Francesco MACINA, Elga MARETTO
(This article was originally published in the component magazine; the author works in the STMicroelectronics Linear and Interface Products Division of STMicroelectronics)
Abstract: Mobile phones, PDAs, and general portable devices must meet the ever-increasing functional requirements in small size limits. Cameras that incorporate video and video are becoming the most popular feature requirements for portable products. The flash function that supports the camera is the key to determining performance and power consumption. This article will explore how to enhance the flash function and show a complete design to illustrate how to drive high power white LEDs .
flash
The functional integration of portable devices, such as the combination of PDAs and mobile phones, is the latest trend that is currently taking place. Today, integration is a very difficult goal if it is not possible to add new features to mobile phones that were originally only available for receiving and speaking, as well as some "prehistoric" terminal devices. This includes MP3/MP4 players; wired (USB and USB-OTG); infrared (IrDA), Bluetooth, Wi-Fi, GPS and other wireless connections; and the most popular digital video broadcasts, all of which are the latest mobile phones. Built-in main and very attractive features.
However, at present, most of the latest mobile phones have built-in most of the common functions, such as high-quality camera modules that can acquire still images or call with video calls. In the past, because the image resolution of the camera module was quite low, it was often used as a simple toy, but now, the precision of the camera module has been greatly improved, and the image resolution has been pushed to the most advanced digital camera. Because the camera module requires a light source that can operate in low-light environments, and the latest high-resolution cameras require higher lumens and brightness, the built-in flash function provides important features and drives the application of the overall camera module. .
Early flashlights built into mobile phones only provided entertainment-type flash functions because they provided almost the same results as no flash. In order to support the latest high-resolution cameras, so that the final product can attract and meet the needs of end users, its lighting function must achieve better decisive photography performance. This article will next explore two different ways to implement a flash.
Flash function
There are currently two options for flash lighting:
(1) flash light;
(2) LEDs.
(Figure 1) is a flash model. Its light bulb is made up of a glass envelope filled with helium. When the electrode is triggered, both the positive and negative electrodes are filled with gas, not connected to the bulb surface.
When the electrical impedance of helium is reduced to a very low value, visible light generated by a large amount of current flows from the positive electrode to the negative electrode. This function can be achieved through the trigger electrode. This feature provides high voltage peaks up to kilovolts to ionize helium and make it a low impedance state.
![]()
(Figure 1) Flash Module
This flash has the outstanding features required for high quality image performance. The light output is very dense and easily spreads over a large area. In addition, since the color temperature of the flash is approximately 5500 to 6000 °K, which is very close to the temperature of natural light, no color correction is required.
On the other hand, since the output illumination involves a very high energy level, typically hundreds of volts are required on the positive pole, so some time is required to raise the battery voltage to meet the power requirements of the mobile phone.
For two consecutive flashes with a time range of 1 to 5 seconds, the standard charging time required depends on the input power, capacitance value, charging circuit characteristics and required energy. The illumination of the flash may be just a pulse that would make the flash a good solution for still image flash, but it is not suitable for video applications.
In addition, because Xenon flash tubes and their additional drive circuits are quite space-consuming for space-constrained mobile phones, they also require a higher and more dangerous voltage to trigger helium to provide sufficient energy. Output lighting, so a precise but expensive design must be used, which limits the use of flash in mobile phones.
For these reasons, mobile phone manufacturers are gradually turning to white LEDs as the light source for flashlights.
LEDs provide continuous illumination, making them ideal for use as a flash source for video telephony in low-light conditions. In addition, the driver circuitry required for LEDs is fairly simple and much cheaper than flash bulbs.
Entry-level camera phone resolutions are typically in the sub-megapixel range, and these phones typically have built-in standard white LEDs with relatively low lighting capabilities. Because of this, multiple standard white LEDs, whether in series or in parallel, must provide a minimum source of light that can be imaged, otherwise the captured image will be almost invisible and unusable.
Standard white LEDs have a standard forward voltage from 3.2V to 4.5V, and a single power supply that drives them typically has a voltage range of 4.2V at full charge and 2.8V to 2.7V at discharge. Lithium Ion Battery.
Standard white LEDs are typically driven using a step (boost) DC-DC converter or a charge pump. Both solutions must control the forward current through the LED, which is proportional to the intensity of the illumination. This article will then explore both methods.
