MIC2873 1.2A High-Brightness Flash LED Driver with Single-Wire Serial Interface General Description Features The MIC2873 is a high-current, high-efficiency flash LED driver. The LED driver current is generated by an integrated inductive boost converter with a 2MHz switching frequency which allows for the use of very small inductor and output capacitor. These features make the MIC2873 an ideal solution for high-resolution camera phone LED flash light driver applications. • Up to 1.2A flash LED driving current • Highly-efficient synchronous boost driver • Control through single-wire serial interface or external control pin • Input voltage range: 2.7V to 5.5V • True load disconnect • Configurable safety time-out protection • Output overvoltage protection (OVP) • LED short-circuitdetection and protection • 1µA shutdown current • Available in 9-bump 1.30mm × 1.30mm WLCSP package MIC2873 operates in either flash or torch modes that can be controlled through the single-wire serial interface and/or external control pin. A robust single-wire serial interface allows the host processor to control the LED current and brightness. The MIC2873 is available in a 9-bump 1.30mm × 1.30mm WLCSP package. Datasheets and support documentation are available on Micrel’s web site at: www.micrel.com. Applications • • • • • • Camera phones/mobile handsets Cell phones/smartphones LED light for image capture/auto focus/white balance Handset video light (torch light) Digital cameras Portable applications Typical Application Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com July 17 2014 Revision 1.0 Micrel, Inc. MIC2873 Ordering Information Part Number Marking Code Operating Ambient Temperature Range MIC2873YCS 73A –40°C to +85°C Package (1) 9-Bump 1.30mm × 1.30mm WLCSP Note: 1. WLCSP bump A1 identifier = “•”. Pin Configuration 9-Bump 1.30mm x 1.30mm WLCSP (CS) (Top View) Pin Description Pin Number Pin Name A1 LED LED Current Sink Pin. Connect the LED anode to OUT and cathode to this pin. A2 DC Single-wire interface serial control input. A3 OUT B1 LGND B2 FEN Flash Mode Enable Pin. Asserting this pin high enables MIC2873 into the flash mode. If this pin is left floating, it is pulled-down internally by a built-in 1µA current source when the device is enabled. B3 SW Inductor Connection Pin. It is connected to the internal power MOSFETs. C1 AGND C2 VIN C3 PGND July 17, 2014 Pin Function Boost converter output pin to be connected to the anode of the LED. Connect a low-ESR ceramic capacitor of at least 4.7µF to PGND. Linear Ground. LED current return path. Analog Ground. Supply Input Pin. Connect a low-ESR ceramic capacitor of at least 4.7µF to AGND. Power Ground. Inductor current return path. 2 Revision 1.0 Micrel, Inc. MIC2873 Absolute Maximum Ratings(2) Operating Ratings(3) Input Voltage (VIN) ........................................ −0.3V to +6.0V General I/O Voltage (VFEN) ................................ −0.3V to VIN VOUT and VLED Voltage .................................. −0.3V to +6.0V Single-Wire I/O Voltage (VDC) ........................... −0.3V to VIN VSW Voltage .................................................. −0.3V to +6.0V Lead Temperature (soldering, 10s) .......................... +260°C Junction Temperature (TJ) ........................ −40°C to +150°C Storage Temperature (Ts) ......................... −40°C to +150°C (5) ESD Rating HBM ......................................................................... 2kV MM ......................................................................... 200V Input Voltage (VIN) .......................................... 2.7V to +5.5V Enable Input Voltage (VFEN) ................................... 0V to VIN Single-Wire I/O Voltage (VDC) ................................ 0V to VIN Junction Temperature (TJ) ........................ −40°C to +125°C Operating Ambient Temperature (TA) ......... −40°C to +85°C Package Thermal Resistance (4) 1.30mm x 1.30mm WLCSP (θJA) .................... 84°C/W (4) Power Dissipation (PD) ........................... Internally Limited Electrical Characteristics(6) VIN = 3.6V; L = 1µH; COUT =4.7µF, IOUT = 100mA, TA = 25°C, bold values indicate –40°C≤ TJ ≤ +125°C, unless otherwise noted. Symbol Parameter Condition Min. Typ. Max. Units 5.5 V 2.68 V Power Supply VIN Supply Voltage Range 2.7 VUVLO UVLO Threshold (rising) 2.41 UVLO Hysteresis 180 ISTB Standby Current VDC = 3.6V, VFEN = 0V, boost regulator and LED current driver both OFF. ISD Shutdown Current VDC = 0V DMAX Maximum Duty Cycle DMIN Minimum Duty Cycle ISW Switch Current Limit PMOS Switch On-Resistance NMOS 2.53 140 82 VIN = VOUT = 2.7V ISW = 100mA mV 170 205 µA 1 2 µA 86 90 % 6.4 % 4.1 A 125 mΩ ISW = 100mA ISW_LK Switch Leakage Current FSW Oscillator Frequency VDC = 0V, VSW = 5.5V 1.8 0.01 1 µA 2 2.2 MHz Notes: 2. Exceeding the absolute maximum rating may damage the device. 