MIC2297. General Description. Features. Applications. Typical Applications. 40V PWM Boost Regulator White LED Driver

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1 40 PWM Boost Regulator White LED Driver General Description The is a 600KHz PWM boost-switchg regulator that is optimized for drivg 6-0 series white LEDs. With its ternal 40 switch and a guaranteed switch current of.2a, the easily drives a strg of 0 white LEDs series at 20mA, ensurg a high level of brightness and elimatg several ballast resistors. The implements constant frequency 600KHz PWM control. The high frequency PWM operation saves board space by reducg external component sizes. The added benefit of the constant frequency PWM operation is much lower noise and put ripple jected back to the battery source than with variable frequency topologies. To optimize efficiency, the feedback voltage is set to 200m. This reduced voltage reduces the power dissipation the current set resistor, and allows the lowest total put voltage, hence mimal current draw from the battery. The is available with put over-voltage protection that protects the C and external components case of open LED conditions. The is available low profile small size 0-p 2.5mm x 2.5mm MLF package. The has a junction temperature range of 40 C to +25 C. Features 2.5 to 0 put voltage range Output voltage up to 40.2A switch current 600KHz PWM operation Trimmed 200m feedback voltage Output over voltage protection (fixed or adjustable) PWM Brightness Control DAC Brightness Control <% le regulation µa shutdown current Over temperature protection ULO 0-p 2.5mm x 2.5mm MLF package 40 o C to +25 o C junction temperature range Applications PDAs GPS systems Smart phones Mi PCs Digital cameras P phones LED flashlights Typical Applications 6.8µH-22µH 6.8µH-22µH -Cell Li on 3 to 4.2 µf N EN SW -42BML OP 0.47µF /50 -Ce ll Li on 3 to 4.2 µf PWM N EN SW -42BML OP 0.47µF /50 µf FB AGND PGND COMP µf 0.µF 0 AGND PGND FB COMP 0.µF 0 0 Series LED Driver with Output OP 0 Series LED Driver with PWM Brightness Control MLF and MicroLeadFrame is a trademark of Amkor Technology, nc. Micrel nc. 280 Fortune Drive San Jose, CA 953 USA tel + (408) fax + (408) March 2008 M

2 , nc. Orderg nformation Part Number Mark Code* Output Over oltage Protection Junction Temp. Range Package Lead Fish -5YML S C to 25 C 0-P 2.5 x2.5 MLF Pb-Free -42YML S C to 25 C 0-P 2.5 x2.5 MLF Pb-Free * ( ) Top bar symbol after P dicator identifyg Pb-Free may not be to scale. P Configuration PGND 0 SW OP 2 9 FB N 3 8 EN 4 7 COMP 5 6 AGND P Description P Number P Name P Function PGND Ground (Return). 2 OP Over oltage Protection (nput): Connect to the put to clamp the maximum put voltage. A resistor divider from this p to ground could be used to raise the OP level of the 5 OP option. 3 N Supply (nput): nput voltage. 4 EN Enable (nput): Logic high enables regulator. Logic low shuts down regulator. 5 COMP Compensation P. 6 AGND Analog Ground. 7 Brightness Control (nput): Either an analog (DAC) or filtered PWM signal can be used. The ga equation is: FB / 5. This p should be left open if the brightness function is not used. n that case, the FB will be set to its default value of 200m. 8 Reference oltage (Output): This node is equal to the voltage on the FB p. A capacitor from to ground should be used to filter the voltage if PWM dimmg is implemented. A capacitor from to ground can also be used to implement a soft-start function. This p can be left open if not used. 9 FB Feedback (nput): Output voltage sense node. Default value is 200m. Connect the cathode of the LED cha to this p. Connect current set resistor from this p to ground. 0 SW Switch Node (Output): nternal power BPOLAR collector. EPad GND Ground (Return): Backside pad. Mach M