â– Step-Up Boost Converters and Charge Pumps Multiple standard white LEDs (usually no less than four) are connected in series to provide the total amount of light required for flash function. (Figure 2) shows a standard application circuit that uses a standard stepper converter to drive the LEDs; and (Figure 3) uses a charge pump circuit.
![]()
(Figure 2) Stepper (Boost) Converter Circuit for Driving Standard White LEDs![]()
(Figure 3) Charge pump converter circuit for driving standard white LEDs
Two parameters affect the choice of design: battery operating voltage and LED forward voltage. As mentioned earlier, today's mobile phones are powered by a single lithium battery, with an initial voltage range of 4.2V to 2.7V at the end, while standard white LEDs for flash lamps typically have a drive current range of hundreds of mA. After conversion to the LED forward voltage, it is approximately 3.3 to 3.5V. However, temperature and process variations may cause considerable changes to these values. Inductive step-up (SU) converters are typically used for driving, which produces an output voltage high enough to supply programming current, and the LEDs are preferably connected in series rather than in parallel. By using these methods, the brightness of the LED is proportional to the forward current, and all LEDs can receive the same current regardless of the forward voltage, so the matching is very good.
On the other hand, the charge pump (CP) uses an external flying capacitor to generate an integer multiple of the input voltage. In order to get more multiplication results, additional flying capacitors and rectifiers must be used, thus limiting the actual CP usage to 2X. Some CPs offer a small multiple (1.5X) input voltage, but require two flying capacitors.
Since 2X is the maximum multiplying factor that is suitable for use, plus CP is usually used to drive white LEDs using parallel spurs, taking into account the standard forward voltage. The current on each leg can be independently controlled, which will result in a slight but unavoidable brightness mismatch in the LED.
The use of the SU converter is more efficient than the integer CP and is almost flat over the entire battery discharge range. The CP score improves CP efficiency, but its value and flatness are still far from the SU design. In addition, since the LEDs using SU are connected in series, only two PCB wirings are required between the SU controller and the LED. This is a considerable advantage in terms of design flexibility. Assuming that the number of LEDs changes, or if these LEDs are installed in each camera flash module, the SU can easily adapt to these changes, and the CP's PCB must be redesigned.
In addition, for the PCB area of ​​the two solutions, even though the SU package of the lower total number of pins is smaller than the CP, since the SU must use the inductor, the total PCB area and thickness are large. On the other hand, this external inductance is usually more expensive than the external flying capacitor used by the CP.
Finally, the standard noise of the SU converter is also higher than that of the CP. In a precise design, interference between different sections must usually be avoided or limited. However, all LED driver topologies will choose standard white LEDs because it has become the only market application focus. The need to improve white LED technology and to meet the demand for better flash performance in higher resolution cameras on the market is driving some LED manufacturers to continuously develop new high-power white LEDs to meet the needs of the mobile phone flash market. These solutions not only provide practical flash function and better spectral performance, but their primary role is to replace a wide variety of white LEDs.
![]()
(Fig. 4) Image results taken under different flash conditions
As the LED flash market moves toward higher currents, CP has struggled to provide the greater total current required to power LEDs, even though they are available, and their overall electrical performance is very low.
In addition, since the SU can only provide a higher output voltage than the input voltage, it cannot accommodate the forward voltage of a high-power white LED (considering all variations, the standard value is about 3.2 to 4.8V), and the wide input voltage of the lithium-ion battery. range.
For these reasons, many IC manufacturers are being asked to offer new solutions that drive advanced high-power LEDs. Based on these needs, a new dual-mode DC-DC converter has also been developed on the market. The complete camera flash design using this dual mode architecture DC-DC converter will be described below.
Camera flash design
A dual-mode, step-up/step-down high-frequency (1.8MHz) dc-to-dc converter that powers a single high-power white LED. If the voltage across the LED is different from the external setpoint, its built-in four-switch architecture allows the DC-DC converter to transition from buck mode to boost or buck, regardless of its forward voltage or battery voltage. - Boost mode to control the forward current through the LED. (Figure 4) shows the different operating methods of the converter, and (Figure 5) is the standard application for driving a single high-power white LED.