3. The device is not guaranteed to function outside its operating rating. 4. The maximum allowable power dissipation of any TA (ambient temperature) is PD(max) = (TJ(max) – TA) / θJA. Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. 5. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF. 6. Specification for packaged product only. July 17, 2014 3 Revision 1.0 Micrel, Inc. MIC2873 Electrical Characteristics(6) (Continued) VIN = 3.6V; L = 1µH; COUT =4.7µF, IOUT = 100mA, TA = 25°C, bold values indicate –40°C≤ TJ ≤ +125°C, unless otherwise noted. Symbol Parameter Condition Min. Typ. Max. Units TSD Overtemperature Shutdown Threshold 155 °C Overtemperature Shutdown Hysteresis 15 °C TTO Safety Timeout Shutdown Default timer setting 1.25 s ITO Safety Timer Current Threshold Default current threshold setting 250 mA VLBVD Low-Battery Voltage Detection Threshold Default LBVD threshold setting 3.0 V Low-Battery Voltage Detection Threshold Accuracy All low-battery voltage detection threshold settings 50 mV VSHORT LED Short-Circuit Detection Voltage Threshold VOUT - VLED ITEST LED Short-Circuit Detection Test Current 1.55 1.7 1.85 V 1.6 2 2.7 mA Current Sink Channel Channel Current Accuracy VLED Current Sink Voltage Dropout VOUT = 4.2V, ILED = 0.20A −6 6 VOUT = 4.2V, ILED = 1.0A -8 8 Boost mode % 250 mV FEN Control Pin VFEN FEN Threshold Voltage FEN Pull-down Current 1.3 FLASH ON V 0.6 FLASH OFF VFEN = 5.5V 1.3 5 µA Electrical Characteristics ̶ Single-Wire Interface (Guaranteed by design) VIN = 3.6V; L = 1µH; COUT =4.7µF, IOUT = 100mA, TA = 25°C, bold values indicate –40°C≤ TJ ≤ +125°C, unless otherwise noted. Symbol VDC Parameter Condition Min. Typ. LOW-Level Input Voltage Units 0.4 V 1.3 HIGH-Level Input Voltage DC Pull-Down Current Max. VDC = 5.5V V 2.8 5 µA TON ON Time 0.1 72 µs TOFF OFF Time 0.1 72 µs TLAT Latch Time 97 324 µs TEND END Time 405 July 17, 2014 4 µs Revision 1.0 Micrel, Inc. MIC2873 Typical Characteristics Shutdown Current vs. Temperature 1.4 1.2 1.0 0.8 0.6 0.4 185 2.8 180 2.7 UVLO THRESHOLD (V) STANDBY CURRENT (µA) 175 170 165 160 FSW = 2MHz VDC = 0V FSW = 2MHz VDC = 3.6V -20 0 20 40 60 100 80 -40 120 -20 MAXIMUM DUTY CYCLE (%) SWITCHING FREQUENCY (MHZ) 20 40 60 80 100 2.15 2.10 2.05 2.00 1.95 1.90 1.85 90 80 70 60 FSW = 2MHz VIN = 3.6V 50 1.80 20 40 60 -40 80 100 -40 120 -20 0 20 40 60 LED Short Test Current vs. Temperature 2.0 1.9 FSW = 2MHz VIN = 3.6V SWITCHING FREQUENCY (MHz) 2.1 80 100 -40 -20 0 20 40 60 80 TEMPERATURE (°C) July 17, 2014 100 120 60 80 100 120 1.8 1.7 1.6 1.5 FSW = 2MHz VIN = 3.6V 1.4 120 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (°C) WLED Power Efficiency vs. Input Voltage (VF = 3.4V) 100 2.15 90 2.10 TA = -40°C 2.05 TA = 25°C 2.00 1.95 TA = 85°C L = 1 µH COUT = 4.7µF ILED = 1.2A 1.90 1.85 80 70 ILED =1.2A ILED =1.0A ILED =600mA ILED =400mA L = 1µH COUT = 4.7µF LED VF = 3.4V 60 1.80 1.7 40 1.9 2.20 2.2 20 2.0 Boost Switching Frequency vs. Input Voltage 2.3 0 LED Short Threshold Voltage vs. Temperature TEMPERATURE (°C) TEMPERATURE (°C) 1.8 -20 TEMPERATURE (°C) VIN = 3.6V 0 2.3 120 100 2.20 LED SHORT TEST CURRENT (mA) 0 Maximum Duty Cycle vs. Temperature Switching Frequency vs. Temperature -20 FALLING TEMPERATURE (°C) TEMPERATURE (°C) -40 2.4 2.2 LED SHORT THRESHOLD VOLTAGE (V) -40 RISING 2.5 FSW = 2MHz 155 0.0 2.6 EFFICIENCY (%) SHUTDOWN CURRENT (µA) 1.6 0.2 UVLO Thresholds vs. Temperature Standby Current vs. Temperature ILED =250mA ILED =100mA 50 2.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 5 4.5 2.6 3.0 3.4 3.8 4.2 4.6 5.0 INPUT VOLTAGE (V) Revision 1.0 Micrel, Inc. MIC2873 Typical Characteristics (Continued) Full Torch ILED Accuracy vs. Input Voltage EFFICIENCY (%) 90 80 ILED =1.2A ILED =1.0A ILED =600mA 70 ILED =400mA 60 L = 1µH COUT = 4.7µF LED VF = 3.8V ILED =250mA ILED =100mA 50 Full Flash ILED Accuracy vs. Input Voltage 10 FULL FLASH ILED ACCURACY (%) 100 FULL TORCH ILED ACCURACY (%) WLED Power Efficiency vs. Input Voltage (VF = 3.8V) 8 6 4 TA = 115°C TA = 25°C 2 0 -2 TA = -40°C -4 -6 FSW = 2MHz ILED = 300mA -8 -10 2.6 3.0 3.4 3.8 4.2 INPUT VOLTAGE (V) July 17, 2014 4.6 5.0 2.5 3 3.5 4 4.5 INPUT VOLTAGE (V) 6 5 5.5 10 8 6 4 TA = 25°C TA = 115°C 2 0 -2 TA = -40°C -4 -6 FSW = 2MHz ILED = 1.2A -8 -10 2.5 3 3.5 4 4.5 5 5.5 INPUT VOLTAGE (V) Revision 1.0 Micrel, Inc. MIC2873 Functional Characteristics July 17, 2014 7 Revision 1.0 Micrel, Inc. MIC2873 Functional Characteristics (Continued) July 17, 2014 8 Revision 1.0 Micrel, Inc. MIC2873 Functional Characteristics (Continued) July 17, 2014 9 Revision 1.0 Micrel, Inc. MIC2873 Functional Diagram Figure 1. Simplified MIC2873 Functional Block Diagram July 17, 2014 10 Revision 1.0 Micrel, Inc. MIC2873 