MIC2298 3.5A Minimum, 1MHz Boost High Brightness White LED Driver General Description Features The MIC2298 is a high power boost-switching regulator that is optimized for constant-current control. The MIC2298 is capable of driving up to 2 series 1A white LED for photoflash, flashlight and other applications. The feedback voltage is only 200mV, minimizing ballast power dissipation in constant-current control applications for improved operating efficiency. The MIC2298 has a brightness pin that allows for a programmable torch mode as well full flash with a single pin when driving high current LEDs. The MIC2298 implements a constant frequency 1MHz PWM control scheme to make the smallest possible design. The 2.5V to 10V input voltage range of MIC2298 allows direct operation from 1- and 2-cell Li Ion as well as 3- to 6cell NiCad/NiMH/Alkaline battery sources. Maximum battery life is assured with a low 1µA shutdown current. The MIC2298 is available in a low profile 12-pin 3x3 MLF® package. Data sheets and support documentation can be found on Micrel’s web site at www.micrel.com. • 3.5A minimum switch current delivers at least 7W of output power over-temperature • 200mV ±10% feedback voltage • 2.5V to 10V input voltage • Output voltage up to 30V (max) • 12-pin 3mm x 3mm leadless MLF® package • Available output over-voltage protection (OVP) • 1MHz operation • Programmable output current • <1% line regulation • 1µA shutdown current • Over temperature protection • Externally programmable soft-start • Under-voltage lockout (UVLO) • –40°C to +125°C junction temperature range Applications • • • • Cell phones PDAs Digital cameras White LED flashlights ___________________________________________________________________________________________________________ Typical Application Figure 1. High Power White LED Driver MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc. Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com October 2007 M9999-101707-B Micrel, Inc. MIC2298 Ordering Information Part Number OVP Frequency Junction Temp. Range Package Lead Finish MIC2298-15YML 15V 1MHz –40° to +125°C 12-Pin 3x3 MLF® Pb-Free ® Note: MLF is a GREEN RoHS compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free. Pin Configuration BRT 1 12 COMP SS/DIM 2 11 EN FB 3 10 VIN AGND 4 9 OVP PGND 5 8 SW PGND 6 7 SW EP 12-Pin 3mm x 3mm MLF (ML) (Top View) Pin Description Pin Number Pin Name 1 BRT 2 SS/DIM Soft start/dimming (input) 40kΩ source. Connect a capacitor to GND for softstart. Clamp the pin to a known voltage to control the internal reference voltage and hence the output current. This can also be done with a resistor to GND 3 FB Feedback (Input): Output voltage sense node. Connect the cathode of the LED to this pin. 4 AGND Analog Ground 5,6 PGND Power Ground 7,8 SW Switch Node: Internal power BIPOLAR collector. 9 OVP Over-Voltage Protection (OVP): Connect to the output voltage to clamp the maximum output voltage. A resistor divider from this pin to ground could be used to raise the OVP level beyond 15V (max) 10 VIN Supply (Input): 2.5V to 10V for internal circuitry. 11 EN Enable (Input): Applying 1.5V or greater enables the regulator. Applying a voltage of 0.4V or less disables the MIC2298. 12 COMP Pad EP October 2007 Pin Function BRT (input): Apply a voltage greater than or equal to 1V to achieve full brightness current as determined by the ballast resistor. A 10µA current source sets the voltage on the resistor. Hence a 50K resistor would yield 0.5V which causes a reduction in LED current to 50% of full brightness. Grounding the BRT reduces the current further to 20% of full brightness. This pin may also be driven by a PWM signal for the same effect. Compensation pin (input): Add external R and C to GND to stabilize the converter. Ground (Return): Backside exposed pad. 2 M9999-101707-B Micrel, Inc. MIC2298 Absolute Maximum Ratings(1) Operating Ratings(2) Supply Voltage (VIN) .......................................................12V Switch Voltage (VSW)....................................... –0.3V to 34V BRT Voltage (VBRT) ........................................... –0.3V to 6V SS/DIM Voltage (VSS)........................................ –0.3V to 6V Enable Voltage (VEN)....................................... –0.3V to 12V FB Voltage (VFB)...............................................................6V Switch Current (ISW) .........................................................6A Ambient Storage Temperature (Ts) ...........–65°C to +150°C ESD Rating(3) .................................................................. 2kV Supply Voltage (VIN).......................................... 2.5V to 10V BRT Voltage (VBRT) ........................................... 0V to 0.6VIN Enable Voltage (VEN).............................................. 0V to VIN Output Voltage (VOUT) ................................... VIN + 1 to VOVP Junction Temperature (TJ) ........................ –40°C to +125°C Package Thermal Impedance 3x3 MLF-12 (θJA) ...............................................60°C/W Electrical Characteristics(4) TA = 25°C; VIN = VEN = 3.6V; VOUT = 7V; IOUT = 1A, unless otherwise noted. Bold values indicate –40°C< TJ < +125°C. Symbol Parameter Condition Min VIN Supply Voltage Range 2.5 VUVLO Under-Voltage Lockout VOVP Over-Voltage Protection IVIN Quiescent Current VFB >200mV, Not Switching ISD Shutdown Current VEN = 0V (Note 5) VFB Feedback Voltage (+/-8%) (+/-10%) (Over Temp) IFB Feedback Input Current VFB = 200mV Line Regulation 2.5V ≤ VIN ≤ 4.5V ILED BRT accuracy (Note 6) RBRT = GND RBRT = 50K DMAX Maximum Duty Cycle ISW Switch Current Limit VIN = 3V VSW Switch Saturation Voltage VIN = 3.6V, ISW = 3.5 A ISW Switch Leakage Current VEN = 0V, VSW = 15V VEN Enable Threshold TURN ON TURN OFF IEN Enable Pin Current VEN = 10V fSW Oscillator Frequency ISS Soft start / DIM current TJ Over-Temperature Threshold Shutdown Typ Max Units 10 V 1.8 2.1 2.4 V 12 13.5 15 V 15 23 mA 0.1 1 µA 200 216 220 mV 184 180 -650 nA 0.5 % 17 45 20 50 85 90 3.5 4.75 8 A 350 500 mV 0.01 10 µA 23 55 % 1.5 0.4 0.8 DIM = 0V Hysteresis % V 20 40 µA 1 1.2 MHz 5 µA 150 °C 10 °C Notes: 1. Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(Max), the junction-to-ambient thermal resistance, θ JA, and the ambient temperature, TA. The maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. 2. The device is not guaranteed to function outside its operating rating. 3. Devices are ESD sensitive. Handling precautions recommended. 4. Specification for packaged product only. 5. ISD = IVIN 6. As percentage of full brightness where VIN = VBRT = 3.6V (100% brightness) October 2007 3 M9999-101707-B Micrel, Inc. MIC2298 Typical Characteristics 20 Input Current vs. Input Voltage 2.4 UVLO (V) 1.1 12 8 2.2 2.1 2.0 1.9 2 4 6 8 10 INPUT VOLTAGE (V) 12 Switching Frequency vs. Temperature 1.8 -40 -20 0 20 40 60 TEMPERATURE (°C) 0.210 1.1 0.205 1.0 0.200 0.9 0.8 -40 -20 0 20 40 60 80 TEMPERATURE (°C) 1.0 LED Current vs. DIM Voltage LED Current vs. Input Voltage (BRT GND) 40 80 120 160 200 DIM VOLTAGE (mV) Max DC LED Current vs. Output Voltage 25°C 0.7 0.6 50°C 0.5 95 85 80 80 75 75 70 70 RSENSE 60 2.5 3 3.5 4 4.5 5 5.5 6 6.5 INPUT VOLTAGE (V) Max DC LED Current vs. Output Voltage 0.9 0.8 25°C 50°C 0.5 0.4 0.3 0.2 0.1 0.2 0.1 October 2007 Efficiency vs. Input Voltage (BRT Open) 90 0.7 0.6 0 0 LED Current vs. Input Voltage (BRT Open) RSENSE 0.91 TCASE = 25°C 0.90 2.5 3 3.5 4 4.5 5 5.5 6 6.5 INPUT VOLTAGE (V) 85 1.0 4 5 6 7 8 9 10 11 INPUT VOLTAGE (V) 0.94 0.93 0.92 90 0.4 0.3 VIN = 3.0V 2 4 6 8 10 12 14 OUTPUT VOLTAGE (V) 1.00 TA = 25°C 3 0.96 0.95 0.190 2.5 3 3.5 4 4.5 5 5.5 6 6.5 INPUT VOLTAGE (V) 95 0.8 2 0.99 0.98 0.97 RSENSE TCASE = 25°C 65 0.9 0.8 0 0 80 0.195 VIN = 5V 1100 1000 900 800 700 600 500 400 300 200 100 0 0 1.0 0.9 4 1.2 1.2 Switching Frequency vs. Input Voltage 2.3 16 0 0 