![]()
(Figure 5) Buck-Boost Operation![]()
(Figure 6) Standard application scheme using STCF02 driver IC
■Inductor Selection Since the switching frequency of the DC-DC converter is 1.8MHz, it is recommended to use an inductor using a high-frequency core material. In addition, this inductor must have a very low DCR to reduce conduction losses and to withstand peak currents without saturation. The basic principle is that 10% to 30% of full load current is a good peak-to-peak inductor current (ΔIL) design option. For white LED flash applications, an inductor value of 4.7μH is recommended.
â– Input Capacitor Selection The input capacitor represents a low-impedance voltage source for the converter, which assists in filtering the pulse input current (buck mode).
The input capacitance is equal to the AC current during operation. According to general rule of thumb, the rated voltage must be as high as 1.5 VIN (MAX) to account for voltage spikes that may occur during transients.
Most of the voltage peaks are due to equivalent series resistance (ESR) and equivalent series inductance (ESL), so low ESR ceramic capacitors and precise PCB layout must be used. This means that the input filter capacitor must be connected as close as possible to the pins of the dc-to-dc converter to reduce PCB trace length and resulting parasitic ESR and ESL. In this design, a 10μF SMD ceramic input capacitor is used.
■Output Capacitor Selecting the output capacitor must provide buck energy storage during the load transient, and must improve the steady state performance by limiting the output chopping voltage during the charging and discharging of the capacitor during each switching cycle. . The following equation provides an input voltage chain (ΔVO) at steady state:
![]()
(Formula 1)
Here CO, FSW and IO represent the output capacitor value, the switching frequency of the converter and the output current value.
Again, ESR capacitors must be as low as possible, so ceramic capacitors are strongly recommended for accurate PCB design. A 4.7μF SMD ceramic capacitor is used in this design.
â– Feedback loop compensation
The DC-DC converter incorporates an already compensated error amplifier with a bandwidth set to 2.5 kHz, which can be used as an option to use an external compensation capacitor. However, if the electrodes of the external LC filter and the zero value cause the system to be unstable, then all the phase and gain edge bode plots must use an appropriate compensation circuit. External compensation networks often need to be designed sequentially for the compensation system so that when passing 0 dB and the phase edge is 45°, the slope will be at -20 dB/decade. The bipolar values ​​introduced through the LC filter are as follows:
![]()
(Formula 2)
Considering the zero value introduced by the capacitor's equivalent series resistance, the following equation can be obtained:
![]()
(Formula 3)
The RESR here is the ESR of the output capacitor. In boost mode, the system will have a right half plane zero, which causes additional phase lag. When the system is stable, you must consider the right half of the zero, the value of which is as follows:
![]()
(Formula 4)
![]()
(Figure 7) Error amplifier for DC-DC converter
(Figure 7) shows the internal architecture of the error amplifier, which is used when designing a compensation network option.
The impedance of Z1 and Z2 is as follows:
![]()
(Formula 5)
Since the DC-DC converter is used as a stable external component in this design, it is no longer necessary to use an external compensation network.
â– Power Flash LED
This design can use a high efficiency white LED with a forward voltage of 2.5V to 3.5V. Considering various variations, the forward voltage range of the LED must include tolerance, temperature, and forward current. In the current design part of the DC-DC converter, the PWF1 Luxeon flash LED series is used.
â– Schottky diodes In order to improve overall system efficiency during internal switching, an external Schottky diode is recommended. In this design, a diode rated at 1A is used.
â– Setting the flash current According to the following equation, the flash current of the converter can be set by an external resistor (RFLASH):
![]()
(Formula 6)
Among them: 160mA is the internal reference voltage, and RFLASH is the external resistor.
This reference voltage value must be compromised in electrical losses (IFLASHRFLASH represents power loss) and in the level of sound suppression (because the extra internal comparator may be triggered, low reference values ​​can be dangerous). RFLASH = 0.27 Ω can be selected to obtain the result of IFLASH = 60 mA. In this design, an SMD thick film resistor is a good choice.