Functional Description DC The DC is a single multiplexed device enable and serial data control pin used for functional control and communication in GPIO limited applications. When the DC pin is used as a hardware device enable pin, a logic high signal on the DC pin enables the device, and a logic low signal on the DC pin disables the device. When the DC pin is used as the single-wire serial interface digital control pin, a combination of bit edges and the period between edges is used to communicate a variable length data word across the single wire. Each word is transmitted as a series of pulses, with each pulse incrementing an internal data counter. A stop sequence consisting of an inactive period is used to latch the data word internally. Two data words in series received are then used to set a specific register with specific value for controlling specific function. The MIC2873 supports five writeable registers for controlling flash mode, torch mode, safety timer duration, safety timer threshold current, and low-battery threshold. VIN The input supply provides power to the internal MOSFETs gate drive and controls circuitry for the switch-mode regulator. The operating input voltage range is from 2.7V to 5.5V. A 4.7µF low-ESR ceramic input capacitor should be connected from VIN to AGND as close to MIC2873 as possible to ensure a clean supply voltage for the device. The minimum voltage rating of 10V is recommended for the input capacitor. SW The MIC2873 has internal low-side and synchronous MOSFET switches. The switch node (SW) between the internal MOSFET switches connects directly to one end of the inductor and provides the current paths during switching cycles. The other end of the inductor is connected to the input supply voltage. Due to the high-speed switching on this pin, the switch node should be routed away from sensitive nodes wherever possible. An address/data frame is used to improve protection against erroneous writes where communications are in error. When DC is in a low state and no data is detected for longer than 405µs, the MIC2873 will automatically go into a low-power SHUTDOWN state, simultaneously resetting all internal registers to their default states. LGND This is the ground path of the LED current sink. It should be connected to the AGND on the PCB. The current loop of the analog ground should be separated from that of the power ground (PGND). LGND and AGND should be connected to PGND at a single point. FEN FEN is the hardware enable pin for flash mode. A logic low-to-high transition on FEN pin can initiate the MIC2873 in flash mode. If FEN is left floating, it is pulled down internally by a built-in 1µA current source when the device is enabled. Flash mode is terminated when FEN is pulled low or left floating, and the flash register is cleared. AGND This is the ground path for the internal biasing and control circuitry. AGND should be connected to the LGND directly. The current loop of the analog ground should be separated from that of the power ground (PGND). The AGND and LGND should be connected to PGND at a single point. PGND The power ground pin is the ground path for the high current in the boost switch. The current loop for the power ground should be as small as possible and separate from the analog ground (AGND) loop as applicable. OUT Boost converter output pin which is connected to the anode of the LED. A low-ESR ceramic capacitor of 4.7µF or larger should be connected from OUT to PGND as close as possible to the MIC2873. The minimum voltage rating of 10V is recommended for the output capacitor. LED The current sink pin for the LED. The LED anode is connected to the OUT pin and the LED cathode is connected to this pin. July 17, 2014 11 Revision 1.0 Micrel, Inc. MIC2873 Application Information Overvoltage Protection When the output voltage rises above an internal OVP threshold, MIC2873 is latched off automatically to avoid permanent damage to the IC. To clear the latched off condition, either power cycle the MIC2873 or assert the DC pin low. MIC2873 can drive a high-current flash WLED in either flash mode or torch mode. Boost Converter The internal boost converter is turned on/off automatically when the LED driver is activated/de-activated without any exception. Short-Circuit Detection Each time before enabling the LED driver, the MIC2873 performs the short circuit test by driving the flash LED with a small (2mA typical) current for 200µs. If (VOUT – VLED) < 1.7V at the end of the short-circuit test, the LED is considered to be shorted and MIC2873 will ignore the flash and/or torch mode command. Note that the short-circuit test is carried out every time prior to flash and torch mode but the result is not latched. The boost converter is an internally-compensated