The quality of camera phones is constantly improving, including features such as higher resolution, better focal length, enhanced image processing software, and anti-shake features. However, one aspect that lags behind is the power of the flash when taking photos in low-light environments. Many mobile phones provide a low-current LED flash or fast flash, so the performance is greatly compromised. In terms of obtaining acceptable image quality photos under low light conditions, such light energy is far from enough.
To be a practical flash technology, the light source must be able to provide sufficient brightness within a certain target range (for example: >50lux @ 1m). The industry’s first-class high-power, high-brightness white LED technology can achieve this goal-each chip drive current up to 1500mA.
Mobile phone devices with integrated functions will be more and more admired by the market, thus presenting a constant demand for miniaturization, general flexibility, form factor and time to market. To meet these needs, TI has introduced a series of easy-to-design and optimized high-power LED flash drivers (TPS61050/2/4). These devices have a solution size of less than 25mm2 and can provide up to 5W of power to the LED.
Figure 1 TPS61050 application overview
In portable applications using a single 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 both buck and boost modes of operation.
The easiest way to implement step-down conversion is to use a linear low-side current regulator. The advantage of this method is low cost and high efficiency, because the LED forward voltage is usually slightly lower than the rated battery voltage.
This article will address LED camera flash applications and related problems, including: high-power LED driver architecture, battery current, and voltage drop.
LED camera flash driver topology
Regardless of the manufacturer, model, size, or power, all LEDs perform best when driven at a constant current. The light output in lumens is proportional to the current, so LED manufacturers specify many characteristics (such as luminosity, color temperature, etc.) of their devices when the forward current IF is specified. High-power LEDs exhibit a steep IV curve, so driving the LED with a constant voltage can cause significant and almost unpredictable changes in forward current.
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 a power stage based on an NMOS switch 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 higher than the diode forward voltage.
Figure 2 TPS61050 functional structure diagram
For the purpose of simplification and reduction of chip area, we use a low-side current detection circuit, which is based on an active current mirror designed to operate in the saturation region. The device will automatically switch between a linear buck mode and an inductive boost mode with a minimum detection voltage of 250mV based on the voltage drop across the current sink.
The advantage of this architecture is that its efficiency is very high under all LED current and battery voltage conditions, 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 detection lies in accuracy and high efficiency, which are two conflicting aspects. The lower the headroom voltage across the current detection/regulation circuit, the more energy can be saved, but this comes at the expense of noise sensitivity.
Figure 3 Typical efficiency
Since the LED flash function is not used so frequently in camera phone applications, we have the idea of using inductive power stages to achieve other functions. TPS6105x devices can not only function as a regulated current source, but also function as a standard boost regulator. Voltage mode operation can be completed by software commands or hardware signals (ENVM).
When powering other high-power devices in the system (such as LED drivers, hands-free audio power amplifiers, or any other components that require a supply voltage higher than the battery voltage), the additional operating mode may be very useful in order to properly synchronize the converter.
Figure 4 White LED flash driver and auxiliary lighting area power supply
In order to support LED current regulation or output voltage regulation, the TPS6105x device implements a new multifunctional regulation scheme (see Figure 2), which implements a seamless and instant conversion between the two control loops.
LED power, battery current, and voltage drop
The output power relationship to be used in the efficiency calculation is PLED = VF x IF. LED drive efficiency (that is, the ratio of electrical LED power to battery power) is equal to:
Figure 5 The relationship between efficiency and input current
For a given LED current, the forward voltage will vary with the process and temperature. This means that the conversion efficiency from battery power to light output will change while the brightness remains the same, because the brightness only depends on the current.
Therefore, efficiency is not a sufficient parameter index (figure of merit) for evaluating power consumption. What we have to consider is the relationship between battery current and LED brightness, that is, LED current. As far as the brightness of a given LED is concerned, the output power is the true yardstick of how much energy the battery can output.
When a large load is applied to the battery, the open-circuit battery voltage will be distorted by the voltage drop, which is caused by the internal impedance of the battery pack. The battery impedance largely depends on the following parameters:
Internal battery impedance. The impedance of a brand new lithium-ion battery is ca 50~70m. The impedance of each battery is not the same, and the impedance change is about 15% according to different production batches.
