MIC2870 1.5A Synchronous Boost Flash LED Driver with I2C Interface General Description Features The MIC2870 is a high-current, high-efficiency flash LED driver for one or two high-brightness camera flash LEDs. • Up to 1.5A flash LED driving current − 2.7V to 5.0V input voltage range • High-efficiency 2MHz VF adaptive boost driver • Configurable 1 or 2 channel(s) WLED driver • LED driving current soft-start 2 • Control through I C interface or external pins • Flash inhibit function for GSM pulse synchronization • True load disconnect • Flash time-out protection • 1µA shutdown current • Available in 16-pin 2mm × 2mm TQFN package The LED driver current is generated by an integrated inductive boost converter with 2MHz switching frequency which allows the use of a very-small inductor and output capacitor. These features make the MIC2870 an ideal solution for high-resolution camera phone LED flashlight driver applications. MIC2870 supports two 750mA white-LEDs (WLEDs) or a single 1.5A WLED configuration. When two WLEDs are connected, their currents are matched automatically. MIC2870 operates in either flash or torch modes that can 2 be controlled through either an I C interface or external pins. The brightness in the flash and torch mode can be adjusted via two external resistors individually. High-speed 2 mode I C interface provides a simple control at a clock speed up to 3.4MHz to support most camera functions such as auto-focus, white balance, and image capture (flash mode). The MIC2870 is available in 16-pin, 2mm × 2mm TQFN package with a junction temperature range of −40°C to +125°C. Applications • • • • • • Camera phones/mobile handsets Cellular phones/smart phones LED light for image capture/auto focus/white balance Handset video light (torch light) Digital cameras Portable applications Datasheets and support documentation are available on Micrel’s web site at: www.micrel.com. 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 September 5, 2013 Revision 1.0 Micrel, Inc. MIC2870 Ordering Information Part Number Marking Temperature Range MIC2870YFT 70H –40°C to +125°C Package (1) 16-Pin 2mm × 2mm TQFN Lead Finish NiPdAu Note: 1. Package is a GREEN, RoHS-compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free. 2. Thin QFN pin 1 identifier = “▲”. Pin Configuration 16-Pin 2mm × 2mm TQFN (Top View) Pin Description Pin Number Pin Name 1 SCL High-Speed Mode (3.4MHz) I²C Clock Input. 2 VIN Supply Input. Connect a low-ESR ceramic capacitor of at least 4.7µF to PGND. A small capacitor of 100nF between VIN and AGND is highly recommended. 3 FEN Flash-Mode Enable Pin. A LOW-to-HIGH transition initiates the flash mode and flash-mode timer. If FEN is left floating, it is pulled-down internally by a built-in 1µA current source when the device is enabled. FI Flash Inhibit. When FI is pulled HIGH, both LED currents are changed from the flash-mode current level to the torch-mode current level. If FI is left floating, it is pulled-down internally by a built-in 1µA current source when the device is enabled. This function is generally used to reduce instantaneous battery load current by synchronizing with the handset’s GSM pulse off time. 5 FRSET Flash-Mode Current Level Programming. Connect a resistor from FRSET to AGND to set the maximum current in the flash mode. For example, a 10kΩ resistor sets the LED sink current to its maximum value of 750mA per channel. FRSET can be grounded if the default maximum flashmode current (750mA) is desired. FRSET, however, cannot be left floating and the maximum resistance is limited to 80kΩ 6 AGND Analog Ground. Reference ground for FRSET and TRSET pins. 4 September 5, 2013 Pin Function 2 Revision 1.0 Micrel, Inc. MIC2870 Pin Description (Continued) Pin Number Pin Name Pin Function 7, 15 PGND Power Ground. PGND is used for the switching NMOS and PMOS of boost converter, and power ground for LED current sinks. 