UVLO vs. Temperature Efficiency vs. Input Voltage (BRT GND) 65 RSENSE 60 2.5 3 3.5 4 4.5 5 5.5 6 6.5 INPUT VOLTAGE (V) Bipolar Saturation Voltage vs. Switch Current 500 450 400 350 300 250 3.0V 3.6V 200 150 100 VIN = 3.6V 2 4 6 8 10 12 14 OUTPUT VOLTAGE (V) 4 50 TA = 25°C 0 0 0.5 1 1.5 2 2.5 3 3.5 4 SWITCH CURRENT (A) M9999-101707-B Micrel, Inc. MIC2298 Functional Characteristics October 2007 5 M9999-101707-B Micrel, Inc. MIC2298 Functional Diagram Figure 2. MIC2298 Block Diagram October 2007 6 M9999-101707-B Micrel, Inc. MIC2298 (D1). Waveforms 4 and 5 show Input Voltage ripple, Output Voltage ripple, SW Voltage, and Inductor Current for 900mA LED current. Regulation is achieved by modulating the pulse width i.e., pulse-width modulation (PWM). Functional Description The MIC2298 is a constant frequency, pulse-widthmodulated (PWM) peak current-mode step-up regulator. The MIC2298 simplified control scheme is illustrated in the block diagram in Figure 2. A reference voltage is fed into the PWM engine where the duty cycle output of the constant frequency PWM engine is computed from the error, or difference, between the REF and FB voltages. The PWM engine encompasses the necessary circuit blocks to implement a current-mode boost switch-mode power supply. The necessary circuit blocks include, but are not limited to, a oscillator/ramp generator, slope compensation ramp generator, gm error amplifier, current amplifier, PWM comparator, and drive logic for the internal 3.5A bipolar power transistor. Inside the PWM engine, the oscillator functions as a trigger for the PWM comparator that turns on the bipolar power transistor and resets the slope compensation ramp generator. The current amplifier is used to measure the power transistor’s current by amplifying the voltage signal from the CS+ and CS- inputs from the sense resistor connected to the emitter of the bipolar power transistor. The output of the current amplifier is summed with the output of the slope compensation ramp generator where the result is connected to one of the inputs of the PWM comparator. The gm error amplifier measures the white LED current through the external sense resistor and amplifies the error between the detected voltage signal from the feedback, or FB pin and the internal reference voltage. The output of the gm error amplifier provides the voltage loop signal that is fed to the other input of the PWM comparator. When the current loop signal exceeds the voltage loop signal the PWM comparator turns off the power transistor. The next oscillator/clock period initiates the next switching cycle, maintaining the constant frequency current-mode PWM control. The maximum white LED current is set by the feedback resistor (the resistor connected from the feedback pin to ground): Figure 3. Typical Application Circuit Duty Cycle Considerations Duty cycle refers to the switch on-to-off time ratio and can be calculated as follows for a boost regulator: D = 1− However, at light loads the inductor will completely discharge before the end of a switching cycle. The current in the inductor reaches 0A before the end of the switching cycle. This is known as discontinuous conduction mode (DCM). DCM occurs when: I out < Vin I peak ⋅ 2 Vout Where I peak = (Vout − Vin ) ⎛ Vin ⋅ ⎜⎜ L⋅f ⎝ Vout ⎞ ⎟ ⎟ ⎠ In DCM, the duty cycle is smaller than in continuous conduction mode. In DCM the duty cycle is given by: D= 200mV I LED = R FB f ⋅ 2 ⋅ L ⋅ I out ⋅ (Vout − Vin ) Vin The duty cycle required for voltage conversion should be less than the maximum duty cycle of 95%. Also, in light load conditions where the input voltage is close to the output voltage, the minimum duty cycle can cause pulse skipping. This is due to the energy stored in the inductor causing the output to slightly overshoot the regulated output voltage. During the next cycle, the error amplifier detects the output as being high and skips the following pulse. This