MIC2291 1.2A PWM Boost Regulator Photo Flash LED Driver General Description Features The MIC2291 is a 1.2MHz Pulse Width Modulation (PWM), boost-switching regulator that is optimized for high-current, white LED photo flash applications. With a guaranteed switch current of 1.2A, the MIC2291 easily drives a string of 3 white LEDs in series at 100mA, ensuring a high level of brightness and eliminating several ballast resistors. The MIC2291 implements a constant frequency, 1.2MHz PWM control scheme. The high frequency PWM operation saves board space by reducing external component sizes. The added benefit of the constant frequency PWM scheme, in contrast to variable frequency topologies, is much lower noise and input ripple injected back to the battery source. To optimize efficiency, the feedback voltage is set to only 95mV. This reduces the power dissipation in the current set resistor, and allows the lowest total output voltage, hence minimal current draw from the battery. The MIC2291 is available with 2 levels of over-voltage protection, 15V, and 34V. This allows designers to choose the smallest possible external components with the appropriate voltage ratings for their applications. The MIC2291 is available in low-profile, Thin SOT23 5-pin and 8-pin 2mm × 2mm MLF® package options. The MIC2291 has a junction temperature range of –40°C to +125°C. Data sheets and support documentation can be found on Micrel’s web site at www.micrel.com. • • • • • • • • • • • • • • 2.5V to 10V input voltage Output voltage up to 34V 1.2A switch current 1.2MHz PWM operation 95mV feedback voltage Overvoltage protection (OVP) – Options for 15V and 34V Stable with ceramic capacitors <1% line and load regulation 1µA shutdown current Over temperature protection UVLO Low-profile Thin SOT23-5 package option 2mm × 2mm MLF® package option –40°C to +125°C junction temperature range Applications • • • • • • • Photo Flash LED driver Cell phones PDAs GPS systems Digital cameras IP phones LED flashlights Typical Application 10µH 10µH 100mA MIC2291-15xML MIC2291xD5 5 1-Cell Li Ion 3V to 4.2V VIN SW 1 1µF 4 FB EN 100mA 1-Cell Li Ion 3V to 4.2V 0.22µF ceramic 3 95mV 1µF VIN SW EN OVP FB GND GND 0.22µF 95mV 2 Thin SOT23 Flash LED Driver 2mm x 2mm Flash LED Driver with Output OVP PowerPAK is a trademark of Siliconix, Inc. 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 May 2007 1 M9999-051507 Micrel, Inc. MIC2291 Ordering Information Part Number Marking* Overvoltage Protection Junction Temp. Range Package Lead Finish MIC2291BD5 SSAA ― –40° to +125°C 5-Pin Thin SOT23 Standard MIC2291YD5 SSAA ― –40° to +125°C 5-Pin Thin SOT23 Pb-Free MIC2291-15BML STA 15 –40° to +125°C 8-Pin 2mm x 2mm MLF® Standard MIC2291-15YML STA** 15 –40° to +125°C 8-Pin 2mm x 2mm MLF® Pb-Free –40° to +125°C ® Standard ® Pb-Free MIC2291-34BML STC MIC2291-34YML STC** 34 34 –40° to +125°C 8-Pin 2mm x 2mm MLF 8-Pin 2mm x 2mm MLF Notes: * Under bar / Over bar symbol may not be to scale. ** Over bar symbol located after Pin 1 identifier. Pin Configuration FB GND SW 1 2 3 4 EN 5 VIN OVP 1 8 PGND VIN 2 7 SW EN 3 6 FB AGND 4 5 NC EP ® 8-Pin 2mm x 2mm MLF (ML) 5-Pin TSOT23 (D5) Pin Description Pin Number TSOT23-5 Pin Number 1 7 Pin Name 2x2 MLF-8 2 May 2007 Pin Name SW GND Switch node (Output): Internal power BIPOLAR collector. Ground (Return): Ground. 3 6 FB Feedback (Input): Output voltage sense node. Connect the cathode of the LED to this pin. Connect current set resistor from this pin to ground. 4 3 EN Enable (Input): Logic high (≥1.5V) enables regulator. Logic low (≤0.4V) shuts down regulator. 5 2 VIN Supply (Input): Input Voltage. ― 1 OVP Overvoltage protection (Input): Connect to the output to clamp the maximum output voltage. ― 4 AGND Analog ground. Internally connected to ground. ― 8 PGND Power ground. ― 5 NC ― EP GND No connect (no internal connection to die). Ground (Return): Exposed backside pad. 2 M9999-051507 Micrel, Inc. MIC2291 Absolute Maximum Ratings(1) Operating Ratings(2) Supply Voltage (VIN) .......................................................12V Switch Voltage (VSW)....................................... –0.3V to 34V Enable Pin Voltage (VEN)................................... –0.3V to VIN FB Voltage (VFB)...............................................................6V Switch Current (ISW) .........................................................2A Storage Temperature (Ts) .........................