8 TRSET Torch-Mode Current Level Programming. Connect a resistor from TRSET to AGND to set the maximum current in the torch mode. For example, a 10kΩ resistor sets the LED sink current to its maximum value of 187.5mA per channel. TRSET can be grounded if the default maximum torchmode current (187.5mA) is desired. TRSET, however, cannot be left floating and the maximum resistance is limited to 80kΩ. 9 LED2 Channel 2 LED Current Sink. Connect the LED anode to OUT and cathode to LED2. 10 LED1 Channel 1 LED Current Sink. Connect the LED anode to OUT and cathode to LED1. 11 TEN Torch-Mode Enable. Initiates torch mode when TEN is HIGH. If TEN is left floating, it is pulleddown internally by a built-in 1µA current source when the device is enabled. 12 OUT Boost Converter Output. 13 EN Enable (IC). The MIC2870 is in standby mode when EN is asserted HIGH. If EN is driven low for 2 more than 1s, the IC is shut down. Alternatively, the I C interface can be used for enabling/disabling the IC through the master control/status register. EN is pulled down by an internal resistor. 14 SW Inductor Connection. It is connected to the internal power MOSFETs. 16 SDA High-Speed Mode (3.4MHz) I²C Data Input/Output. EP ePad Exposed Heat Sink Pad. Connect to PGND ground plane for best thermal performance. This pin is internally connected to PGND. September 5, 2013 3 Revision 1.0 Micrel, Inc. MIC2870 Absolute Maximum Ratings(3) Operating Ratings(4) Supply Voltage (VIN) ..................................... −0.3V to +6.0V Enable Input Voltage (VEN, VFEN, VFI, VTEN) ....................... −0.3V to VIN + 0.3V VOUT, VLED1, and VLED2 .................................... −0.3V to 6.0V 2 I C I/O (VSCL, VSDA) ................................ −0.3V to VIN + 0.3V VFRSET and VTRSET .................................. −0.3V to VIN + 0.3V VSW ................................................................. −0.3V to 6.0V (5) Power Dissipation (PDISS) ....................... Internally Limited Lead Temperature (soldering, 10s) .......................... +260°C Junction Temperature (TJ) ........................ −40°C to +150°C Storage Temperature (TS) ......................... −40°C to +150°C (6) ESD Rating ................................. 2kV HBM and 150V MM Supply Voltage (VIN) ..................................... +2.7V to +5.0V Enable Input Voltage (VEN, VFEN, VFI, VTEN) ............ 0V to VIN 2 I C I/O (VSCL, VSDA) ................................................. 0V to VIN Junction Temperature (TJ) .......................... −40°C to 125°C (5) Package Thermal Resistance 2mm × 2mm TQFN (θJA) .................................... 80°C/W Electrical Characteristics(7) VIN = 3.6V; L = 1μH, COUT = 2.2μF, RFRSET = 10kΩ, RTRSET = 10kΩ, ILED = 100mA; TA = 25°C, bold values indicate −40°C ≤ TJ ≤ 125°C, unless otherwise noted. Symbol Parameter Condition Min. Typ. Max. Units 5.0 V Power Supply VIN Input Voltage IVIN Quiescent Current IVIN(SD) Shutdown Current ISW(SD) SW Pin Shutdown Current UVLO_Rise UVLO Threshold (Rising) UVLO_Hyst UVLO Hysteresis VOUT Output Voltage Overvoltage Protection Threshold VOVP 2.7 VLED1 = VLED2 > 200mV, not switching 0.9 VLED1 = VLED2 = 70mV, boost keeps switching 4.2 VEN = 0 V 0.6 VEN = 0 V 2.35 mA µA 1 5 µA 2.5 2.65 V 300 mV VIN ≥ VOUT VIN VOUT > VIN 5.2 5.26 VOUT > VIN 5.38 5.6 V V Overvoltage Protection Hysteresis 60 mV OVP Blanking Time 24 µs DMAX Maximum Duty Cycle DMIN Minimum Duty Cycle ISW_OC Switch Current Limit 80 85 90 5.5 3.35 VIN = VOUT = 2.7V 4.5 % % 5.65 A Notes: 3. Exceeding the absolute maximum ratings may damage the device. 4. The device is not guaranteed to function outside its operating ratings. 5. The maximum allowable power dissipation of any TA (ambient temperature) is PDISS(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. 