effect can be reduced by increasing the minimum load or by increasing the inductor value. Increasing the inductor value also reduces the peak current. The enable pin shuts down the output switching and disables control circuitry to reduce input current-toleakage levels. Enable pin input current is zero at zero volts. DC-to-DC PWM Boost Conversion The MIC2298 is a constant-frequency boost converter. It operates by taking a DC input voltage and regulating a DC output voltage. Figure 3 shows a typical circuit. Boost regulation is achieved by turning on an internal switch, which draws current through the inductor (L1). When the switch turns off, the inductor’s magnetic field collapses. This causes the current to be discharged into the output capacitor through an external Schottky diode October 2007 Vin Vout 7 M9999-101707-B Micrel, Inc. MIC2298 Hence, a 200mΩ sense resistor will achieve nominally 1A when both DIM and BRT pins are left open. Over-Voltage Protection The MIC2298 offers over-voltage protection functionality. If an LED is disconnected from the circuit or the feedback pin is shorted to ground, the feedback pin will fall to ground potential. This will cause the MIC2298 to switch at full duty cycle in an attempt to maintain the feedback voltage. As a result, the output voltage will climb out of control. This may cause the switch node voltage to exceed its maximum voltage rating, possibly damaging the IC and the external components. To ensure the highest level of protection, the MIC2298 OVP pin will shut the switch off when an over-voltage condition is detected, saving itself and the output capacitor from damage. The OVP threshold can be increased by adding a resistor divider between the output and ground. Be careful not to exceed the 30V rating of the switch. PWM control of brightness A control signal can be driven into the enable pin to vary average current through the LED for applications not sensitive to low frequency (~100Hz) light modulation. For such applications, the SS/DIM pin capacitance should be minimized to achieve a fast turn on time. 0nF will achieve approximately 1.5ms with a CCOMP value of 33nF. For other applications, where no analog control voltage is available, the BRT pin can be driven through a low pass filter (18kΩ and 470nF) at a PWM frequency of >5kHz to set the FB voltage, and therefore, the LED current from 20% to 100% of Nominal LED current (Figure 5). Figure 4. Adjustable OVP Circuit Figure 5. High Frequency PWM Programming Via BRT Pin Brightness Control Since the DIM pin is typically utilized for soft start, it is recommended to use the enable and BRT pins for the PWM method of adjusting the average LED current. Figures 6 and 7 below show typical results for this method. Pin Brightness Functionality BRT Pin VFB (V) OPEN 200mV or VSS/DIM GND 40mV ≥20kΩ to 100kΩ [RBRT] to GND (10µA × RBRT)/5 SS/DIM Pin VFB (V) OPEN 200mV Voltage v VFB = v Table 1. BRT and SS/DIM Brightness Control Functionality The MIC2298 has built in brightness/dimming functionally for white LED applications. The BRT and SS/DIM pins are available for brightness/dimming control functionality. Table 1 illustrates the different modes of dimming functionality afforded by the BRT and SS/DIM pins. The resulting LED current is then calculated as: ILED = VFB/RSENSE October 2007 Figure 6. LED Current vs. DC (on enable pin) 8 M9999-101707-B Micrel, Inc. MIC2298 Figure 7. LED Current vs. DC (on BRT pin) Should the SS/DIM pin be used for voltage programming the LED current, note that there will be a small offset due mismatches between the 30k FB input and the impedance driving the SS/DIM pin. Soft Start Functionality The soft start time is dependant up on both CSS and the comp capacitor values. CCOMP is fixed for stable operation (typically 33nF); therefore, if any increases in soft start are desired, this should be done using the CSS capacitor. The approximate total