–65°C to +150°C ESD Rating(3) .................................................................. 2kV Supply Voltage (VIN).......................................... 2.5V to 10V Junction Temperature (TJ) ........................ –40°C to +125°C Package Thermal Resistance 2x2 MLF-8 (θJA) .................................................93°C/W Thin SOT23-5 (θJA)..........................................256°C/W Electrical Characteristics(4) TA = 25°C, VIN = VEN = 3.6V; VOUT = 10V; IOUT = 40mA, bold values indicate –40°C< TJ < +125°C, unless noted. Symbol Parameter Condition Min Typ Max Units 10 V 2.1 2.4 V VIN Supply Voltage Range 2.5 VUVLO Under Voltage Lockout 1.8 IVIN Quiescent Current VFB > 200mV, (not switching) 2.8 5 mA ISD Shutdown Current VEN = 0V(5) 0.1 1 µA VFB Feedback Voltage (±5%) 95 100 mV IFB Feedback Input Current VFB = 95mV Line Regulation(7) 3V ≤ VIN ≤ 5V Load Regulation(7) 5mA ≤ IOUT ≤ 40mA DMAX Maximum Duty Cycle ISW Switch Current Limit VSW Switch Saturation Voltage ISW = 1.0A ISW Switch Leakage Current VEN = 0V, VSW = 10V VEN Enable Threshold TURN ON TURN OFF IEN Enable Pin Current fSW Oscillator Frequency VOVP Overvoltage Protection TJ Overtemperature Threshold Shutdown 90 –450 0.5 85 nA 1 0.5 % 90 % 1.2 A 550 mV 0.01 5 µA 0.4 V V 1.5 VEN = 10V(6) MIC2291BML- 15 only MIC2291BML- 34 only 20 40 µA 1.05 1.2 1.35 MHz 13 30 14 32 16 34 V V 150 10 Hysteresis % °C °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, θJA, 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. Human body model. 4. Specification for packaged product only. 5. ISD = IVIN. 6. See “Typical Characteristics” section for other VEN. 7. Guaranteed by design. May 2007 3 M9999-051507 Micrel, Inc. MIC2291 Typical Characteristics May 2007 4 M9999-051507 Micrel, Inc. MIC2291 Functional Characteristics May 2007 5 M9999-051507 Micrel, Inc. MIC2291 Functional Diagram VIN FB OVP* EN OVP* SW PWM Generator gm VREF 95mV 1.2MHz Oscillator GND Ramp Generator *OVP available on MLF® package option only Figure 1. MIC2291 Block Diagram The gm error amplifier measures the LED current through the external sense resistor and amplifies the error between the detected signal and the 95mV reference voltage. The output of the gm error amplifier provides the voltage-loop signal that is fed to the other input of the PWM generator. When the current-loop signal exceeds the voltage-loop signal, the PWM generator turns off the bipolar output transistor. The next clock period initiates the next switching cycle, maintaining the constant frequency current-mode PWM control. The LED is set by the feedback resistor: Functional Description The MIC2291 is a constant frequency, PWM current mode boost regulator. The block diagram is shown above. The MIC2291 is composed of an oscillator, slope compensation ramp generator, current amplifier, gm error amplifier, PWM generator, and a 500mA bipolar output transistor. The oscillator generates a 1.2MHz clock. The clock’s two functions are to trigger the PWM generator that turns on the output transistor and to reset the slope compensation ramp generator. The current amplifier is used to measure the switch cur-rent by amplifying the voltage signal from the internal sense resistor. The output of the current amplifier is summed with the output of the slope compensation ramp generator. This summed current-loop signal is fed to one of the inputs of the PWM generator. May 2007 ILED = 95mW R FB 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. 6 M9999-051507 Micrel, Inc. MIC2291 Over Voltage Protection For MLF® package of MIC2291, there is an over voltage protection function. If the feedback resistors are disconnected from the circuit or the feedback pin is shorted to ground, the feedback pin will fall to ground potential. This will cause the MIC2291 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 MIC2291 OVP pin will shut the switch off when an over-voltage condition is detected saving itself and other sensitive circuitry downstream. Application Information DC to DC PWM Boost Conversion The MIC2291 is a constant frequency boost converter. It operates by taking a DC input voltage and regulating cur-rent through series LED’s by