6. Devices are ESD sensitive. Handling precautions are recommended. Human body model, 1.5kΩ in series with 100pF. 7. Specification for packaged product only September 5, 2013 4 Revision 1.0 Micrel, Inc. MIC2870 Electrical Characteristics(7) (Continued) VIN = 3.6V; L = 1μH, COUT = 2.2μF, RFRSET = 10kΩ, RTRSET = 10kΩ, ILED = 100mA; TA = 25°C, bold values indicate −40°C ≤ TJ ≤ 125°C, unless otherwise noted. Symbol Parameter FSW Oscillator Frequency RON(N) RON(P) Switch On-Resistance Condition Min. Typ. Max. Units 1.8 2.0 2.2 MHz VVIN = 2.7 V, ISW = 750 mA 80 VSW = 2.7 V, IOUT = 750 mA 80 ILK(N) NMOS Switch Leakage Current VEN = 0 V, VIN = Vsw = VOUT = 5 V ILK(P) PMOS Switch Leakage Current VEN = 0 V, VIN = VOUT = 5 V, Vsw = 0 V RDCHG Auto-Discharge NMOS Resistance VEN = 0 V, IOUT = −1 mA TSD mΩ 1 5 µA 1 5 µA 160 Ω Overtemperature Shutdown Threshold 160 °C TSD_HYST Overtemperature Shutdown Hysteresis 25 °C TFLASH_TIMEOUT Flash Safety Timeout Shutdown 1.25 s Maximum timeout setting Current Sink Channels AccuLED_Ch Channel Current Accuracy VLED1 = VLED2 = 890mV, ILED1 = ILED2 = 750mA −10 10 % MatchLED_Ch Channel Current Matching VLED1 = VLED2 = 890mV, ILED1 = ILED2 = 750mA −5 5 % VDROPOUT Current Sink Dropout Boost is in regulation 100 mV ILK_LED1 LED1 Leakage Current VIN = 3.6 V, VEN = 0 V, VLED1 = 3.6 V 0.05 µA ILK_LED2 LED2 Leakage Current VIN = 3.6 V, VEN = 0 V, VLED2 = 3.6 V 0.05 µA VFRSET FRSET Pin Voltage RFRSET = 10kΩ, flash mode IFRSET FRSET Current Sourcing FRSET pin is shorted to ground, flash mode VTRSET TRSET Pin Voltage RTRSET = 10kΩ, torch mode ITRSET TRSET Current Sourcing TRSET pin is shorted to ground, torch mode September 5, 2013 5 0.970 1.00 1.030 V 90 100 110 µA 0.970 1.00 1.030 V 90 100 110 µA Revision 1.0 Micrel, Inc. MIC2870 Electrical Characteristics(7) (Continued) VIN = 3.6V; L = 1μH, COUT = 2.2μF, RFRSET = 10kΩ, RTRSET = 10kΩ, ILED = 100mA; TA = 25°C, bold values indicate −40°C ≤ TJ ≤ 125°C, unless otherwise noted. Symbol Parameter Condition Min. Typ. Max. Units EN / FEN / TEN / FI Control Pins VEN_ON EN On Threshold Boost converter and chip logic ON VEN_OFF EN Off Threshold Boost converter and chip logic ON VFEN_ON FEN On Threshold FLASH ON VFEN_OFF FEN Off Threshold FLASH OFF VTEN_ON TEN On Threshold TORCH ON VTEN_OFF TEN Off Threshold TORCH OFF VFI_ON FI On Threshold FLASH INHIBIT ON VFI_OFF FI Off Threshold FLASH INHIBIT OFF EN Pin Current VEN = 5V FEN/TEN/FI Pin Current VFEN = VTEN = VFI = 5V EN OFF Blanking Time EN pin should be driven low for more than this time before the IC enters sleep mode tBlank_EN_Off 1.5 V 0.4 1.5 V 0.4 1.5 V V 0.4 1.5 0.90 V V V 0.4 V 2 5 µA 1 5 µA 1.10 1.30 s 2 I C Interface (SCL / SDA Pins) (Guaranteed by Design) fSCL Maximum Operating Frequency 3.4 MHz VIL Low-Level Input Voltage 0.4 V VIH High-Level Input Voltage RSDA_DN SDA Pulled-Down Resistance 1.5 V Ω 20 Additional Protection Features VTH_LEDOPEN LED1 / LED2 Open Detect Threshold TBLANK_OPEN Open Detect Blanking Time 65 µs TRETRY_OPEN Open Retry Timeout 100 ms VTH_LEDSHORT Short Trigger Threshold VHYST_LEDSHORT Short Trigger Hysteresis 200 mV TBLANK_SHORT Short Trigger Blanking Time 30 µs TRETRY_ SHORT Short Retry Timeout 100 ms September 5, 2013 15 VOUT − MAX[VLED1,VLED2], VOUT = 3.6V 6 400 25 600 40 800 mV mV Revision 1.0 Micrel, Inc. MIC2870 Typical Characteristics Shutdown Current vs. Temperature Quiescent Current (Linear Mode) vs. Temperature 1.2 1.0 0.8 0.6 0.4 0.2 0.94 4.50 0.93 4.45 QUIESCENT CURRENT (µA) QUIESCENT CURRENT (µA) SHUTDOWN CURRENT (µA) 1.4 Quiescent Current (Boost Mode) vs. Temperature 0.92 0.91 0.90 0.89 0.88 LINEAR MODE NOT SWITCHING BOOST MODE SWITCHING VLED1 = VLED2 = 70mV 4.40 4.35 4.30 4.25 4.20 4.15 VLED1 = VLED2 > 200mV 0.0 0.87 -40 -20 0 20 40 60 80 100 120 4.10 -40 -20 0 40 60 80 100 120 188 TORCH MODE L = 1µH COUT = 2.2µF ILED = 187.5mA VLED = 890mV RTRSET = 10kΩ 185 0 20 40 60 80 100 800 750 VLED = 890mV RFRSET = 10kΩ 600 ILED PER CHANNEL TA = 25°C -20 0 20 40 60 80 100 120 400 400 1600 350 1400 200 L = 1 µH COUT = 2.2µF SINGLE LED ILED1+ILED2 100 100 50 TA = 25°C 0 0 40 50 60 FRSET RESISTOR (kΩ) September 5, 2013 70 80 20 30 40 50 60 70 80 1000 150 30 10 1200 200 300 20 0 Flash Mode ILED(MAX) (Single LED) vs. FRSET Resistor 250 10 50 TRSET RESISTOR (kΩ) 300 500 0 ILED PER CHANNEL TA = 25°C Torch Mode ILED(MAX) (Single LED) vs. TRSET Resistor TORCH MODE ILED(MAX) (mA) 600 FLASH MODE ILED(MAX) (mA) 700 120 L = 1 µH COUT = 2.2µF DUAL LEDs