startup time is given by the larger of: TSS = 1ms + 175k ⋅ C SS Or TSS = 1ms + CCOMP / 44 ⋅ 10 −6 E.g. for CCOMP = 33nF, use values of CCOMP > 4.3nF to increase startup time from 1.75ms. The soft start capacitor should be connected from the SS/DIM pin to ground. October 2007 9 M9999-101707-B Micrel, Inc. MIC2298 Diode Selection The MIC2298 requires an external diode for operation. A Schottky diode is recommended for most applications due to their lower forward voltage drop and reverse recovery time. Ensure the diode selected can deliver the peak inductor current and the maximum reverse voltage is rated greater than the output voltage. Component Selection Inductor Inductor selection is a balance between efficiency, stability, cost, size, and rated current. For most applications, a 4.7µH is the recommended inductor value. It is usually a good balance between these considerations. Larger inductance values reduce the peak-to-peak ripple current, affecting efficiency. This has the effect of reducing both the DC losses and the transition losses. There is also a secondary effect of an inductor’s DC resistance (DCR). The DCR of an inductor will be higher for more inductance in the same package size. This is due to the longer windings required for an increase in inductance. Since the majority of input current (minus the MIC2298 operating current) is passed through the inductor, higher DCR inductors will reduce efficiency. To maintain stability, increasing inductor size will have to be met with an increase in output capacitance. This is due to the unavoidable “right half plane zero” effect for the continuous current boost converter topology. The frequency at which the right half plane zero occurs can be calculated as follows: Input capacitor A minimum 1µF ceramic capacitor with an X5R or X7R dielectric is recommended for designing with the MIC2298. Increasing input capacitance will improve performance and greater noise immunity on the source. The input capacitor should be as close as possible to the inductor and the MIC2298, with short traces for good noise performance. The MIC2298 utilizes a feedback pin to compare the LED current to an internal reference. The LED current is adjusted by selecting the appropriate feedback resistor value. The desired output current can be calculated as follows: I LED = 2 f rhpz = VOUT VIN ⋅ L ⋅ I OUT ⋅ 2π Compensation The comp pin is connected to the output of the voltage error amplifier. The voltage error amplifier is a transconductance amplifier. Adding a series RC-toground adds a zero at: The right half plane zero has the undesirable effect of increasing gain, while decreasing phase. This requires that the loop gain is rolled off before this has significant effect on the total loop response. This can be accomplished by either reducing inductance (increasing RHPZ frequency) or increasing the output capacitor value (decreasing loop gain). f zero = f pole = 1 2πR 2C 3 This capacitor typically ranges from 100pF to 10nF. Generally, an RC to ground is all that is needed. The RC should be placed as close as possible to the compensation pin. The capacitor should be a ceramic with a X5R, X7R, or COG dielectric. Refer to the MIC2298 evaluation board document for component location. 1 C ⋅ R esr ⋅ 2π For ceramic capacitors, the ESR is very small. This puts the zero at a very high frequency where it can be ignored. Fortunately, the MIC2298 is current mode in operation which reduces the need for this output capacitor zero when compensating the feedback loop. The frequency of the pole caused by the output capacitor is given by. fp = 1 2πR 2C 4 The resistor typically ranges from 1kΩ to 50kΩ. The capacitor typically ranges from 1nF to 100nF. Adding an optional capacitor from comp pin-to-ground adds a pole at approximately Output Capacitor Output capacitor selection is also a trade-off between performance, size, and cost. Increasing output capacitance