L DESIGN IDEAS Complete Power Solution for Digital Cameras and Other Complex Compact Portable Applications by Brian Shaffer Introduction Digital cameras, portable GPS systems, MP3 players and other feature-rich mobile devices have complicated power requirements. In these complex devices, the flow of power must be carefully managed between a number of specialized sources and loads, including charging/discharging the battery, current-limited USB power and a set of multivoltage power supply rails, including negative rails for CCDs or LCDs. The supply rails must be sequenced and tracked and faults must be handled cleanly and communicated to a microcontroller. When these requirements are added together, the task of squeezing an efficient and robust power system into a handheld device can seem near impossible. Linear Technology solves this problem with a family of devices called PMICs (Power Management Integrated Circuits) that greatly simplify the design of complex rechargeable battery power systems. Some Linear Technology PMICs use a switching PowerPath controller topology with the unique Bat-Track feature, which allows charge currents above the USB limit (see Figure 1) for faster battery charging. The power solution for digital cameras presented 700 CHARGE CURRENT (mA) 600 500 400 VBUS = 5V RPROG = 1k RCLPROG = 3k 300 200 100 5x USB SETTING, BATTERY CHARGER SET FOR 1A 0 3.0 3.3 3.6 2.7 3.9 BATTERY VOLTAGE (V) 4.2 Figure 1. Battery charge current vs battery voltage 34 The LTC3586 implements Linear Technology’s unique Bat-Track™ technology, which can use more power from a USB source than traditional linear chargers, resulting in faster charging. here takes advantage of this and other powerful PMIC features. Complete Digital Camera Power System Figure 2 shows a complete digital camera power solution using the LTC3586 PMIC as the power traffic control center. Its 4mm × 6mm QFN package includes a USB PowerPath manager, a battery charger, plus a boost DC/DC converter, a buck-boost and two buck converters. The LT3587 in a 3mm × 3mm package is used to drive a CCD and an LED backlight for an LCD screen with a high voltage monolithic inverter and dual boost converter. Switching PowerPath Controller Maximizes Available Power The LTC3586 implements Linear Technology’s unique Bat-Track™ technology, which maximizes the use of available power from a USB source for either providing current to the load or charging the battery at rates greater than achievable from linear chargers. The switching PowerPath controller maintains accurate control of the average input current for USB applications. The average level of input current is controlled by the state of two digital inputs and can be set to 100mA, 500mA, 1A or suspend (500µA). The switching PowerPath controller is highly efficient, which results in battery charge currents of well over 600mA from a 500mA USB source (Figure 1). The battery charging efficiency is between 85% and 90% for the entire battery voltage range. In contrast, the efficiency of a traditional linear charger falls as low as 57%, generating the losses as heat. See Figure 3 for a graph of the battery charger efficiency as a function of battery voltage. Instant-On Operation The LTC3586 also features instant-on operation, which allows the camera to function immediately when external power is applied even if the battery voltage is below the system cutoff voltage. This is achieved by generating a separate voltage rail, VOUT, which is decoupled from the battery voltage when the battery is below 3.3V. When external power is applied, the PowerPath controller prioritizes load current over battery charge current and regulates VOUT to 3.6V, enabling the system to operate immediately upon the application of external power. The instant-on feature is important in camera applications because important moments do not wait for batteries to charge. Fault Handling The FAULT signals on both of these devices are designed to work together for seamless fault handling. By making the fault signals both an input and an output, the two chips can communicate fault events to each other. If either of the devices has a fault then all the outputs turn off, protecting the system and battery from damage. The enable lines and the fault signal Linear Technology Magazine • September 2008 DESIGN IDEAS L USB/WALL 4.5V TO 5.5V L1 3.3µH C1 22µF SW VBUS GATE NTC T VOUT VOUT 100k 2k CLPROG 2.94k 0.1µF + RED CHRG 3.3V 1A VOUT3 3.3V, 20mA LDO3V3 1µF 10k FAULT 2 PUSHBUTTON MICROCONTROLLER Li-Ion GND 510Ω C2 22µF MP1 BAT PROG 10pF 15k VC3 33pF 330pF 324k VIN3 2.2µF 105k L2 2.2µH SWAB3 L3 4.7µH LTC3586 1.8V 400mA SW2 MODE SYSTEM RAIL/ I/O 22µF FB3 SWCD3 ILIM 121k 1.02M I/O/MEMORY 10pF FB2 4 10µF 1µF 806k EN VIN2 L4 4.7µH MICROPROCESSOR 1.6V 400mA SW1 806k CORE 10pF FB1 806k 10µF 1µF C1, C2: TDK C2012X5R0J226M L1: COILCRAFT LPS4018-332LM L2, L5: TOKO 