TEMPERATURE (°C) L = 1 µH COUT = 2.2µF DUAL LEDs 100 0 -40 TEMPERATURE (°C) 800 80 100 FLASH MODE L = 1µH COUT = 2.2µF ILED = 750mA 650 Flash Mode ILED(MAX) (Dual LEDs) vs. FRSET Resistor 60 150 700 120 40 200 FLASH MODE ILED(MAX) (mA) -20 20 250 TORCH MODE ILED(MAX) (mA) FLASH MODE LED CURRENT (mA) 189 -40 0 Torch Mode ILED(MAX) (Dual LEDs) vs. TRSET Resistor 850 190 186 -20 TEMPERATURE (°C) Flash Mode LED1 and LED2 Current vs. Temperature Torch Mode LED1 and LED2 Current vs. Temperature 187 -40 TEMPERATURE (°C) TEMPERATURE (°C) TORCH MODE LED CURRENT (mA) 20 800 600 400 L = 1 µH COUT = 2.2µF SINGLE LED 200 ILED1+ILED2 TA = 25°C 0 0 10 20 30 40 50 60 TRSET RESISTOR (kΩ) 7 70 80 0 10 20 30 40 50 60 70 80 FRSET RESISTOR (kΩ) Revision 1.0 Micrel, Inc. MIC2870 Typical Characteristics (Continued) WLED Output Power Efficiency vs. Input Voltage Boost Switching Frequency vs. Input Voltage 100 2.20 RTRSET = 75kΩ RTRSET = 82kΩ 2.0 RTRSET = 62kΩ RTRSET = 51kΩ 1.5 1.0 0.5 0.0 RTRSET = 39kΩ RTRSET = 30kΩ RTRSET = 20kΩ -0.5 -1.0 RTRSET = 10kΩ -1.5 -2.0 -2.5 -40°C 2.15 90 25°C 2.10 EFFICIENCY (%) 2.5 SWITCHING FREQUENCY (MHz) TORCH MODE ILED(MAX) ACCURACY (%) Torch Mode ILED(MAX) Accuracy vs. Input Voltage 2.05 2.00 75°C 1.95 125°C 1.90 3.7 3.9 INPUT VOLTAGE (V) September 5, 2013 4.1 4.3 70 ILED = 780mA ILED = 375mA L = 1µH COUT = 2.2µF TA = 25°C ILED = 150mA 50 1.80 3.5 ILED = 1.5A ILED = 1.2A 60 L = 1 µH COUT = 2.2µF ILED1+ ILED2 = 1.5A 1.85 80 2.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 8 4.5 2.6 3.0 3.4 3.8 4.2 4.6 5.0 INPUT VOLTAGE (V) Revision 1.0 Micrel, Inc. MIC2870 Functional Characteristics September 5, 2013 9 Revision 1.0 Micrel, Inc. MIC2870 Functional Characteristics (Continued) September 5, 2013 10 Revision 1.0 Micrel, Inc. MIC2870 Functional Characteristics (Continued) September 5, 2013 11 Revision 1.0 Micrel, Inc. MIC2870 Functional Diagram Figure 1. Simplified MIC2870 Functional Block Diagram September 5, 2013 12 Revision 1.0 Micrel, Inc. MIC2870 Functional Description FEN FEN is the hardware enable pin for flash mode. A logic low-to-high transition on FEN pin initiates the flash mode. If FEN pin 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 control register is cleared. VIN The input supply provides power to the internal MOSFETs gate drive and controls circuitry for the switchmode regulator. The operating input voltage range is from 2.7V to 5.0V. A 4.7µF low-ESR ceramic input capacitor should be connected from VIN to AGND as close to MIC2870 as possible to ensure a clean supply voltage for the device. The minimum voltage rating of 10V is recommended for the input capacitor. TEN TEN is the hardware enable pin for torch mode. A logic low-to-high transition on TEN pin initiates the torch mode. If TEN pin is left floating, it is pulled down internally by a built-in 1µA current source when the device is enabled. Torch mode is terminated when TEN is pulled low or left floating, and the torch control register is cleared. SW The MIC2870 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 highspeed switching on this pin, the switch node should be routed away from sensitive nodes wherever possible. FI FI is the flash inhibit pin. When this pin is high in flash mode, both LED1 and LED2 currents are changed from the flash-mode current level to the torch-mode current level. When this pin is low, both LED1 and LED2 currents are changed from torch-mode current level back to the original flash-mode current level. AGND This is the ground path for the internal biasing and control circuitry. The current loop of the analog ground should be separated from that of the power ground (PGND). AGND should be connected to PGND at a single point. FRSET The flash-mode maximum LED current level is programmed through FRSET. A resistor connected from FRSET to AGND sets the maximum current in the flash mode. FRSET can be grounded for the default flashmode current of 0.75A. For best current accuracy, a 0.1% tolerance resistor is recommended. FRSET cannot be left floating and the maximum resistance