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mic2297 40v pwm boost regulator white led driver mlf and micro leadframe is a trademark 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 march 2008 m9999-032708 general description the mic2297 is a 600khz pwm boost-switching regulator that is optimized for driving 6-10 series white leds. with its internal 40v switch and a guaranteed switch current of 1.2a, the mic2297 easily drives a string of 10 white leds in series at 20ma, ensuring a high level of brightness and eliminating several ballast resistors. the mic2297 implements cons tant frequency 600khz pwm control. the high frequency pwm operation saves board space by reducing external component sizes. the added benefit of the constant frequency pwm operation is much lower noise and input ripple injected back to the battery source than with variable frequency topologies. to optimize efficiency, the feedback voltage is set to 200mv. this reduced voltage reduces the power dissipation in the current set re sistor, and allows the lowest total output voltage, hence mini mal current draw from the battery. the mic2297 is available wi th output over-voltage protection that protects the ic and external components in case of open led conditions. the mic2297 is available in low profile small size 10-pin 2.5mm x 2.5mm mlf ? package. the mic2297 has a junction temperature range of C40c to +125c. features ? 2.5v to 10v input voltage range ? output voltage up to 40v ? 1.2a switch current ? 600khz pwm operation ? trimmed 200mv feedback voltage ? output over voltage protec tion (fixed or adjustable) ? pwm brightness control ? dac brightness control ? <1% line regulation ? 1a shutdown current ? over temperature protection ? uvlo ? 10-pin 2.5mm x 2.5mm mlf ? package ? C40 o c to +125 o c junction temperature range applications ? pdas ? gps systems ? smart phones ? mini pcs ? digital cameras ? ip phones ? led flashlights ___________________________________________________________________________________________________________ typical applications vin en sw fb mic2297 - 42bm l 0.47f /50v 6.8h-22h 10 ovp pgnd 1f 1-cell li i on 3v to 4.2v brt ref agnd comp 1f pwm vin en sw fb mic2297 - 42bm l 0.47f /50v 6.8h-22h 10 ovp pgnd 1f 1-ce ll li i on 3v to 4.2v brt ref agnd comp 0.1f 1f 0.1f 10 series led driver with output ovp 10 series led driver with pwm brightness control downloaded from: http:///
micrel, inc. mic2297 mach 2008 2 m9999-032708 ordering information part number mark code* output over voltage protection junction temp. range package lead finish mic2297-15yml s115 15v C40c to 125c 10-pin 2.5 x2.5 mlf ? pb-free mic2297-42yml s142 42v C40c to 125c 10-pin 2.5 x2.5 mlf ? pb-free * ( ) top bar symbol after pi indicator identifying pb-free may not be to scale. pin configuration comp agnd 5 1 pgnd ovp vin en 10 sw fbref brt 98 7 23 4 6 pin description pin number pin name pin function 1 pgnd ground (return). 2 ovp over voltage protection (input ): connect to the output to clamp the maximum output voltage. a resistor divider from this pin to ground could be used to raise the ovp level of the 15v ovp option. 3 vin supply (input): input voltage. 4 en enable (input): logic high enables regulator. logic low shuts down regulator. 5 comp compensation pin. 6 agnd analog ground. 7 brt brightness control (input): ei ther an analog (dac) or filtered pwm signal can be used. the gain equation is: vfb = vbrt / 5. this pin should be left open if the brightness function is not used. in that case, the fb will be set to its default value of 200mv. 8 ref reference voltage (output): this node is equal to the voltage on the fb pin. a capacitor from ref to ground should be used to filter the brt voltage if pwm dimming is implemented. a capacitor from ref to ground can also be used to implement a soft-start function. this pin can be left open if not used. 9 fb feedback (input): output vo ltage sense node. default value is 200mv. connect the cathode of the led chain to this pin. connect current set resistor from this pin to ground. 10 sw switch node (output): in ternal power bipolar collector. epad gnd ground (return): backside pad. downloaded from: http:///