currentmode PWM boost converter running at 2MHz. It is for stepping up the supply voltage to a high enough value at the OUT pin to drive the LED current. If the supply voltage is high enough, the synchronous switch of the converter is then fully turned on. In this case, all the excessive voltage is dropped over the linear LED driver. Flash Mode The maximum current level in the flash mode is 1.2A. The flash mode current can be initiated by asserting FEN pin high, or by setting the flash control register (Address 1), for the desired flash duration, subjected to the safety timeout setting. The flash mode current is terminated when the FEN pin is brought low and the flash register is cleared, or when the configurable safety timer expires. Thermal Shutdown When the internal die temperature of MIC2873 reaches 155°C, the LED driver is disabled until the die temperature falls below 140°C and either FEN pin, FEN register, TEN register, or VIN is toggled. Single-Wire Interface The single-wire interface allows the use of a single multiplexed enable and data pin (DC) for control and communication in GPIO-limited applications. The interface is implemented using a simple mechanism allowing any open drain or directly driven GPIO to control the MIC2873. Flash mode current can be adjusted to a fraction of the maximum flash mode current level by selecting the desired value in the flash control register through the single-wire serial interface. Torch Mode By default, the maximum torch mode level is 300mA. The torch mode operation is activated by setting the torch control register (Address 2) for the desired duration. The torch mode current is terminated when the torch register is cleared or when the configurable safety timer expires. The MIC2873 uses the single-wire interface for simple command and control functions. The interface provides fast access to write only registers with protection features to avoid potentially erroneous data writes and improve robustness. When DC is in a low state and no data is detected for longer than 405µs, the MIC2873 will automatically go into a low-power SHUTDOWN state, simultaneously resetting internal registers to default states. Like the flash mode current, the torch mode current can be set to a fraction of the maximum torch mode current level by selecting the desired torch current in the torch control register (Address 2) via the single-wire serial interface. Overview The single-wire interface relies on a combination of bit edges and the period between edges in order to communicate across a single wire. Each word is transmitted as a series of pulses, with each pulse incrementing an internal data counter. A stop sequence consisting of an inactive period of DC pin remaining high is used to latch the data word internally. An address and data framing format is used to improve protection against erroneous writes by enforcing address and data field lengths as well as the timing duration between them. Configurable Safety Timer The flash safety timeout feature automatically shuts down the LED current after the safety timer duration is expired if the programmed LED current exceeds a certain current threshold. Both the current threshold and the timer duration are programmable via the safety timer registers (Addresses 3 and 5). Low-Battery Voltage Detection (LBVD) When the VIN voltage drops below the LBVD threshold (default = 3.0V) in flash or torch mode, the LED current driver is disabled. The LED driver can be resumed by raising the VIN above the LBVD threshold and toggling the corresponding flash or torch command. The LBVD threshold is adjustable thru the LBVD control register (Address 4). July 17, 2014 12 Revision 1.0 Micrel, Inc. MIC2873 Timing is designed such that when communicating with a device using a low cost on chip oscillator, the worst case minimum and maximum conditions can be easily met within the wide operating range of the oscillator. Using this method guarantees that the device can always detect the delay introduced by the communication master. IDLE < TEND - TLAT VH VL TLAT TEND Idle States and Error Conditions In shutdown mode, the MIC2873 is in a reset condition with all functions off while consuming minimal power. Register settings are reset to default state when coming out of shutdown state. In idle mode, all register settings persist and all MIC2873 functions continue in their current state. Table 1 summarises the difference between the two idle modes: VH VL TLAT TEND SHUTDOWN IDLE VH VL Table 1. Differences between Idle Modes TLAT Mode Shutdown Idle VDC Low High ISUPPLY (all functions off) 1μA 230μA Register State Default Persist Start-Up Time 1μs 100ns TEND IDLE Figure 2. Abort, Shutdown, and Idle Timing Waveforms Communication Details The serial