With two additional currents, the intensity of the LED's source is changeable, with a choice of mid-range flash and Torch mode. Mid-range flash mode is often used to reduce red-eye effects or for autofocus. One or more mid-range flashes before the main flash will help reduce the effects of the retina's reverberation on the flash; at the same time, the LED flash can be used to allow the camera to perform the desired autofocus adjustment and avoid overheating. The lower current of the flash is activated. (Fig. 8) For the mid-level flash current setting graph, the straight line in the middle-order area can be calculated by:
![]()
(Formula 7)
The RMF here is an external resistor. RMF=8200Ω can be used to set the mid-range flash current to 300mA. The Torch mode can be used for video telephony or for simple lighting in the surrounding environment.
![]()
(Figure 8) Mid-range flash current Vs. RMF (as a percentage of IFLASH)
The current level of Torch mode can be set by the following related programs:
![]()
(Formula 8)
The 250mA torch current is programmable, so RTORCH = 0.37 Ω must be set.
â– Detecting LED Temperature If the temperature of the LED increases beyond its maximum allowable junction temperature, the external negative temperature coefficient (NTC) used to connect the DC-DC converter may detect this condition and immediately turn off the driver. Avoid system damage or overheating.
![]()
(Figure 9) LED temperature detection and NTC resistance
The over temperature trigger point can be set by:
![]()
(Formula 9)
EXTVREF in the above equation is an externally stable voltage source, and Rx and RNTC(T) represent the constant and temperature depending on the resistance, respectively. Since the maximum contact temperature of the selected LED is 85 °C, to ensure a safe edge, the 75 °C comparator trigger point can be set with RX = 10KΩ, while the NTC has a 14KΩ @ 75 °C resistor. Here, Murata's NCP18WF104J03RB resistance (100kΩ@25°C) was selected.
â– ENABLE pin The three logic input signals can be selected between five modes of operation:
(1) off (stationary current less than 1μA);
(2) Off, NTC sensing function is turned on;
(3) Torch mode;
(4) mid-level flash;
(5) Flash mode.
![]()
(Figure 10) Diagram of the efficiency of the DC-DC converter and the battery input voltage
â– Experimental results The components suggested above can measure the electrical effects. (Figure 9) shows the experimental results of this design.
The efficiency is completely fixed throughout the life of the lithium-ion battery, while ensuring that its efficiency value is not less than 85%. This is a very important feature for battery powered devices because it saves power and helps extend battery life. (Table 1) summarizes the above conditions:
![]()
(This article was originally published in the component magazine; the author works in the STMicroelectronics Linear and Interface Products Division of STMicroelectronics)
Abstract: Mobile phones, PDAs, and general portable devices must meet the ever-increasing functional requirements in small size limits. Cameras that incorporate video and video are becoming the most popular feature requirements for portable products. The flash function that supports the camera is the key to determining performance and power consumption. This article will explore how to enhance the flash function and show a complete design to illustrate how to drive high power white LEDs .
flash
The functional integration of portable devices, such as the combination of PDAs and mobile phones, is the latest trend that is currently taking place. Today, integration is a very difficult goal if it is not possible to add new features to mobile phones that were originally only available for receiving and speaking, as well as some "prehistoric" terminal devices. This includes MP3/MP4 players; wired (USB and USB-OTG); infrared (IrDA), Bluetooth, Wi-Fi, GPS and other wireless connections; and the most popular digital video broadcasts, all of which are the latest mobile phones. Built-in main and very attractive features.
However, at present, most of the latest mobile phones have built-in most of the common functions, such as high-quality camera modules that can acquire still images or call with video calls. In the past, because the image resolution of the camera module was quite low, it was often used as a simple toy, but now, the precision of the camera module has been greatly improved, and the image resolution has been pushed to the most advanced digital camera. Because the camera module requires a light source that can operate in low-light environments, and the latest high-resolution cameras require higher lumens and brightness, the built-in flash function provides important features and drives the application of the overall camera module. .
Early flashlights built into mobile phones only provided entertainment-type flash functions because they provided almost the same results as no flash. In order to support the latest high-resolution cameras, so that the final product can attract and meet the needs of end users, its lighting function must achieve better decisive photography performance. This article will next explore two different ways to implement a flash.
Flash function
There are currently two options for flash lighting:
(1) flash light;
(2) LEDs.
(Figure 1) is a flash model. Its light bulb is made up of a glass envelope filled with helium. When the electrode is triggered, both the positive and negative electrodes are filled with gas, not connected to the bulb surface.