is limited to 80kΩ. PGND The power ground pin is the ground path for the high current in the boost switch and the ground path of the LED current sinks. The current loop for the power ground should be as small as possible and separate from the AGND loop as applicable. TRSET The torch-mode maximum LED current level is programmed through TRSET. A resistor connected from the TRSET pin to AGND sets the maximum current in the torch mode. TRSET can be grounded for the default torch-mode current of 187.5mA. For best current accuracy, a 0.1% tolerance resistor is recommended. TRSET cannot be left floating and the maximum resistance is limited to 80kΩ. OUT Boost converter output pin which is connected to the anode of the LED. A low-ESR ceramic capacitor of 2.2µF or larger should be connected from OUT to PGND as close as possible to the MIC2870. The minimum voltage rating of 10V is recommended for the output capacitor. LED1/LED2 The current sink pins for the LED(s). The LED anode is connected to the OUT pin and the LED cathode is connected to the LED1/LED2 pin(s). SCL 2 I C clock input pin provides a reference clock for clocking in the data signal. This is a high-speed mode up to 3.4MHz input pin, and requires a 4.7kΩ pull-up resistor. EN This is the enable pin of the MIC2870. The MIC2870 is in standby mode when the EN pin is asserted high. If this pin is driven low for more than 1s, the IC is shutdown. 2 Alternatively, the I C interface can be used for enabling/disabling the IC through the master control/status register. EN is pulled down by an internal resistor. September 5, 2013 SDA 2 I C data input/output pin allows for data to be written to and read from the MIC2870. This is a high-speed mode 2 up to 3.4MHz I C pin, and requires a 4.7kΩ pull-up resistor. 13 Revision 1.0 Micrel, Inc. MIC2870 Application Information Torch Mode The maximum torch-mode current level can be adjusted through an external resistor connecting to the TRSET pin according to Equation 2: The MIC2870 can drive one or two high-current flash WLEDs in either flash mode or torch mode. Two WLEDs can be used to optimize the light output and beam shaping through the optical lens/reflector assembly. In this case, the two channels, up to 750mA each, are matched to within 10% for optimal flash illumination. When the two channels are combined to drive a single high-brightness WLED, the maximum current is 1.5A. If one of the channels is left floating, MIC2870 senses the circuit condition automatically and allows the other channel to operate. ILED(MAX) = ILED(MAX) Like the flash-mode current, the torch-mode current can be set to a fraction of the maximum torch-mode level (either default or set by the TRSET resistor) by selecting the desired torch current level percentage in the torch 2 register (address 02h) through the I C interface. The torch current is the product of the maximum torch current setting and the percentage selected in the torch register. Eq. 1 Alternatively, the default maximum value of 750mA per channel is used when FRSET is grounded. Overvoltage Protection When the output voltage rises above the overvoltage protection (OVP) threshold, the MIC2870 is turned off automatically to avoid permanent damage to the IC. The flash-mode current can be initiated at the preset FRSET brightness level by asserting FEN high or by 2 setting the I C flash control register (address 01h) for the desired flash duration, subjected to the flash safety timeout setting. The flash-mode current is terminated 2 when FEN is brought low and the I C flash register is cleared. Open-Circuit Detection The open-circuit detector (OCD) is active only when the LED current regulator is turned on. When the external LED is missing or fails open, the LED1/2 pin voltage is pulled to near the ground potential by the internal current sink. If both LEDs are open or missing, the open-circuit detector would force the boost regulator and LED current regulator to turn off. The MIC2870 will try to turn on the boost regulator and LED current regulator again after a 