interface requires delimiters to indicate the start of frame, data as a series of pulses, and end of frame indicated by a lack of activity for longer than TLAT. The start of frame is the first high to low transition of DC when in idle mode. The first rising edge resets the internal data counter to 0. Idle mode is entered automatically at the end of a communication frame by holding DC high for ≥TEND, by enabling the device by bringing DC high when in shutdown mode, or when an error is detected by the single-wire interface logic. END OF FRAME 1 COUNT Shutdown mode can be entered at any time by pulling down DC for ≥TEND, discarding any current communication and resetting the internal registers. If a communication is received before the shutdown period but after the TLAT period, the communication is discarded. This state is also used to create an internal error state to avoid erroneously latching data where the communication process cannot be serviced in time. Additionally, each register has a maximum value associated with it. If the number of bits clocked in exceeds the maximum value for the register, the data is assumed to be in error and the data is discarded. VH VL TOFF TON TON+TOFF<TLAT START TLAT AUTOMATIC LATCH AFTER TLAT EXPIRES Figure 3. Data Word Pulse Timing A pulse is delimited by the signal first going below VL and then above VH within the latch timeout TLAT. During this transition, minimum on (TON) and off (TOFF) periods are observed to improve tolerance to glitches. Each rising edge increments the internal data register. Data is automatically latched into internal shadow address or data registers after an inactivity period of DC remaining high for longer than TLAT. To send register write commands, the address and data are entered in series as two data words using the above pattern, with the second word starting after the first latch period has expired. After the second word is entered, the IDLE command should be issued by leaving the DC pin high for ≥TEND to indicate the stop sequence of the address/data words frame. July 17, 2014 13 Revision 1.0 Micrel, Inc. MIC2873 Table 3. Flash Current Register Mapping into Internal FEN plus FCUR Registers and Relationship between Flash Current and the FCUR Register Setting After receiving the stop sequence, the internal registers decode and update cycle is started, with the shadow register values being transferred to the decoder. Figure 4 shows an example of entering a write of data 5 to Address 3. FEN/FCUR[4:0] Value IFLASH (A) Dec. Binary FEN[4] FCUR[3:0] 0 00000 0 0000 1.200 1 00001 0 0001 1.150 2 00010 0 0010 1.100 3 00011 0 0011 1.050 4 00100 0 0100 1.000 5 00101 0 0101 0.950 6 00110 0 0110 0.900 7 00111 0 0111 0.850 Figure 4. Communication Timing Example of Entering Write for Data 5 to Address 3 8 01000 0 1000 0.800 9 01001 0 1001 0.750 Only correctly formatted address/data combination will be treated as a valid frame and processed by the MIC2873. Any other input, such as a single data word followed by TEND, or three successive data words will be discarded by the target hardware as an erroneous entry. Additionally, any register write to either an invalid register or with invalid register data will also be discarded. 10 01010 0 1010 0.700 11 01011 0 1011 0.650 12 01100 0 1100 0.600 13 01101 0 1101 0.550 14 01110 0 1110 0.400 15 01111 0 1111 0.250 16 10000 1 0000 1.200 17 10001 1 0001 1.150 18 10010 1 0010 1.100 19 10011 1 0011 1.050 20 10100 1 0100 1.000 21 10101 1 0101 0.950 22 10110 1 0110 0.900 23 10111 1 0111 0.850 11000 1 1000 0.800 ADDRESS/DATA FRAME LATCH START LATCH START TLAT TLAT 0 1 2 3 END REGISTER WRITE < TEND > TEND 0 1 2 3 4 5 MIC2873 Registers The MIC2873 supports five writeable registers for controlling the torch and the flash modes of operation as shown in Table 2. Note that register addressing starts at 1. Writing any value above the maximum value shown for each registers will cause an invalid data error and the frame will be discarded. Table 2. Five Writable Registers of MIC2873 Address Name Max. Value Description 1 FEN/FCUR 31 Flash Enable/Current 24 2 TEN/TCUR 31 Torch Enable/Current 25 11001 1 1001 0.750 3 STDUR 7 Safety Timer Duration 26 11010 1 1010 0.700 4 LB_TH 9 Low-Battery Voltage Detection Threshold 27 11011 1 1011 0.650 28 11100 1 1100 0.600 5 ST_TH 5 Safety Timer Threshold 29 11101 1 1101 0.550 30 11110 1 1110 0.400 31 11111 1 1111 0.250 Flash Current Register (FEN/FCUR: default 0) The flash current register enables and sets the flash mode current level. Valid values are 0 to 31; values 0-15 will set the flash current without enabling the flash (such that it can be triggered externally), values 16-31 will set the flash current and enable the flash. The flash current register maps into the internal FEN and