will lead to an improved transient response, but also an increase in size and cost. X5R or X7R dielectric ceramic capacitors are recommended for designs with the MIC2298. The output capacitor sets the frequency of the dominant pole and zero in the power stage. The zero is given by: fz = 0.2V R I OUT C ⋅ VOUT ⋅ 2 ⋅ π October 2007 10 M9999-101707-B Micrel, Inc. MIC2298 The graph in Figure 9 shows the peak LED current which can be pulsed at a given duty cycle (DC) to stay within SOA limits of 400mA to 700mA. Application Information Grounding Both the AGND and PGND must be connected to the exposed backside pad. The exposed backside pad also improves thermal performance. A large ground plane decreases thermal resistance to ambient air. Thermal Considerations and the SOA The SOA (safe operating area) of the MIC2298 is shown in the typical characteristics sub-section. This graph represents the maximum continuous output power capability of the part when used in the evaluation board layout. An alternative layout with more copper area for heat sinking will increase the area under the SOA curve. Note that the SOA is for continuous power and not peak power and is effectively a thermal limitation. The SOA is true for a time constant of approximately >1 second. Therefore, any load transient with a period of < 3s can exceed the SOA curve power up to a maximum limited by the current limit of the MIC2298. Figure 8 shows the theoretical output current limit of the MIC2298 using the Evaluation Board inductor value of 4.7µH. Figure 9. Duty Cycle vs. Peak Current for Fixed RMS Current Example Two series connected high brightness white LEDs with a Vf max of 4.2V and peak current of 800mA require pulses of 300ms at 3 second intervals. Power source is a Li-ion cell of 3v min. • Looking at the SOA curves (Page 4, Max DC LED Current), these cannot be driven continuously. • The time constant of the driver is < 3 seconds, so one can look at the peak current capability of the driver in Figure 8. • Looking at Figure 8, the MIC2298 can achieve more than the required 800mA peak current at 8.4V. • Reading from the SOA curve in the typical characteristics sub-section (Page 4, Max DC LED Current), the MIC2298 at 3V, 50oC and 8.4V output voltage, can provide 580mA RMS. • Now looking at the curve in Figure 9, using the next lower value of 500mA RMS current, one can see that the 850mA peak can be driven at a duty cycle of ~33% (or 1 second out of every 3 seconds). That is well within our target of 300ms. Figure 8. Peak Output Current vs. VOUT If our load is within these limits, it is possible to drive the load at some repetition rate or duty cycle (DC). This is allowed as long as we limit the RMS current to below the SOA limit. The RMS current for a pulsed current is known to be I RMS = I PK − PK ⋅ DC + I DC where the current pulse IPK-PK sits on a DC level of IDC. ( ) This simplifies to I RMS = I PK ⋅ DC when there is no DC level. October 2007 11 M9999-101707-B Micrel, Inc. MIC2298 LED Protection The operation of the Power LED must be limited to short pulses to prevent overheating. This is usually controlled by the micro controller in a typical application. For further protection, or where a micro controller is not used, the temperature of the LED can be limited by the addition of an NTC thermistor. The value should be > 100kΩ at its maximum safe temperature. This will then limit current drive to the LED as temperature rises further and prevents overheating. This thermistor should be connected directly from BRT to GND. Reference Figure 10. October 2007 Figure 10. LED Thermal Protection 12 M9999-101707-B Micrel, Inc. MIC2298 Package Information 12-Pin 3mm x 3mm MLF® (ML) 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 The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. 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. © 2007 Micrel, Incorporated. October 2007 13 M9999-101707-B