3 , nc. Absolute Maximum Ratg () Supply voltage ( N )...2 Switch voltage ( SW ) to 50 Enable p voltage ( EN ) to N FB oltage ( FB ) Switch Current ( SW )...3A Ambient Storage Temperature (T S ) C to +50 C ESD Ratg (3)... 2K Operatg Range (2) Supply oltage ( N ) to 0 Maximum Output oltage ( OUT )...40 Junction Temperature Range (T J ) C to +25 C Package Thermal mpedance MLF -0 (θ JA )...65 C/W Electrical Characteristics T A 25 o C, N EN 3.6, OUT 30, OUT 20mA, unless otherwise noted. Bold values dicate -40 C T J 25 C. Symbol Parameter Condition M Typ Max Units N Supply oltage Range ULO Under-oltage Lock N Quiescent Current FB 200m (not switchg) 4 7 ma SD Shutdown Current EN 0 (4) 0. µa FB Feedback oltage (+/-5%) (+/±6.5%) (Over Temp) m FB Feedback nput Current FB 200m -450 na Le Regulation (5) 2.5 N % Load Regulation (5) 5mA OUT 20mA 0.5 % D MAX Maximum Duty Cycle 93 % SW Switch Current Limit N A SW Switch Saturation oltage N 2.5, SW 0.5A 220 m SW Switch Leakage Current EN 0, N µa EN Enable Threshold TURN ON.5 TURN OFF 0.4 EN Enable P Current EN µa Brightness Control Accuracy 0 5 OPEN f SW Oscillator Frequency KHz -42BML (nomal voltage) OP Over oltage protection -5BML (nomal voltage) T J Over-Temperature Threshold 50 C Shutdown Hysteresis 0 C Notes:. Absolute maximum ratgs dicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operatg the device side of its operatg ratgs. The maximum allowable power dissipation is a function of the maximum junction temperature, T J(Max), the junction-to-ambient thermal resistance, θ JA, and the ambient temperature, T A. The maximum allowable power dissipation will result excessive die temperature, and the regulator will go to thermal shutdown. 2. This device is not guaranteed to operate beyond its specified operatg ratg. 3. C devices are herently ESD sensitive. Handlg precautions required. Human body model. 4. SD N. 5. Guaranteed by design. Mach M

4 Typical Characteristics 6 Supply Current vs. nput oltage 00 Max Duty Cycle vs. nput oltage 800 Frequency vs. nput oltage Not Switchg FB NPUT OLTAGE () NPUT OLTAGE () NPUT OLTAGE () 200 Switch Current vs. Switch oltage 220 Switch oltage vs. nput oltage 20 LED Current vs. nput oltage N SWTCH CURRENT (ma) FSWTCH 0.5A 90 NPUT OLTAGE () NPUT OLTAGE () oltage vs. nput oltage NPUT OLTAGE () FB oltage vs. nput oltage 00 NPUT OLTAGE () LED Current vs. oltage RSENSE OLTAGE (m) FB oltage vs. oltage OLTAGE (m) Efficiency for 0 20mA vs. nput oltage L 6.8µH 70 NPUT OLTAGE () Efficiency for 0 LEDs vs. LED Current N 4.2 N 3.6 N L 6.8µH LED CURRENT (ma) Mach M

5 Typical Characteristics (contued) Efficiency for 0 LEDs vs. LED Current N 4.2 N 3.2 N L 5µH LED CURRENT (ma) Efficiency for 9 LEDs vs. LED Current N 4.2 N 3.2 N L 6.8µH LED CURRENT (ma) Efficiency for 9 LEDs vs. LED Current N 4.2 N 3.2 N L 5µH LED CURRENT (ma) LED Current vs. Duty Cycle peak PWM 5 PWM 20kHz RSENSE DUTY CYCLE (%) C3 0.22µF, 0 C µf, 6 PWM L Murata LQH32CN00K L N EN SW OP BML COMP AGND PGND FB C4 0.µF, 0 C2 0.47µF, 50 Mach M

6 Functional Characteristics Enable Characteristics Waveform Enable oltage (2/div) Output oltage (0/div) Output oltage (00m/div) SW oltage (20/div) N mA L 5µH nput Current (00mA/div) N mA L 5µH C OUT 0.47µ nductor Current (00mA/div) 0A TME (2ms/div) TME (µs/div) Waveform 2 SW oltage (20/div) nductor Current (200mA/div) Output oltage (00m/div) N mA L 5µH 0A TME (µs/div) Mach M