FCUR registers as shown in the table below. Table 3 describes the relationship between the flash current, and the FCUR register setting. July 17, 2014 14 Revision 1.0 Micrel, Inc. MIC2873 Table 4. Torch Current Register Mapping into Internal TEN and TCUR Registers, and Relationship between Torch Current and the TCUR Register Setting Torch Current Register (TEN/TCUR: default 0) The torch current register enables and sets the torch mode current level. Valid values are 0 to 31; values 0 − 15 will set the torch current without enabling the torch (such that it can be triggered by setting the internal TEN register value to 1), values 16 − 31 will set the torch current and enable the torch. A value of 0 at the internal TEN register will disable the torch. The torch current register maps into the internal TEN and TCUR registers as shown in Table 4. The table also describes the relationship between the torch current, and the TCUR register setting. July 17, 2014 TEN/TCUR[4:0] Value 15 ITORCH (mA) Dec. Binary TEN[4] TCUR[3:0] 0 00000 0 0000 300.0 1 00001 0 0001 287.5 2 00010 0 0010 275.0 3 00011 0 0011 262.5 4 00100 0 0100 250.0 5 00101 0 0101 237.5 6 00110 0 0110 225.0 7 00111 0 0111 212.5 8 01000 0 1000 200.0 9 01001 0 1001 187.5 10 01010 0 1010 175.0 11 01011 0 1011 162.5 12 01100 0 1100 150.0 13 01101 0 1101 137.5 14 01110 0 1110 100.0 15 01111 0 1111 62.5 16 10000 1 0000 300.0 17 10001 1 0001 287.5 18 10010 1 0010 275.0 19 10011 1 0011 262.5 20 10100 1 0100 250.0 21 10101 1 0101 237.5 22 10110 1 0110 225.0 23 10111 1 0111 212.5 24 11000 1 1000 200.0 25 11001 1 1001 187.5 26 11010 1 1010 175.0 27 11011 1 1011 162.5 28 11100 1 1100 150.0 29 11101 1 1101 137.5 30 11110 1 1110 100.0 31 11111 1 1111 62.5 Revision 1.0 Micrel, Inc. MIC2873 Safety Timer Threshold Current Register (ST_TH: default 4) Safety timer threshold current determines the amount of LED current flowing through the external LED before the internal LED safety timer is activated. Setting ST_TH to 0 disables the safety timer function, and setting the register to values 1 to 5 set the safety time threshold current 100mA to 300mA in 50mA steps. Safety Timer Duration Register (STDUR: default 7) The safety timer duration register sets the duration of the flash and torch mode when the LED current exceeds the programmed threshold current. Valid values are 0 for the minimum timer duration to 7 for the maximum duration. Table 5. Safety Timer Duration Register Setting and Safety Timer Duration Value Dec. Binary FDUR[2:0] (binary) Timeout (ms) 0 000 000 156.25 1 001 001 312.5 Dec. Binary 2 010 010 468.75 0 3 011 011 625 4 100 100 781.25 5 101 101 937.5 6 110 110 1093.75 7 111 111 1250 Table 7. Safety Timer Threshold Current Register Setting and Safety Timer Threshold Current Value ST_TH[2:0] Safety Timer Threshold Current (mA) 000 000 Disabled 1 001 001 100 2 010 010 150 3 011 011 200 4 100 100 250 5 101 101 300 Low-Battery Threshold Register (LB_TH: default 1) The LB_TH register sets the supply threshold voltage below which the internal low battery flag is asserted and flash functions are inhibited. Table 6 shows the threshold values that correspond to the register settings. Setting 0 is reserved for disabling the function, and settings between 1 and 9 inclusively enable and set the LB_TH value between 3.0V and 3.8V with 100mV resolution. Table 6. Low-Battery Threshold Register Setting and Supply Threshold Voltage Value LB_TH[3:0] VBAT Threshold (V) 0000 0000 Disabled 1 0001 0001 3.0 2 0010 0010 3.1 3 0011 0011 3.2 4 0100 0100 3.3 5 0101 0101 3.4 6 0110 0110 3.5 7 0111 0111 3.6 8 1000 1000 3.7 9 1001 1001 3.8 Dec. Binary 0 July 17, 2014 16 Revision 1.0 Micrel, Inc. MIC2873 Component Selection Output Capacitor Output capacitor selection is also a trade-off between performance, size, and cost. Increasing output capacitor will lead to an improved transient response, however, the size and cost also increase. The output capacitor is preferred in the range of 2.2µF to 10µF with ESR from 10mΩ to 50mΩ, and a 4.7µF ceramic capacitor is typically recommended. X5R or X7R type ceramic capacitors are recommended for better tolerance over temperature. The Y5V and Z5U type ceramic capacitors are not recommended due to their wide variation in capacitance over temperature and increased resistance at high frequencies. The rated voltage of the output capacitor should be at least 20% higher than the maximum operating output voltage over the operating temperature range. Inductor Inductor selection is a balance between efficiency, stability, cost, size, and rated current. Since the boost converter is compensated internally, the recommended inductance of L is limited from 1µH to 2.2µH to ensure system stability. It is usually a good balance between these considerations. A