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Rev. 1 — 28 October 2011 Application note Document information Info Content Keywords Photo flash, dual LED, 500 mA LED current, soft start, SSL3252 Abstract Application guidelines for a photo flash driver for mobile applications, including an application setup.
Application note Rev. 1 — 28 October 2011 2 of 14 Contact information For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: salesaddresses@nxp.com NXP Semiconductors AN10712 Photo flash LED driver
Revision history
v.1 20111028 application note; initial version
Application note Rev. 1 — 28 October 2011 3 of 14 NXP Semiconductors AN10712 Photo flash LED driver 1. Introduction The SSL3252 is a photo flash LED driver designed for battery operated mobile devices such as mobile phones and PDAs. The boost converter delivers high performance and drives a single or dual high brightness LED at up to 500 mA with over 85 % efficiency. The driver can be programmed to operate in Flash mode, Torch mode, Assist light mode, or Indicator mode. The small silicon size and the high internal switching frequency of 2 MHz minimizes the size of the application and makes the SSL3252 very suitable for mobile phones where space is limited, and only requiring three external components. System protection has been a very important part of the SSL3252 design so a time-out function can be programmed to prevent overstressing the LED, and the driver itself is protected from overheating. 2. Application information
2.1 General description
The SSL3252 is a boost converter intended to drive either a single high power flash LED or two high power flash LEDs in series. The main LED current is controlled by the output voltage of the boost converter and the integrated linear current source. The SSL3252 has two interface modes and six operational modes. The Interface mode is selected by the interface select pin IF_SEL. Depending on the interface mode selected, the device can either be controlled by an I 2C-bus interface, or external enable lines.
2.2 Application diagrams
2.2.1 I 2C control mode
Using the SSL3252 as shown in Figure 1 is the typical application for the SSL3252 in I2C control mode, and therefore gives the advantage of maximum flexibility of the operating features of the SSL3252. Apart from setting the driver into the different operating modes, all of the operating modes can be activated and settings can be altered to match the behavior of the driver to the application, e.g., adjusting the LED brightness intensity to meet the required level for a clear picture. The device cannot enter Shut-down mode when in I 2C mode. The lowest power consumption can be achieved in Standby mode. When using I2C, the device can still be put in Shut-down mode by first making all control pins LOW (SDA = SCL = TORCH = 0) and then going to Direct enable Shut-down mode by making IF_SEL LOW.
Application note Rev. 1 — 28 October 2011 4 of 14 NXP Semiconductors AN10712 Photo flash LED driver
2.2.2 Direct enable control mode
Using the SSL3252 as shown in Figure 2 is the typical application for the SSL3252 in Direct enable control mode. It has the advantage of operating the driver without using I2C communication. This provides a short response time and a less complicated operation, minimizing flash-ON latency. However, it has less flexible control features for the different operating modes. The STRB/2LED pin is functioning as 2LED output. (1) The selection of the mode of the IC is done with I 2C. IF_SEL must be connected to VIN. Fig 1. Typical application diagram of SSL3252 using I 2C control 002aaf151 L 2.2 μH LX VIN Cin 10 μF PGND Vbat battery PGND TORCH STRB/2LED SDA/EN2 SCL/EN1 IF_SEL (1) GND control signals VO Cout 10 μF PGNDLED PGND flash LEDs I_IND PGNDPGND indicator LED SSL3252 (1) The selection of the operational mode of the IC is done with EN1 and EN2. IF_SEL must be connected to GND. Fig 2. Typical application diagram of SSL3252 using Direct enable control 002aaf152 L 2.2 μH LX VIN Cin 10 μF PGND Vbat battery PGND TORCH STRB/2LED SDA/EN2 SCL/EN1 IF_SEL (1) GND control signals VO Cout 10 μF PGNDLED PGND flash LEDs I_IND PGNDPGND indicator LED SSL3252
data sheet. In Direct enable control mode the maximum flash time is fixed at 850 ms. and EN2 and stop the Flash mode.