When the electrical impedance of helium is reduced to a very low value, visible light generated by a large amount of current flows from the positive electrode to the negative electrode. This function can be achieved through the trigger electrode. This feature provides high voltage peaks up to kilovolts to ionize helium and make it a low impedance state.
(Figure 1) Flash Module
This flash has the outstanding features required for high quality image performance. The light output is very dense and easily spreads over a large area. In addition, since the color temperature of the flash is approximately 5500 to 6000 °K, which is very close to the temperature of natural light, no color correction is required.
On the other hand, since the output illumination involves a very high energy level, typically hundreds of volts are required on the positive pole, so some time is required to raise the battery voltage to meet the power requirements of the mobile phone.
For two consecutive flashes with a time range of 1 to 5 seconds, the standard charging time required depends on the input power, capacitance value, charging circuit characteristics and required energy. The illumination of the flash may be just a pulse that would make the flash a good solution for still image flash, but it is not suitable for video applications.
In addition, because Xenon flash tubes and their additional drive circuits are quite space-consuming for space-constrained mobile phones, they also require a higher and more dangerous voltage to trigger helium to provide sufficient energy. Output lighting, so a precise but expensive design must be used, which limits the use of flash in mobile phones.
For these reasons, mobile phone manufacturers are gradually turning to white LEDs as the light source for flashlights.
LEDs provide continuous illumination, making them ideal for use as a flash source for video telephony in low-light conditions. In addition, the driver circuitry required for LEDs is fairly simple and much cheaper than flash bulbs.
Entry-level camera phone resolutions are typically in the sub-megapixel range, and these phones typically have built-in standard white LEDs with relatively low lighting capabilities. Because of this, multiple standard white LEDs, whether in series or in parallel, must provide a minimum source of light that can be imaged, otherwise the captured image will be almost invisible and unusable.
Standard white LEDs have a standard forward voltage from 3.2V to 4.5V, and a single power supply that drives them typically has a voltage range of 4.2V at full charge and 2.8V to 2.7V at discharge. Lithium Ion Battery.
Standard white LEDs are typically driven using a step (boost) DC-DC converter or a charge pump. Both solutions must control the forward current through the LED, which is proportional to the intensity of the illumination. This article will then explore both methods.
â– Step-Up Boost Converters and Charge Pumps Multiple standard white LEDs (usually no less than four) are connected in series to provide the total amount of light required for flash function. (Figure 2) shows a standard application circuit that uses a standard stepper converter to drive the LEDs; and (Figure 3) uses a charge pump circuit.
(Figure 2) Stepper (Boost) Converter Circuit for Driving Standard White LEDs
(Figure 3) Charge pump converter circuit for driving standard white LEDs
Two parameters affect the choice of design: battery operating voltage and LED forward voltage. As mentioned earlier, today's mobile phones are powered by a single lithium battery, with an initial voltage range of 4.2V to 2.7V at the end, while standard white LEDs for flash lamps typically have a drive current range of hundreds of mA. After conversion to the LED forward voltage, it is approximately 3.3 to 3.5V. However, temperature and process variations may cause considerable changes to these values. Inductive step-up (SU) converters are typically used for driving, which produces an output voltage high enough to supply programming current, and the LEDs are preferably connected in series rather than in parallel. By using these methods, the brightness of the LED is proportional to the forward current, and all LEDs can receive the same current regardless of the forward voltage, so the matching is very good.
On the other hand, the charge pump (CP) uses an external flying capacitor to generate an integer multiple of the input voltage. In order to get more multiplication results, additional flying capacitors and rectifiers must be used, thus limiting the actual CP usage to 2X. Some CPs offer a small multiple (1.5X) input voltage, but require two flying capacitors.
Since 2X is the maximum multiplying factor that is suitable for use, plus CP is usually used to drive white LEDs using parallel spurs, taking into account the standard forward voltage. The current on each leg can be independently controlled, which will result in a slight but unavoidable brightness mismatch in the LED.
The use of the SU converter is more efficient than the integer CP and is almost flat over the entire battery discharge range. The CP score improves CP efficiency, but its value and flatness are still far from the SU design. In addition, since the LEDs using SU are connected in series, only two PCB wirings are required between the SU controller and the LED. This is a considerable advantage in terms of design flexibility. Assuming that the number of LEDs changes, or if these LEDs are installed in each camera flash module, the SU can easily adapt to these changes, and the CP's PCB must be redesigned.