large inductance value reduces the peak-to-peak inductor ripple current hence the output ripple voltage and the LED ripple current. This also reduces both the DC loss and the transition loss at the same inductor’s DC resistance (DCR). However, the DCR of an inductor usually increases with the inductance in the same package size. This is due to the longer windings required for an increase in inductance. Since the majority of the input current passes through the inductor, the higher the DCR the lower the efficiency is, and more significantly at higher load currents. On the other hand, inductor with smaller DCR but the same inductance usually has a larger size. The saturation current rating of the selected inductor must be higher than the maximum peak inductor current to be encountered and should be at least 20% to 30% higher than the average inductor current at maximum output current. Input Capacitor A ceramic capacitor of 4.7µF or larger with low ESR is recommended to reduce the input voltage ripple to ensure a clean supply voltage for the device. The input capacitor should be placed as close as possible to the MIC2873 VIN pin with short trace for good noise performance. X5R or X7R type ceramic capacitors are recommended for better tolerance over temperature. The Y5V and Z5U type temperature rating ceramic capacitors are not recommended due to their large reduction in capacitance over temperature and increased resistance at high frequencies. These reduce their ability to filter out highfrequency noise. The rated voltage of the input capacitor should be at least 20% higher than the maximum operating input voltage over the operating temperature range. July 17, 2014 17 Revision 1.0 Micrel, Inc. MIC2873 Power Dissipation Consideration As with all power devices, the ultimate current rating of the output is limited by the thermal properties of the device package and the PCB on which the device is mounted. There is a simple, Ohm’s law type relationship between thermal resistance, power dissipation and temperature which are analogous to an electrical circuit: Now replacing the variables in the equation for VX, we can find the junction temperature (TJ) from the power dissipation, ambient temperature and the known thermal resistance of the PCB (θCA) and the package (θJC). TJ = PDISS × (θ JC + θ CA ) + TA Eq. 2 As can be seen in the diagram, total thermal resistance θJA = θJC + θCA. Hence this can also be written as in Equation 3: TJ = PDISS × (θ JA ) + TA Since effectively all of the power losses (minus the inductor losses) in the converter are dissipated within the MIC2873 package, PDISS can be calculated thus: Figure 5. Series Electrical Resistance Circuit From this simple circuit we can calculate VX if we know ISOURCE, VZ and the resistor values, RXY and RYZ using Equation 1: V X = ISOURCE × (R XY + R YZ ) + VZ Eq. 3 Linear Mode: 1 PDISS = [POUT × Eq. 1 η Thermal circuits can be considered using this same rule and can be drawn similarly by replacing current sources with power dissipation (in watts), resistance with thermal resistance (in °C/W) and voltage sources with temperature (in °C). 2 − 1] − IOUT × DCR Eq. 4 Boost Mode: 2 PDISS 1 I = [POUT × − 1 ] − OUT × DCR η 1− D Eq. 5 Duty Cycle in Boost Mode: D= VOUT − VIN VOUT Eq. 6 where: Figure 6. Series Thermal Resistance Circuit η = Efficiency taken from efficiency curves and DCR = inductor DCR. θJC and θJA are found in the operating ratings section of the data sheet. July 17, 2014 18 Revision 1.0 Micrel, Inc. MIC2873 Where the real board area differs from 1 inch square, θCA (the PCB thermal resistance) values for various PCB copper areas can be taken from Figure 7. Figure 7 is taken from Designing with Low Dropout Voltage Regulators available from the Micrel website (“LDO Application Hints”). Figure 7. Graph to Determine PC Board Area for a Given PCB Thermal Resistance Figure 7 shows the total area of a round or square pad, centered on the device. The solid trace represents the area of a square, single sided, in horizontal orientation, solder masked, copper PC board trace heat sink, measured in square millimeters. No airflow is assumed. The dashed line shows PC boards trace heat sink covered in black oil-based paint and with 1.3m/s (250 feet per minute) airflow. This approaches a “best case” pad heat sink. Conservative design dictates using the solid trace data, which indicates that a maximum pad size of 5000 2 mm is needed. This is a pad 71mm × 71mm (2.8 inches per side). July 17, 2014 19 Revision 1.0 Micrel, Inc. MIC2873 PCB Layout Guidelines Output Capacitor PCB layout is critical to achieve reliable, stable and efficient performance. A ground plane is required to control EMI and minimize the inductance in power, signal and return paths. The