2.3 PCB design and component placement
- Component placement
- Track width and length
- Use of vias
- Thermal restrictions
Table 1. Enable definition
Application note Rev. 1 — 28 October 2011 6 of 14 NXP Semiconductors AN10712 Photo flash LED driver In general, the PCB should be designed to accommodate all components as close as possible to the SSL3252 to minimize track resistance. When connecting capacitors the ground connection (GND) should be as short as possible. The pin arrangement of the device can make the usage of vias unavoidable. However it is advised to, if possible, avoid the use vias in tracks to and from those capacitors. If vias are used, use at least two or three to establish a low ohmic connection. In order to minimize EMI and to maximize the efficiency, the high current tracks to the coil, Co and to the Flash LED should be short and wide. If routing to another layer is unavoidable for one of the high current tracks, it is advised to use low ohmic vias and where possible to use two or more vias to lower overall track resistance. In general, high-current tracks to and from the same component should run close to each other to minimize the surface of the accompanying current loop. On Figure 4 the main current loops are shown for Ci, L and Co. The tracks used for these loops should either be in the same plane close to each other or in different planes on top of each other. Figure 4 also shows the importance of the position of the output capacitor. Within every switching cycle the loop area that carries a large inductor charge and discharge currents changes. This change will cause a change in the magnetic field, which will generate currents in other circuits near this loop. Therefore, to keep the distance between the two current paths to a minimum, Co must be as close as possible to the SSL3252. Co has a higher priority to be placed close to the driver than Ci. Ci can best be placed near the inductor L. The positive node of Ci should form a star connection. From this node a separate track (assuming this is supposed to be 1 track) should go to the inductor and to VIN of the SSL3252. Fig 3. Typical PCB design for SSL3252 019aac675
the high current pins for better heat dissipation.
2.4 Preferred components
2.4.1 C i capacitor
Table 2. Component list
Application note Rev. 1 — 28 October 2011 8 of 14 NXP Semiconductors AN10712 Photo flash LED driver into account, a 10 μF (X5R/X7R) is preferred. In the typical application, the maximum battery input inrush current is less than 200 mA at lower battery voltage levels (worst case). Although the component count will increase, a further improvement is obtained by placing a smaller capacitor of 100 nF (X5R/X7R) parallel to the input capacitor. When the circuit is used in other than battery powered applications and the input capacitor is located relatively far from the DC buffer capacitors, it is recommended to add a 150 μF tantalum or 470 μF electrolytic capacitor in parallel to the input capacitor. The ESR of this buffer capacitor is preferably higher as it also dampens the oscillations on the power supply caused by a possible high Q resonant LC circuit formed by the long power lines at the ceramic input capacitor. This electrolytic capacitor is also needed when doing efficiency measurements to obtain a good averaged input current from the supply.
2.4.2 C o capacitor
Co supplies the current in the LED when the SSL3252 is charging energy in the inductor. Although a 4.7 μF / 16 V capacitor is sufficient for Co, it is advised to use 10 μF / 16 V / X5R. The voltage derating characteristics of capacitors show a drop in value near the upper voltage limit of well above 50 %. Therefore a capacitor 4.7 μF / 10 V / X5R will not have sufficient capacitance left when operated near the upper voltage limit. A 16 V capacitor, however, will not be operated near its upper voltage limit and therefore it will still have sufficient capacitance. Currently, the smallest available mass production case size for 4.7 μF / 16 V / X5R is 0805. To squeeze the maximum out of size and performance the Panasonic 10 μF / 10 V / X5R is in 0603 case size. Although it will be driven to the maximum operating voltage the capacitance value left is still higher than that of a 4.7 μF type. Another advantage of this capacitor is that the same part can be used as an input and an output capacitor.
2.4.3 L inductor
The inductor that has the smallest footprint and has proven to be very suitable for LED flash application (large power for short period) is the Toko FDSE 0312-2R2M. If sustained high output power is of main importance, the Coilcraft LPS4012-222_L has proven to be a suitable inductor. It has small overall dimensions and a small footprint, but a higher temperature rise current and lower restive losses. Another option is the Taiyo Yuden NR4012T2R2N. It is similar to Toko inductor for maximum peak current and similar to Coilcraft inductor for continuous output current capability. When selecting a suitable inductor, not only the inductance is important, but the saturation current is also an important parameter. It should be matched to the maximum coil peak current, which is set to 2 A. Since the coil only carries the large flash current for a short period, the temperature rise current is less important.
3.1 I 2C control mode: I2C-bus software controlled 500 ms, 400 mA flash
supply is OFF between the flashes, then new initialization of the device is necessary. of the driver like shorted or open LED pin, and also will clear the fault register. Table 3. Initialization for 400 mA flash
2 Initialize digital lines IF_SEL HIGH - -
3 Init device
4 Init device
5 Init device
6 Trigger flash, Output On bit SDA/SCL write 30 04 8B Set Output On bit to ON
7 Read status register SDA/SCL read 30 05 XX Read (auto clear) status register
3.2 I 2C control mode: Edge-sensitive strobe controlled 500 ms, 400 mA
initialization of the device is necessary. of the driver, like shorted or open LED pin, and also will clear the fault register. Table 4. Initialization for 400 mA flash
6 Trigger flash STRB LOW to
3.3 I 2C control mode: 100 mA Assist light mode
OFF, then new initialization of the device is necessary. of the driver, like shorted or open LED pin, and also will clear the fault register.