1098AS-2R2M L3, L4: TOKO 1098AS-4R7M MP1: SILICONIX Si2333 VIN1 VIN4 L5 2.2µH SW4 10µF 5V 800mA VOUT4 88.7k 10pF AUDIO/ MOTOR DRIVE FB4 16.9k 10µH 22µF 15µH 1µF 2.2µF 8.06k 1µF SW3 CAP3 VIN SW1 CAP1 IFB3 VOUT1 GND FB1 LT3587 10µF 15V 50mA 1M 2.7pF BACKLIGHT LEDs FLT EN/SS3 100nF 15µH = IR05H40CSPTR FB2 SW2 100nF LED DRIVER 20mA, UP TO 6 LEDS VOUT3 VFB3 EN/SS1 CCD POSITIVE CCD 1M 6.8pF 2.2µF 15µH 22µF –8V 100mA CCD NEGATIVE Figure 2. Complete power solution for portable cameras Linear Technology Magazine • September 2008 35 L DESIGN IDEAS 100 EFFICIENCY (%) 90 VVIN 2.5V TO 5V 10µH RCLPROG = 3.01K RPROG = 1K IVOUT = 0mA IR05H40CSPTR 1x CHARGING EFFICIENCY VIN 80 1µF SW3 CAP3 5x CHARGING EFFICIENCY LT3587 VOUT3 IFB3 70 60 2.7 3 3.5 3.9 3.3 BATTERY VOLTAGE (V) 4.2 Figure 3. Battery charging efficiency vs battery voltage with no external load (PBAT/PBUS) should be pulled-up to the same voltage. In Figure 2 the LDO3V3 regulator is used as the pull-up voltage for the FAULT signal and the power supply for the low power microcontroller used to process pushbutton events and sequence the power supplies. The FAULT pin also acts as an input and hence, must be high before any outputs are enabled. DAC LTC2630 VDAC-OUT EN/SS3 8.06k RIFB3 MN1 Si1304BDL PWM 2.5V FREQ 0V Figure 4. Six white LED driver with PWM and analog dimming the current regulation loop increases voltage in an attempt to regulate the current. The integrated LED driver in the LT3587 is capable of accepting a direct PWM dimming signal into its enable input (EN/SS3) and/or accommodates analog dimming via an external DAC. See Figure 4 for a partial application circuit showing the LED driver with direct PWM and analog dimming. LEDs can change color when the current through them changes, but PWM dimming maintains color consistency over the dimming range, as the ON part of the PWM cycle is always the same current. In PWM dimming, the brightness of the LEDs is a function of average current, adjusted by changing the duty cycle of the PWM signal. In analog dimming, the constant current through the LEDs is adjusted, which causes variations in color. The LT3587 accepts PWM signals with frequencies over 60Hz to assure flicker-free operation. High PWM frequencies are achievable because of the internal disconnect FET between CAP3 and VOUT3. This FET ensures that CAP3 maintains its steady-state value while the PWM signal is low, resulting in minimal startup delays. For a 100Hz PWM dimming signal and allowing for 10% deviation from linearity at the lowest duty cycle, the LT3587 allows for a dimming ratio of 30:1. If the maximum amount of adjustment range is desired, an external DAC, such as the LTC2630, can be used to feed an adjustment voltage onto the IFB3 resistor, creating an LED current range of 20,000:1. LTC3851, continued from page 17 Conclusion protects against insufficient turn-on voltage for the top MOSFET. efficiency vs load for all three modes of operation with an input voltage of 12V. 3.3V/15A Regulator with DCR Sensing 1.5V/15A Regulator Synchronized at 350kHz Compact LED Driver The LT3587 LED driver is designed to drive up to six LEDs with average LED currents between 20mA and 1µA. When the LT3587’s VOUT3 is used as a current regulated LED driver, the VFB3 pin can be used as an overvoltage protection function. By connecting a resistor between VOUT and VFB3 the device limits the maximum allowable output voltage on VOUT3. This feature is extremely important in LED applications because without it the client device may be damaged if one of the LEDs were to open; in such a case, the output would continue to rise as Figure 2 shows a 400kHz, 3.3V output regulator using DCR current sensing. The DC resistance of the inductor is used as the current sense element, eliminating the need for a discrete sense resistor and thus maximizing efficiency. Figure 3 shows a plot of the 36 Figure 4 illustrates a 1.5V output regulator that is synchronized to an external clock. The loop filter components connected to the FREQ/ PLLFLTR pin are optimized to achieve a jitter free oscillator frequency and reduced lock time. Conclusion Two highly integrated devices, the LTC3586 and LT3587 can be combined to create a complete USB compatible power solution for portable cameras and other feature-rich portable devices. The solution is robust, high performance and compact, with efficient battery charging, instant-on capability and LED protection. L The LTC3851 combines high performance, ease of use and a comprehensive feature set in a 3mm × 3mm 16-pin package. DCR current sensing and Burst Mode® operation keep efficiency high. With a broad 4V to 38V input range, strong MOSFET drivers, low minimum on-time and tracking, the LTC3851 is ideal for automotive electronics, server farms, datacom and telecom power supply systems and industrial equipment. L Linear Technology Magazine • September 2008