following guidelines should be followed to ensure proper operation of the device: • The output capacitor must be placed close to the OUT pin and PGND pin of the IC and preferably connected directly and closely to the OUT pin and PGND pin without going through any via to minimize the switching current loop during the main switch off-cycle, and the switching noise. IC (Integrated Circuit) • Place the IC close to the point-of-load (in this case, the flash LED). • • Use fat traces to route the input and output power lines. Use wide and short traces to connect the output capacitor to the OUT and PGND pins. • • Analog grounds (LGND and AGND) and power ground (PGND) should be kept separate and connected at a single location. Place several vias to the ground plane close to the output capacitor ground terminal. • • 6 to 12 thermal vias must be placed on the PCB top layer PGND copper from the PGND pin and connected it to the ground plane to ensure a good PCB thermal resistance can be achieved. Use either X5R or X7R temperature rating ceramic capacitors. Do not use Y5V or Z5U type ceramic capacitors. • Flash LED Since all the top copper areas connected directly to the CSP package bumps are used as the immediate PCB heat sink, these top copper areas should be spread out from the bumps in funnel-shape to maximize the top copper PCB heat sink areas. • Use wide and short trace to connect the LED anode to the OUT pin. • Use wide and short trace to connect the LED cathode to the LED pin. • Make sure that the LED’s PCB land pattern can provide sufficient PCB pad heat sink to the flash LED, such as sufficient copper areas and thermal vias. VIN Decoupling Capacitor • The VIN decoupling capacitor must be placed close to the VIN pin of the IC and preferably connected directly to the pin and not through any via. The capacitor must be located right at the IC. • The VIN decoupling capacitor should be connected to analog ground (AGND). • The VIN terminal is noise sensitive and the placement of capacitor is very critical. Inductor • Keep both the inductor connections to the switch node (SW) and input power line short and wide enough to handle the switching current. Keep the areas of the switching current loops small to minimize the EMI problem. • Do not route any digital lines underneath or close to the inductor. • Keep the switch node (SW) away from the noise sensitive pins. • To minimize noise, place a ground plane underneath the inductor. July 17, 2014 20 Revision 1.0 Micrel, Inc. MIC2873 Typical Application Schematic L1 1µH VIN VBAT 2.7V to 5.5V C1 4.7µF 10V GND AGND LGND SINGLE-WIRE SERIAL I/F FLASH ENABLE SW OUT PGND DC FEN C2 4.7µF 10V D1 FLASH WHITE LED LED U1 MIC2873YCS Bill of Materials Item C1, C2 Part Number C1608X5R1A475K080AC SPFCW04301BL Manufacturer ( ) TDK 7 Capacitor 4.7 µF, 10V, 10%, X5R, 0603 High-Power Flash LED, 4.1mm × 3.9mm × 2.1mm, 220lux @ ILED = 1A ( ) Samsung 8 D1 LXCL-MN06-3002 L1 PIFE25201B-1R0MS-39 U1 MIC2873YCS Description LUXEON Flash 6 Module, 4mm × 4mm × 2.2mm, 180lux @ ILED = 1A ( ) Philips 9 ( ) Cyntec 10 ( ) Micrel, Inc. 11 Qty. 2 1 Inductor 1µH, 3.55A, SMD, 2.5mm × 2.0mm × 1.2mm 1 1.2A High-Brightness Flash LED Driver with Single-Wire Serial Interface 1 Notes: 7. TDK: www.tdk.com. 8. Samsung: www.samsung.com. 9. Philips: www.philipsluminleds.com. 10. Cyntec: www.cyntec.com. 11. Micrel, Inc.: www.micrel.com. July 17, 2014 21 Revision 1.0 Micrel, Inc. MIC2873 PCB Layout Recommendations Top Layer Bottom Layer July 17, 2014 22 Revision 1.0 Micrel, Inc. MIC2873 Package Information and Recommended Landing Pattern(12, 13) 9-Bump 1.3mm x 1.3mm WLCSP (CS) Notes: 12. Package information is correct as of the publication date. For updates and most current information, go to www.micrel.com. 13. Disclaimer: This is only a recommendation based on information available to Micrel from its suppliers. Actual land pattern may have to be significantly different due to various materials and processes used in PCB assembly. Micrel makes no representation or warranty of performance based on the recommended land pattern. July 17, 2014 23 Revision 1.0 Micrel, Inc. MIC2873 MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This information is not intended as a warranty and Micrel does not assume responsibility for its use. Micrel reserves the right to change circuitry, specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual property rights is granted by this document. Except as provided in Micrel’s terms and conditions of sale for such products, Micrel assumes no liability whatsoever, and Micrel disclaims any express or implied warranty relating to the sale and/or use of Micrel products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. © 2014 Micrel, Incorporated. July 17, 2014 24 Revision 1.0