micrel, inc. mic2297 mach 2008 3 m9999-032708 absolute maximum rating (1) supply voltage (v in )........................................................12v switch voltage (v sw ) ........................................ -0.3v to 50v enable pin voltage (v en )....................................... -0.3 to v in fb voltage (v fb )...............................................................6v v brt ..................................................................................6v switch current (i sw ) .........................................................3a ambient storage temperature (t s )............-65 c to +150 c esd rating (3) ................................................................ 2kv operating range (2) supply voltage (v in ).......................................... 2.5v to 10v maximum output voltage (v out )....................................40v junction temperature range (t j )..............-40 c to +125 c package thermal impedance mlf ? -10 ( ja ).....................................................65 c/w electrical characteristics t a =25 o c, v in =v en = 3.6v, v out = 30v, i out = 20ma, unless otherwise noted. bold values indicate -40 c t j 125 c. symbol parameter condition min typ max units v in supply voltage range 2.5 10 v v uvlo under-voltage lockout 1.8 2.1 2.4 v i vin quiescent current v fb = 200mv (not switching) 4 7 ? ma i sd shutdown current v en = 0v (4) 0.1 1 a (+/-5%) 190 200 210 v fb feedback voltage (+/6.5%) (over temp) 187 213 mv i fb feedback input current v fb = 200mv -450 na line regulation (5) 2.5v v in 4.5v 0.5 1 % load regulation (5) 5ma i out 20ma 0.5 % d max maximum duty cycle 93 % i sw switch current limit v in 2.5v 1.2 1.7 2.5 a v sw switch saturation voltage v in = 2.5v, i sw = 0.5a 220 mv i sw switch leakage current v en = 0v, v in = 10v 0.01 1 a v en enable threshold turn on turn off 1.5 0.4 v i en enable pin current v en = 10v 20 40 a v ref brightness control accuracy v brt = 0v v brt = 1v v brt = 5v v brt = open 0.185 0.93 0.19 0.2 1.0 0.2 0.015 0.215 1.05 0.21 v f sw oscillator frequency 525 600 675 khz v ovp over voltage protection mic2297-42bml (nominal voltage) mic2297-15bml (nominal voltage) 40.5 15 42 16 47 18 v 150 c t j over-temperature threshold shutdown hysteresis 10 c notes: 1. absolute maximum ratings indicate limits beyond which damage to the component may occur. electr ical specifications do not a pply when operating the device outside of its operating ratings. the maximu m allowable power dissipation is a function of the maximum ju nction temperature, t j(max) , the junction-to-ambient thermal resistance, ja , and the ambient temperature, t a . the maximum allowable power dissipation will result in excessive die temperatur e, and the regulator will go into thermal shutdown. 2. this device is not guaranteed to operate beyond its specified operating rating. 3. ic devices are inherently esd sensitive. handling precauti ons required. hu man body model. 4. i sd = i vin . 5. guaranteed by design. downloaded from: http:///
micrel mic2297 mach 2008 4 m9999-032708 typical characteristics 3 3.5 4 4.5 5 5.5 6 2345678910 input voltage (v) supply current vs. input voltage not switchingv fb = 1v 95 96 97 98 99 100 2345678910 input voltage (v) max duty cycle vs. input voltage 400 500 600 700 800 2345678910 input voltage (v) frequency vs. input voltage 0 200 400 600 800 1000 1200 0 500 1000 1500 switch current (ma) switch current vs. switch voltage v in = 3.6v 190 195 200 205 210 215 220 2345678910 input voltage (v) switch voltage vs. input voltage i fswitch = 0.5a 10 12 14 16 18 20 2345678910 input voltage (v) led current vs. input voltage 