In addition, for the PCB area of ​​the two solutions, even though the SU package of the lower total number of pins is smaller than the CP, since the SU must use the inductor, the total PCB area and thickness are large. On the other hand, this external inductance is usually more expensive than the external flying capacitor used by the CP.
Finally, the standard noise of the SU converter is also higher than that of the CP. In a precise design, interference between different sections must usually be avoided or limited. However, all LED driver topologies will choose standard white LEDs because it has become the only market application focus. The need to improve white LED technology and to meet the demand for better flash performance in higher resolution cameras on the market is driving some LED manufacturers to continuously develop new high-power white LEDs to meet the needs of the mobile phone flash market. These solutions not only provide practical flash function and better spectral performance, but their primary role is to replace a wide variety of white LEDs.
(Fig. 4) Image results taken under different flash conditions
As the LED flash market moves toward higher currents, CP has struggled to provide the greater total current required to power LEDs, even though they are available, and their overall electrical performance is very low.
In addition, since the SU can only provide a higher output voltage than the input voltage, it cannot accommodate the forward voltage of a high-power white LED (considering all variations, the standard value is about 3.2 to 4.8V), and the wide input voltage of the lithium-ion battery. range.
For these reasons, many IC manufacturers are being asked to offer new solutions that drive advanced high-power LEDs. Based on these needs, a new dual-mode DC-DC converter has also been developed on the market. The complete camera flash design using this dual mode architecture DC-DC converter will be described below.
Camera flash design
A dual-mode, step-up/step-down high-frequency (1.8MHz) dc-to-dc converter that powers a single high-power white LED. If the voltage across the LED is different from the external setpoint, its built-in four-switch architecture allows the DC-DC converter to transition from buck mode to boost or buck, regardless of its forward voltage or battery voltage. - Boost mode to control the forward current through the LED. (Figure 4) shows the different operating methods of the converter, and (Figure 5) is the standard application for driving a single high-power white LED.
(Figure 5) Buck-Boost Operation
(Figure 6) Standard application scheme using STCF02 driver IC
■Inductor Selection Since the switching frequency of the DC-DC converter is 1.8MHz, it is recommended to use an inductor using a high-frequency core material. In addition, this inductor must have a very low DCR to reduce conduction losses and to withstand peak currents without saturation. The basic principle is that 10% to 30% of full load current is a good peak-to-peak inductor current (ΔIL) design option. For white LED flash applications, an inductor value of 4.7μH is recommended.
â– Input Capacitor Selection The input capacitor represents a low-impedance voltage source for the converter, which assists in filtering the pulse input current (buck mode).
The input capacitance is equal to the AC current during operation. According to general rule of thumb, the rated voltage must be as high as 1.5 VIN (MAX) to account for voltage spikes that may occur during transients.
Most of the voltage peaks are due to equivalent series resistance (ESR) and equivalent series inductance (ESL), so low ESR ceramic capacitors and precise PCB layout must be used. This means that the input filter capacitor must be connected as close as possible to the pins of the dc-to-dc converter to reduce PCB trace length and resulting parasitic ESR and ESL. In this design, a 10μF SMD ceramic input capacitor is used.
■Output Capacitor Selecting the output capacitor must provide buck energy storage during the load transient, and must improve the steady state performance by limiting the output chopping voltage during the charging and discharging of the capacitor during each switching cycle. . The following equation provides an input voltage chain (ΔVO) at steady state:
(Formula 1)
Here CO, FSW and IO represent the output capacitor value, the switching frequency of the converter and the output current value.
Again, ESR capacitors must be as low as possible, so ceramic capacitors are strongly recommended for accurate PCB design. A 4.7μF SMD ceramic capacitor is used in this design.
â– Feedback loop compensation
The DC-DC converter incorporates an already compensated error amplifier with a bandwidth set to 2.5 kHz, which can be used as an option to use an external compensation capacitor. However, if the electrodes of the external LC filter and the zero value cause the system to be unstable, then all the phase and gain edge bode plots must use an appropriate compensation circuit. External compensation networks often need to be designed sequentially for the compensation system so that when passing 0 dB and the phase edge is 45°, the slope will be at -20 dB/decade. The bipolar values ​​introduced through the LC filter are as follows:
(Formula 2)
Considering the zero value introduced by the capacitor's equivalent series resistance, the following equation can be obtained:
(Formula 3)
The RESR here is the ESR of the output capacitor. In boost mode, the system will have a right half plane zero, which causes additional phase lag. When the system is stable, you must consider the right half of the zero, the value of which is as follows:
(Formula 4)
(Figure 7) Error amplifier for DC-DC converter
(Figure 7) shows the internal architecture of the error amplifier, which is used when designing a compensation network option.