7 Block Diagram FB COMP OP EN SW PWM Generator kHz Oscillator Ramp Generator GND Block Diagram Mach M

8 Functional Description The is a constant frequency, PWM current mode boost regulator. The uses peak current mode control. The block diagram is shown above. The is composed of an oscillator, slope compensation ramp generator, current amplifier, gm error amplifier, PWM generator, and a.2a bipolar put transistor. The oscillator generates a 600kHz clock. The clock s two functions are to trigger the PWM generator that turns on the put transistor and to reset the slope compensation ramp generator. The current amplifier is used to measure the switch current by amplifyg the voltage signal from the ternal sense resistor. The put of the current amplifier is summed with the put of the slope compensation ramp generator. This summed current-loop signal is fed to one of the puts of the PWM generator. The gm error amplifier measures the LED current through the external sense resistor and amplifies the error between the detected signal and the 200m reference voltage. The put of the gm error amplifier provides the voltage-loop signal that is fed to the other put of the PWM generator. When the current-loop signal exceeds the voltage-loop signal, the PWM generator turns off the bipolar put transistor. The next clock period itiates the next switchg cycle, matag the constant frequency current-mode PWM control. The LED current is set by the feedback resistor: 200m LED R The Enable p shuts down the put switchg and disables control circuitry to reduce put current-to-leakage levels. Enable p put current is zero at zero volts. DC-to-DC PWM Boost Conversion The is a constant-frequency boost converter. t operates by takg a DC put voltage and regulatg a higher DC put voltage. Figure 2 shows a typical circuit. Boost regulation is achieved by turng on an ternal switch, which draws current through the ductor (L). When the switch turns off, the ductor s magnetic field collapses. This causes the current to be discharged to the put capacitor through an external Schottky diode (D). Waveforms and 2 show Output oltage ripple, SW oltage, and ndicator Current for 5mA and 20mA LED current respectively. oltage regulation is achieved by modulatg the pulse width or pulse-width modulation (PWM). FB -Cell Li on 3 to 4.2 µf µf 6.8µH-22µH N EN SW -42BML OP FB AGND PGND COMP 0.µF Figure 2. Typical Application Circuit µF /50 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 However, at light loads the ductor will completely discharge before the end of a switchg cycle. The current the ductor reaches 0A before the end of the switchg cycle. This is known as discontuous conduction mode (DCM). DCM occurs when: Where peak < 2 ( ) L f peak n DCM, the duty cycle is smaller than contuous conduction mode. n DCM the duty cycle is given by: f D 2 L ( The duty cycle required for voltage conversion should be less than the maximum duty cycle of 95%. Also, light load conditions where the put voltage is close to the put voltage, the mimum duty cycle can cause pulse skippg. This is due to the energy stored the ductor causg the put to overshoot slightly over the regulated put voltage. Durg the next cycle, the error amplifier detects the put as beg high and skips the followg pulse. This effect can be reduced by creasg the mimum load or by creasg the ductor value. ncreasg the ductor value reduces peak current. ) Mach M