100ms timeout. However, in most practical cases, the boost output voltage would rise above the OVP threshold, when both LED channels have an open fault. The OVP function would cause the MIC2870 to shut down. The flash inhibit (FI) pin can be used to synchronize the flash current to a handset GSM pulse event to prevent excessive battery droop. When FEN and FI pins are both HIGH, the flash-mode current is limited to the torch-mode current setting. The FI pin is also functional when the 2 flash-mode current is enabled through the I C flash register. Flash-mode current can be adjusted to a fraction of the maximum flash-mode level (either default or set by the FRSET resistor) by selecting the desired flash current level percentage in the flash control register (address 2 01h) through the I C interface. The flash current is the product of the maximum flash current setting and the percentage selected in the flash register. Short-Circuit Detection Like the OCD, the short-circuit detector is active only when the current regulator is turned on. If either one or both of the external LEDs fail a short, the short-circuit detector would force the MIC2870 to turn off. The MIC2870 will try to turn on the boost regulator and LED current regulator again after a 100ms timeout. If the short condition persists, the whole cycle repeats again. Prolonged operation in short-circuit condition is not recommended as it can damage the device. The flash safety timeout feature automatically shuts down the flash current if the flash mode is enabled for an extended period of time. Refer to the flash safety timer setting in Table 4. September 5, 2013 Eq. 2 Alternatively, the default maximum value of 187.5 mA per channel is used when the TRSET pin is grounded. The torch-mode operation is activated by asserting TEN high 2 or by setting the I C torch register (address 02h) for the desired duration. The torch-mode current is terminated 2 when TEN is brought low and the I C torch register is cleared. Flash Mode The maximum current level in the flash mode is 750mA per channel. This current level can be adjusted through an external resistor connecting to FRSET according to the following equation: 7500 = R FRSET 7500 4R TRSET 14 Revision 1.0 Micrel, Inc. MIC2870 2 I C Interface Figure 2 shows the communications required for write 2 and read operations via the I C interface. The black lines show master communications and the red lines show the slave communications. During a write operation the master must drive SDA and SCL for all stages except the acknowledgement (A) shown in red, which are provided by the slave (MIC2870): Table 1. MIC2870 Register Map Register Address Register Name Description 00h Master Control/ Status Chip enable control and status register 01h Flash Control Flash-mode current, flashmode enable, and flash timeout control register 02h Torch Control Torch-mode current and torchmode enable control register Master Control / Status Register [00h] The master control / status register allows the MIC2870 2 to be enabled by the I C interface -- setting the ON [ ] bit high has the same effect as asserting EN pin. The LED short bit, LED_SHT[ ] is set if any or both of the LED is shorted to OUT, while the LED open bit, LED_OP[ ] is asserted only when both LED are open circuit. The thermal shutdown bit, TSD[ ] is set when the junction temperature of the MIC2870 is higher than 160°C. Flash Control Register [01h] The flash safety timer and flash-mode current are configurable via the flash control register. Refer to flash timeout duration setting and flash-mode current setting in Table 4 and Table 5. 