3.4 Direct enable control mode: As sist light mode followed by
Table 5. Initialization for 400 mA flash
6 Switch on LED output SDA/SCL write 30 04 8A Assist light is ON
8 Switch off LED output SDA/SCL writ e 30 04 82 Assist light is OFF
9 Read status register SDA/SCL read 30 05 XX Read (auto clear) status register
Table 6. Initialization for a direct en able controlled Assist light and flash
2 Initialize digital lines IF_SEL LOW - Connect to GND
3 Assist light ON EN2 HIGH - Turn on LED9s) with default
5 Flash ON EN1 HIGH - LED(s) current ramp-up to the
6 Flash OFF EN1 LOW - Turn off LED(s) and clear faults
Table 7. Abbreviations
Application note Rev. 1 — 28 October 2011 13 of 14 NXP Semiconductors AN10712 Photo flash LED driver 5. Legal information
5.1 Definitions
Draft — The document is a draft version only. The content is still under internal review and subject to formal approval, which may result in modifications or additions. NXP Semiconductors does not give any representations or warranties as to the accuracy or completeness of information included herein and shall have no liability for the consequences of use of such information.
5.2 Disclaimers
Limited warranty and liability — Information in this document is believed to be accurate and reliable. However, NXP Semiconductors does not give any representations or warranties, expressed or implied, as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information. In no event shall NXP Semiconductors be liable for any indirect, incidental, punitive, special or consequential damages (including - without limitation - lost profits, lost savings, business interruption, costs related to the removal or replacement of any products or rework charges) whether or not such damages are based on tort (including negligence), warranty, breach of contract or any other legal theory. Notwithstanding any damages that customer might incur for any reason whatsoever, NXP Semiconductors’ aggregate and cumulative liability towards customer for the products described herein shall be limited in accordance with the Terms and conditions of commercial sale of NXP Semiconductors. Right to make changes — NXP Semiconductors reserves the right to make changes to information published in this document, including without limitation specifications and product descriptions, at any time and without notice. This document supersedes and replaces all information supplied prior to the publication hereof. Suitability for use — NXP Semiconductors products are not designed, authorized or warranted to be suitable for use in life support, life-critical or safety-critical systems or equipment, nor in applications where failure or malfunction of an NXP Semiconductors product can reasonably be expected to result in personal injury, death or severe property or environmental damage. NXP Semiconductors accepts no liability for inclusion and/or use of NXP Semiconductors products in such equipment or applications and therefore such inclusion and/or use is at the customer’s own risk. Applications — Applications that are described herein for any of these products are for illustrative purposes only. NXP Semiconductors makes no representation or warranty that such applications will be suitable for the specified use without further testing or modification. Customers are responsible for the design and operation of their applications and products using NXP Semiconductors products, and NXP Semiconductors accepts no liability for any assistance with applications or customer product design. It is customer’s sole responsibility to determine whether the NXP Semiconductors product is suitable and fit for the customer’s applications and products planned, as well as for the planned application and use of customer’s third party customer(s). Customers should provide appropriate design and operating safeguards to minimize the risks associated with their applications and products. NXP Semiconductors does not accept any liability related to any default, damage, costs or problem which is based on any weakness or default in the customer’s applications or products, or the application or use by customer’s third party customer(s). Customer is responsible for doing all necessary testing for the customer’s applications and products using NXP Semiconductors products in order to avoid a default of the applications and the products or of the application or use by customer’s third party customer(s). NXP does not accept any liability in this respect. Export control — This document as well as the item(s) described herein may be subject to export control regulations. Export might require a prior authorization from competent authorities.
5.3 Trademarks
Notice: All referenced brands, product names, service names and trademarks are the property of their respective owners. I2C-bus — logo is a trademark of NXP B.V.
NXP Semiconductors AN10712 Photo flash LED driver © NXP B.V. 2011. All rights reserved. For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: salesaddresses@nxp.com Date of release: 28 October 2011 Document identifier: AN10712 Please be aware that important notices concerning this document and the product(s) described herein, have been included in section ‘Legal information’. 6. Contents
3.1 I 2C control mode: I2C-bus software controlled
3.2 I 2C control mode: Edge-sensitive strobe
3.3 I 2C control mode: 100 mA Assist light mode . 11
3.4 Direct enable control mode:
Assist light mode followed by Flash mode . . . 11