the state of the load and the temperature of the battery, we will have to consider the following procedure of self-regulation. This algorithm can be integrated in the algorithms of automatic exhibition, automatic balance of the target or preflash for reduction of red eyes. Characterization “in situ” of the direct tension of the LED, realised during the final test of production of the processor of the camera. With converter A/D of 3 bits integrated an approach of first order of the direct tension of LED can be realised (VF). It is enough with three brief firings of the flash (a few tens of milliseconds are sufficient) with three different currents: 200 mA, 500 mA and 1000 mA. These data allow more accurately to consider the electric power of the LED in relation to the current of the flash.
Preflash to evaluate the impedance of the battery During a firing of flash of high power, the tension of the battery falls hundreds of millivolts normally. In short durations, the voltage drop is not influenced by the intrinsic capacity of the battery, that is to say, by the effect of relaxation, but by the impedance of the elements of the same. The digital processors of the camera and the baseband are generally able to before measure the tension of the battery and after the firing of the flash. With this information the system can calculate the impedance of the battery, that is defined of the following way: Image , with IBAT = 1.15xIF, operation in linear reducing way (VF < VBAT) Image , operation in elevating way (VF > VBAT) Being based on data like real the electrical characteristics of the LED and parameters of the battery like the impedance to MFs, the state of load and the temperature, the software of the processor of the camera can optimize dynamically the current of flash LED to avoid a dangerous collapse of the battery.
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Scheme of application of the TPS61050.
Functional block diagram of the TPS61050.
Nominal effectiveness.
Feeding of white flash LED and the auxiliary illumination.
Efficiency and current of entrance.
Equivalent circuit of a battery.
Transitory answer of a battery Li-ion of 900 mSh with respect to the state of load and to the temperature. 
Approach of the direct tension of the LED.
Sequence of capture of image with operation of the LED in way of impulses.
The author: Image Christophe Vaucourt is systems engineer of Texas Instruments Germany, where it has worked more than seven years in the definition of new products and the attendance of applications for converters CC/CC of low power. Before integrating itself in TI, it worked for Alcatel like designer of the power supply of the Internet ScreenPhone. Christophe Vaucourt is titled in electrical engineering by the National School Superior of Physics of Strasbourg, France.