Relaxation effect. The battery voltage keeps changing after applying/removing the pulse load.
temperature. Battery impedance has a close relationship with temperature, and the impedance will increase by 50% for every 10C drop in temperature.
charging. The internal impedance depends on the state of charge (SoC), and the internal impedance increases at the end of the discharge.
protect the circuit. The lithium-ion battery pack has a back-to-back protection MOSFET connected in series with the battery, and its resistance range is ca 50~70m.
Connector. Usually the battery pack is connected to the system through a pair of spring connectors (each connector has a DC resistance of 25m).
From an electrical point of view, a battery is usually just a voltage source, or a voltage source connected in series with a resistor that represents the internal impedance of the battery. In order to correctly represent the transient behavior of the battery, we should use an equivalent circuit, not just a resistance. .
When the battery is fully charged or discharged, its open circuit voltage will change. Therefore, from an electrical point of view, it can be regarded as a capacitor with a variable capacitance value (CO).
In Figure 6, RA and RC are the total diffusion, conduction, and charge transfer resistances of the corresponding cathode and anode. CA and CC are surface capacitances. RSER is a series resistance including electrolyte, current collector, and wire resistance.
Figure 6 Battery equivalent circuit
Each stage is associated with its time constant, which leads to complex electrical behavior.
Figure 7 The relationship between transient response of 900mAh, lithium-ion battery and SOC and temperature
As shown in Figure 7, although the response of the battery voltage to the current step is delayed, after a period of time, it begins to approach the capacitive behavior with a series resistor. After the current is terminated, the battery voltage will not immediately return to the no-current state. On the contrary, it will slowly increase until it finally reaches the equivalent capacitor voltage level, which is the open circuit voltage.
Even in the case of insufficient battery capacity, the voltage drop across the high internal impedance will cause the system to reach its cut-off voltage and the “low battery” indicator to trigger. As a result, the mobile device resets and/or stops working. We should fully consider this factor when calculating the camera engine cut-off voltage and the maximum LED flash current level.
In TDMA-based systems (such as GSM/GPRS mobile phones), the RF power amplifier (PA) can also draw high peak currents from the battery. The TPS61050 device integrates a general-purpose I/O pin (GPIO), which can be configured as a standard logic input/output, or as a flash masking input (Tx- MASK).
This blanking function changes the LED from a camera flash to a flashlight light, thus reducing the peak current load of the battery almost instantaneously. This system-level feature prevents the phone from shutting down by preventing two high-power loads (PA and flash LED) from turning on at the same time.
LED flash current level optimization
In mobile phone applications, we usually specify the camera engine to work at a temperature as low as 0C or -10C. In order to achieve stable system operation, the LED flash current needs to be adjusted according to the maximum allowable battery voltage drop (that is, the highest battery impedance, the lowest ambient temperature).
In order to dynamically optimize the relationship between LED flash current (ie light output) and battery charging status and temperature, we can consider using the following self-adjusting program. This algorithm can be embedded in the automatic exposure white balance or red-eye reduction pre-flash algorithm.
LED forward voltage “catalysis” characteristics-carried out during the production test of the camera engine.
The first-order approximation of the LED forward voltage (VF) can be done by the integrated 3-bit A/D converter.
For three different flash currents (200mA, 500mA, and 1000mA), only three short flash strobe pulses (a few tenths of a millisecond is sufficient).
These data help us more accurately estimate the true electrical power of the LED compared to the flash current.
Figure 8 LED forward voltage approximation
Pre-flash function to estimate battery impedance
In a high-power flash strobe pulse, the battery voltage usually drops by hundreds of millivolts. As far as the short-time high-power flash strobe pulse is concerned, the voltage drop is not greatly affected by the battery’s own capacitance (that is, the relaxation effect), but is affected by its battery impedance.
Figure 9 Image capture sequence when pulsed LED is running
The camera and/or baseband engine can usually measure the battery voltage before the flash strobe pulse and at the end of the flash strobe pulse. With this information system, the approximate battery impedance can be calculated as follows:
According to the actual LED electrical characteristics, intermediate frequency battery impedance, charging status and temperature information, the camera engine software can dynamically optimize the LED flash current to avoid the risk of battery collapse.
About the Author
Christophe Vaucourt is currently a system engineer at TI (Germany). He has worked at TI for more than 7 years and is mainly responsible for new product definitions and low-power DC/DC converter application support. Before joining TI, he worked for Alcatel as a power designer for Internet screen phone equipment. He graduated from Ecole National Superieure de Physique de Strasbourg (France) with a bachelor of science degree in electrical engineering.
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