2 Figure 2. I C Timing Example The read operation begins firstly with a data-less write to select the register address from which to read. Then a restart sequence is issued, and then a read command followed by the data read. Torch Control Register [02h] The torch-mode current is configurable via the torch control register. Refer to torch-mode current setting in Table 7. The FI[ ] bit has the same function as the FI pin. When the FI[ ] bit is set, the flash-mode current is reduced to the torch-mode current setting. The MIC2870 responds to a slave address of hex 0xB4 and 0xB5 for write and read operations respectively, or binary 1011010X (where X is the read/write bit). The register address is eight bits wide and carries the address of the MIC2870 register to be operated upon. Only the lower three bits are used. I²C Registers MIC2870 contains three 8-bit read/write registers having an address from 00h to 02h for operation control as shown in Table 1. These registers are reset to their default values in power-on-reset (POR) event. In other words, they hold their previous contents when the chip is shutdown as long as supply voltage is above 1.5V (typical). September 5, 2013 15 Revision 1.0 Micrel, Inc. MIC2870 Table 2. Master Control Register [00h] Bit D7 D6 D5 D4 D3 D2 D1 D0 LED_SHT LED_OP TSD Name Reserved ON Access R R/W Default Value R 0 Table 3. Flash Control Register [01h] Bit D7 D6 Name D5 D4 FTMR D3 D2 FEN Access D1 D0 FCUR R/W Default Value 111 0 0000 Table 4. Flash Safety Timer Setting (FTMR) Register Value [D7:D5] of 01h Flash Timeout Duration (ms) 111 1250 110 1093.75 101 937.5 100 781.25 011 625 010 468.75 001 312.5 000 156.25 September 5, 2013 16 Revision 1.0 Micrel, Inc. MIC2870 Table 5. Flash-Mode Current Setting (FCUR) Percentage of Maximum Current / % Register Value [D3:D0] of 01h Current per Channel (mA) (RFRSET = 0Ω) Combined Current (mA) (RFRSET = 0Ω) 100 0000 750.0 1500.0 90 0001 675.0 1350.0 80 0010 600.0 1200.0 70 0011 525.0 1050.0 63 0100 472.5 945.0 56 0101 420.0 840.0 50 0110 375.0 750.0 44.7 0111 335.3 670.5 39.8 1000 298.5 597.0 35.5 1001 266.3 532.5 31.6 1010 237.0 474.0 28.2 1011 211.5 423.0 25.1 1100 188.3 376.5 22.4 1101 168.0 336.0 20 1110 150.0 300.0 18 1111 135.0 270.0 Table 6. Torch Control Register [02h] Bit D7 D6 Name Reserved Access RO Default Value September 5, 2013 D5 D4 FI TEN D3 D2 D1 D0 TCUR R/W 0 0000 17 Revision 1.0 Micrel, Inc. MIC2870 Table 7. Torch-Mode Current Setting (TCUR) Percentage of Maximum Current (%) Register Value [D3:D0] of 02h Current per Channel (mA) (RTRSET = 0Ω) Combined Current (mA) (RTRSET = 0Ω) 100 0000 187.5 375.0 90 0001 168.8 337.5 80 0010 150.0 300.0 70 0011 131.3 262.5 63 0100 118.1 236.3 56 0101 105.0 210.0 50 0110 93.8 187.5 44.7 0111 83.8 167.6 39.8 1000 74.6 149.3 35.5 1001 66.6 133.1 31.6 1010 59.3 118.5 28.2 1011 52.9 105.8 25.1 1100 47.1 94.1 22.4 1101 42.0 84.0 20 1110 37.5 75.0 18 1111 33.8 67.5 September 5, 2013 18 Revision 1.0 Micrel, Inc. MIC2870 Component Selection 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. 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. 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. FRSET/TRSET Resistor Since FRSET/TRSET resistor is used for setting the maximum LED current in flash mode and torch mode respectively, resistor type with 0.1% tolerance is recommended for more accurate LED current setting. 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 MIC2870 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 high-frequency 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. 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. X5R or X7R type ceramic capacitors are recommended for better tolerance over temperature. September 5, 2013 19 Revision 1.0 Micrel, Inc. MIC2870 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. 4 As can be seen in the diagram, total thermal resistance θJA = θJC + θCA. Hence this can also be written as in Equation 5: TJ = PDISS × (θ JA ) + TA Figure 3. Series Electrical Resistance Circuit Eq. 5 Since effectively all of the power losses (minus the inductor losses) in the converter are dissipated within the MIC2870 package, PDISS can be calculated thus: From this simple circuit we can calculate VX if we know ISOURCE, VZ and the resistor values, RXY and RYZ using Equation 3: 1 VX = ISOURCE × (R XY + R YZ ) + VZ Linear Mode: PDISS = [POUT × η Eq. 3 2 − 1 ] − IOUT × DCR Eq. 6 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). 