monitoring voltage across the sense resistor (R2). LED current 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, causing the current to be discharged into the output capacitor through an external schottkey diode (D1). Regulation is then achieved by pulse width modulation (PWM) to maintain a constant voltage on the FB pin. This in turn provides constant LED current. VIN D1 1A/40V Schottky 10µH Component Selection VOUT Inductor Inductor selection is a balance between efficiency, stability, cost, size and rated current. For most applications a 10µH is the recommended inductor value. It is usually a good balance between these considerations. Efficiency is affected by inductance value in that larger inductance values reduce the peak to peak ripple current. This has an effect of reducing both the DC losses and the transition losses. There is also a secondary effect of an inductors 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 MIC2291 operating current) is passed through the inductor, higher DCR inductors will reduce efficiency. Also, 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; MIC2291-34xML 1-Cell Li Ion VIN SW EN OVP 3xLED C2 1µF FB GND R2 GND GND Figure 2. DC to DC PWM Boost Conversion 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 VIN VOUT The duty cycle required for voltage conversion should be less than the maximum duty cycle of 85%. Also, in light load conditions where the input voltage is close to the output volt-age, the minimum duty cycle can cause pulse skipping. This is due to the energy stored in the inductor causing the output to overshoot slightly over 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 reduces peak current, which in turn reduces energy transfer in each cycle. May 2007 2 frhpz = VOUT VIN × L × IOUT × 2π 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). 7 M9999-051507 Micrel, Inc. MIC2291 VIN Output Capacitor A 1µF or greater output capacitor is sufficient for most designs. An X5R or X7R dielectric ceramic capacitors are recommended for designs with the MIC2291. Y5V values may be used, but to offset their tolerance over temperature, more capacitance is required. PWM Diode Selection The MIC2291 requires an external diode for operation. A schottkey 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 cur-rent, the maximum output current and the maximum reverse voltage is rated greater than the output voltage. EN FB Figure 3. PWM Dimming Method 2. Continuous dimming control is implemented by applying a DC control voltage to the FB pin of the MIC2291 through a series resistor as shown in Figure 2. The LED intensity (current) can be dynamically varied applying a DC voltage to the FB pin. The DC voltage can come from a DAC signal, or a filtered PWM signal. The advantage of this approach is that a high frequency PWM signal (>10kHz) can be used to control LED intensity. Feedback Resistors The MIC2291 utilizes a feedback pin to compare the output to an internal reference. The LED current is adjusted by selecting the appropriate feedback resistor value. The desired current can be calculated as follows; VIN VREF ILED Where VREF is equal to 95mV. VIN SW EN FB GND Dimming Control There are two techniques for dimming control. One is PWM dimming, and the other is continuous dimming. 1. PWM dimming control is implemented by applying a PWM signal on EN pin as shown in Figure 1. The MIC2291 is turned on and off by the PWM signal. With this method, the LEDs operate with either zero or full current. The average LED current is increased proportionally to the duty-cycle of the PWM signal. This technique has high-efficiency because the IC and the LEDs consume no current during the off cycle of the PWM signal. Typical PWM frequency should be between 100Hz and 10kHz. May 2007 SW GND Input Capacitor A minimum 1µF ceramic capacitor is recommended for designing with the MIC2291. 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 MIC2291, with short traces for good noise performance. R2 = VIN 5.11k 49.9k DC Equivalent Figure 4. Continuous Dimming 8 M9999-051507 Micrel, Inc. MIC2291 Package Information 5-Pin Thin SOT23 (D5) 8-Pin 2mm x 2mm MLF® (ML) May 2007 9 M9999-051507 Micrel, Inc. MIC2291 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. © 2004 Micrel, Incorporated. May 2007 10 M9999-051507