9 Over-oltage Protection The has an over-voltage protection function. f an LED is disconnected from the circuit or the feedback p is shorted to ground, the feedback p will fall to ground potential. This will cause the to switch at full duty cycle an attempt to mata the feedback voltage. As a result, the put voltage will climb of control. This may cause the switch node voltage to exceed its maximum voltage ratg, possibly damagg the C and the external components. To ensure the highest level of protection, the OP p will shut the switch off when an overvoltage condition is detected, savg itself and the put capacitor. Brightness Control n the, the reference to the voltage error amplifier is pned. The p and p form a voltage divider off the ternal.245 reference. The voltage is such that with nothg connected to the p, the voltage is 0.2 and the voltage is. The voltage is /5 the voltage. The mimum voltage with pulled to ground is typically 0m. With a 0Ω sense resistor, the LED current is typically ma with the p pulled to ground. An analog DC voltage can be connected to the p. The will create an LED current proportional to the voltage accordg to the followg equation: LED 5 R sense Where is the voltage applied to the p, and Rsense is the sense resistor used the LED strg. t s important to use a uf ceramic capacitor on the p to filter any noise. An external PWM signal can be applied to the for dimmg. The uf capacitor and ternal 24kΩ resistor form an RC that filters the voltage to the p. The LED current is proportional the PWM duty cycle accordg to the followg equation: peak D LED 5 R sense Where peak is the peak PWM voltage and D is the duty cycle of the PWM signal. Component Selection nductor nductor selection is a balance between efficiency, stability, cost, size, and rated current. For most applications a 22µH is the recommended ductor value. t is usually a good balance between these considerations. Larger ductance values reduce the peak-to-peak ripple current, affectg efficiency. This has the effect of reducg both the DC losses and the transition losses. There is also a secondary effect of an ductor s DC resistance (DCR). The DCR of an ductor will be higher for more ductance the same package size. This is due to the longer wdgs required for an crease ductance. Sce the majority of put current (mus the operatg current) is passed through the ductor, higher DCR ductors will reduce efficiency. To mata stability, creasg ductor size will have to be met with an crease put capacitance. This is due to the unavoidable right half plane zero effect for the contuous current boost converter topology. The frequency at which the right half plane zero occurs can be calculated as follows: f rhpz L 2 2π The right half plane zero has the undesirable effect of creasg ga, while decreasg phase. This requires that the loop ga is rolled off before this has significant effect on the total loop response. This can be accomplished by either reducg ductance (creasg RHPZ frequency) or creasg the put capacitor value (decreasg loop ga). Output Capacitor Output capacitor selection is also a trade-off between performance, size, and cost. ncreasg put capacitance will lead to an improved transient response, but also an crease size and cost. X5R or X7R dielectric ceramic capacitors are recommended for designs with the. The put capacitor sets the frequency of the pole and zero the power stage. The zero is given by: f z C R 2π For ceramic capacitors, the ESR is very small. This puts the zero at a very high frequency where it can be ignored. The frequency of the pole caused by the put capacitor is given by. f p esr C π Mach M

10 Reference Capacitor A uf ceramic should be used on the reference p to prevent noise from gettg to this node. A uf ceramic is needed when a PWM signal is connected to the p. Diode Selection The 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 ductor current and the maximum reverse voltage is rated greater than the put voltage. nput capacitor A mimum F ceramic capacitor with an X5R or X7R dielectric is recommended for designg with the. ncreasg put capacitance will improve performance and greater noise immunity on the source. The put capacitor should be as close as possible to the ductor and the, with short traces for good noise performance. The utilizes a feedback p to compare the LED current to an ternal reference. The LED current is adjusted by selectg the appropriate feedback resistor value. The desired put current can be calculated as follows: 0. 2 LED R Compensation The comp p is connected to the put of the voltage error amplifier. The voltage error amplifier is a transconductance amplifier. Addg a series RC to ground adds a zero at: f zero 2 πr C The resistor typically ranges from 0kOhm to 50kOhm. The capacitor typically ranges from nf to 00nF. Addg a capacitor from comp to ground adds a pole at f pole 2 πr C 2 This capacitor typically ranges from 00pF to 0nF. Generally an RC to ground is all that is needed. The RC should be placed as close as possible to the comp p. The capacitor should be a ceramic with a X5R, X7R, or COG dielectric. Groundg 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. Mach M

11 Package nformation 0-P Package MLF (ML) MCREL, NC. 280 FORTUNE DRE SAN JOSE, CA 953 USA TEL + (408) FAX + (408) WEB The formation furnished by Micrel 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 with notification to the customer. Micrel Products are not designed or authorized for use as components life support appliances, devices or systems where malfunction of a product can reasonably be expected to result personal jury. Life support devices or systems are devices or systems that (a) are tended for surgical implant to the body or (b) support or susta life, and whose failure to perform can be reasonably expected to result a significant jury to the user. A Purchaser s use or sale of Micrel Products for use life support appliances, devices or systems is a Purchaser s own risk and Purchaser agrees to fully demnify Micrel for any damages resultg from such use or sale Micrel, ncorporated. Mach 2008 M