1 Boost Mode: PDISS = [POUT × η 2 IOUT × DCR 1− D − 1 ] − Eq. 7 Duty Cycle in Boost Mode: D = VOUT − VIN VOUT Eq. 8 where: η = Efficiency taken from efficiency curves and DCR = inductor DCR. θJC and θJA are found in the operating ratings section of the data sheet. Figure 4. Series Thermal Resistance Circuit September 5, 2013 Where the real board area differs from 1” square, θCA (the PCB thermal resistance) values for various PCB copper areas can be taken from Figure 5. Figure 5 is taken from Designing with Low Dropout Voltage Regulators available from the Micrel website. 20 Revision 1.0 Micrel, Inc. MIC2870 Figure 5. Graph to Determine PC Board Area for a Given PCB Thermal Resistance Figure 5 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, horizontal, 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 oilbased paint and with 1.3m/sec (250 feet per minute) airflow. This approaches a “best case” pad heat sink. Conservative design dictates using the solid trace data, 2 which indicates that a maximum pad size of 5000 mm is needed. This is a pad 71mm × 71mm (2.8 inches per side). September 5, 2013 21 Revision 1.0 Micrel, Inc. MIC2870 PCB Layout Guidelines 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: Output Capacitor • Use wide and short traces to connect the output capacitor to the OUT and PGND pins. • Place several vias to the ground plane close to the output capacitor ground terminal. • Use either X5R or X7R temperature rating ceramic capacitors. Do not use Y5V or Z5U type ceramic capacitors. 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. • Analog ground (AGND) and power ground (PGND) should be kept separate and connected at a single location. • The exposed pad (ePad) on the bottom of the IC must be connected to the PGND ground plane of the PCB. • 4 to 6 thermal vias must be placed on the PCB pad for exposed pad and connected it to the PGND ground plane to ensure a good PCB thermal resistance can be achieved. Flash LED • 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 LED1/LED2 pins. • Make sure that the LED’s PCB land pattern can provide sufficient PCB pad heat sink to the flash LED. FRSET/TRSET Resistor • The FRSET/TRSET resistor should be placed close to the FRSET/TRSET pin and connected to AGND. 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. September 5, 2013 22 Revision 1.0 Micrel, Inc. MIC2870 Typical Application Schematic Bill of Materials Item C3 Part Number C5 AVX C1608X5R1A475K080AC TDK C6 L1 R1, R2 R3, R4 D1, D2 U1 (9) (10) Murata C1608X5R1A225K080AC TDK 06033D104KAJ2A AVX Murata C1608X5R1E104K TDK 0603ZD105KAT2A AVX GRM188R61A105KA61D Murata C1608X5R1A105K080AC TDK CDRH4D28CLDNP-1R0PC LQH44PN1R0NP0L CRCW06034K70FKEA ERA3AEB103V SML-LXL99UWC-TR/5 MIC2870YFT Qty. Ceramic Capacitor 4.7µF, 10V, X5R, 0603 1 Ceramic Capacitor 2.2µF, 10V, X5R, 0603 1 Ceramic Capacitor 0.1µF, 25V, X5R, 0603 1 Ceramic Capacitor 1µF, 10V, X5R, 0603 1 AVX GRM188R61A225KE34D GRM188R61C104KA01D Description (8) 0603ZD475KAT2A 0603ZD225KAT2A C4 Manufacturer Sumida (11) Inductor 1µH, 2.45A, 36mΩ, L4.0mm × W4.0mm × H1.65mm Murata Vishay/Dale Panasonic Inductor 1µH, 3.0A, 14mΩ, L5.1mm × W5.1mm × H3.0mm (12) (13) (14) Lumex (15) Micrel, Inc. 1 Resistor 4.7kΩ, 1%, 1/10W, 0603 2 Resistor 10kΩ, 0.1%, 1/10W, 0603 2 LED SQ 5W COOL WHT 6000K SMD, 190lm 2 2 1.5A Synchronous Boost Flash LED Driver with I C Interface 1 Notes: 8. AVX: www.avx.com. 9. TDK: www.tdk.com. 10. Murata: www.murata.com. 11. Sumida: www.sumida.com. 12. Vishay: www.vishay.com. 13. Panasonic: www.panasonic.com. 14. Lumex: www.lumex.com. 15. Micrel, Inc.: www.micrel.com. September 5, 2013 23 Revision 1.0 Micrel, Inc. MIC2870 PCB Layout Recommendations Top Layer Bottom Layer September 5, 2013 24 Revision 1.0 Micrel, Inc. MIC2870 Package Information(16) 16-Pin 2mm × 2mm TQFN (FT) Note: 16. Package information is correct as of the publication date. For updates and most current information, go to www.micrel.com. 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. © 2013 Micrel, Incorporated. September 5, 2013 25 Revision 1.0