The impedance of Z1 and Z2 is as follows:
(Formula 5)
Since the DC-DC converter is used as a stable external component in this design, it is no longer necessary to use an external compensation network.
â– Power Flash LED
This design can use a high efficiency white LED with a forward voltage of 2.5V to 3.5V. Considering various variations, the forward voltage range of the LED must include tolerance, temperature, and forward current. In the current design part of the DC-DC converter, the PWF1 Luxeon flash LED series is used.
â– Schottky diodes In order to improve overall system efficiency during internal switching, an external Schottky diode is recommended. In this design, a diode rated at 1A is used.
â– Setting the flash current According to the following equation, the flash current of the converter can be set by an external resistor (RFLASH):
(Formula 6)
Among them: 160mA is the internal reference voltage, and RFLASH is the external resistor.
This reference voltage value must be compromised in electrical losses (IFLASHRFLASH represents power loss) and in the level of sound suppression (because the extra internal comparator may be triggered, low reference values ​​can be dangerous). RFLASH = 0.27 Ω can be selected to obtain the result of IFLASH = 60 mA. In this design, an SMD thick film resistor is a good choice.
With two additional currents, the intensity of the LED's source is changeable, with a choice of mid-range flash and Torch mode. Mid-range flash mode is often used to reduce red-eye effects or for autofocus. One or more mid-range flashes before the main flash will help reduce the effects of the retina's reverberation on the flash; at the same time, the LED flash can be used to allow the camera to perform the desired autofocus adjustment and avoid overheating. The lower current of the flash is activated. (Fig. 8) For the mid-level flash current setting graph, the straight line in the middle-order area can be calculated by:
(Formula 7)
The RMF here is an external resistor. RMF=8200Ω can be used to set the mid-range flash current to 300mA. The Torch mode can be used for video telephony or for simple lighting in the surrounding environment.
(Figure 8) Mid-range flash current Vs. RMF (as a percentage of IFLASH)
The current level of Torch mode can be set by the following related programs:
(Formula 8)
The 250mA torch current is programmable, so RTORCH = 0.37 Ω must be set.
â– Detecting LED Temperature If the temperature of the LED increases beyond its maximum allowable junction temperature, the external negative temperature coefficient (NTC) used to connect the DC-DC converter may detect this condition and immediately turn off the driver. Avoid system damage or overheating.
(Figure 9) LED temperature detection and NTC resistance
The over temperature trigger point can be set by:
(Formula 9)
EXTVREF in the above equation is an externally stable voltage source, and Rx and RNTC(T) represent the constant and temperature depending on the resistance, respectively. Since the maximum contact temperature of the selected LED is 85 °C, to ensure a safe edge, the 75 °C comparator trigger point can be set with RX = 10KΩ, while the NTC has a 14KΩ @ 75 °C resistor. Here, Murata's NCP18WF104J03RB resistance (100kΩ@25°C) was selected.
â– ENABLE pin The three logic input signals can be selected between five modes of operation:
(1) off (stationary current less than 1μA);
(2) Off, NTC sensing function is turned on;
(3) Torch mode;
(4) mid-level flash;
(5) Flash mode.
(Figure 10) Diagram of the efficiency of the DC-DC converter and the battery input voltage
â– Experimental results The components suggested above can measure the electrical effects. (Figure 9) shows the experimental results of this design.
The efficiency is completely fixed throughout the life of the lithium-ion battery, while ensuring that its efficiency value is not less than 85%. This is a very important feature for battery powered devices because it saves power and helps extend battery life. (Table 1) summarizes the above conditions:
Conclusion
This article describes an innovative DC-DC converter architecture designed to drive the latest generation of high-power white LEDs that enhance flash functionality in portable applications. The experimental results show that the DC-DC converter has excellent performance in practical applications.
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