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 2345678910 input voltage (v) brt voltage vs. input voltage 100 120 140 160 180 200 220 240 260 280 300 2345678910 input voltage (v) fb voltage vs. input voltage 0 5 10 15 20 25 30 1000 1200 brt voltage (mv) led current vs. brt voltage r sense 0 50 100 150 200 250 300 fb voltage vs. brt voltage 1000 1200 brt voltage (mv) 70 72 74 76 78 80 2345678910 input voltage (v) efficiency for 10 leds @ 20ma vs. input voltage l = 6.8h 40 45 50 55 60 65 70 75 80 0 5 10 15 20 25 30 35 led current (ma) efficiency for 10 leds vs. led current l = 6.8h v in = 3.2v v in = 3.6v v in = 4.2v downloaded from: http:///
micrel mic2297 mach 2008 5 m9999-032708 typical characteristics (continued) 35 40 45 50 55 60 65 70 75 80 85 0 1 02 03 04 0 led current (ma) efficiency for 10 leds vs. led current v in = 3.6v v in = 3.2v v in = 4.2v l = 15h 40 45 50 55 60 65 70 75 80 0 5 10 15 20 25 30 35 led current (ma) efficiency for 9 leds vs. led current l = 6.8h v in = 3.6v v in = 3.2v v in = 4.2v 35 40 45 50 55 60 65 70 75 80 85 0 5 10 15 20 25 30 35 40 led current (ma) efficiency for 9 leds vs. led current v in = 3.6v v in = 3.2v v in = 4.2v l = 15h 0 5 10 15 20 25 30 35 0 20406080100 duty cycle (%) led current vs. duty cycle r sense 5v 4v 3v 2v pwm = 20khz 2v peak pwm vin en ref brt agnd pgnd sw ovp comp fb mic2297bml c3 0.22f, 10v pwm c4 0.1f, 10v c2 0.47f, 50 v c1 1f, 16v l1 = murata lqh32cn100k11 l1 downloaded from: http:///
micrel mic2297 mach 2008 6 m9999-032708 functional characteristics enable characteristics output volta g e (10v/div) input current (100ma/div) time (2ms/div) v in = 3.6v 10 leds @ 20ma l = 15h c out = 0.47v enable volta g e (2v/div) wavefor m 1 sw volta g e (20v/div) inductor current (100ma/div) time (1s/div) v in = 3.6v 10 leds @ 5ma l = 15h output volta g e (100mv/div) 0a wavefor m 2 sw volta g e (20v/div) inductor current (200ma/div) time (1s/div) v in = 3.6v 10 leds @ 20ma l = 15h outpu t volta g e (100mv/div) 0a downloaded from: http:///
micrel mic2297 mach 2008 7 m9999-032708 block diagram + + 1.245v 1.245v pwm generator ramp generator 600khz oscillator fb comp ovp en sw gnd ref brt mic2297 block diagram downloaded from: http:///
micrel mic2297 mach 2008 8 m9999-032708 functional description the mic2297 is a constant frequency, pwm current mode boost regulator. the mic2297 uses peak current mode control. the block diagram is shown above. the mic2297 is composed of an oscillator, slope compensation ramp generator, current amplifier, gm error amplifier, pwm generator, and a 1.2a bipolar output transistor. the oscillator generates a 600khz clock. the clocks 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 current by amplifyi ng the voltage signal from the internal sense resistor. the out put 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. the gm error amplifier measures the led current through the external sense resistor and amplifies the error between the detected signal and the 200mv reference voltage. the output of the gm error amplifier provides the voltage-loop signal that is fed to the othe r 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 curr ent is set by the feedback resistor: fb led r mv i 200 = the enable pin shuts down the output switching and disables control circuitry to reduce input current-to-leakage levels. enable pin input current is zero at zero volts. dc-to-dc pwm boost conversion the mic2297 is a constant-fre quency boost converter. it operates by taking a dc input voltage and regulating a higher dc output voltage. figur e 2 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, t he inductors magnetic field collapses. this causes the current to be discharged into the output capacitor through an external schottky diode (d1). waveforms 1 and 2 show output voltage ripple, sw voltage, and indicator curr ent for 5ma and 20ma led current respectively. voltage regulation is achieved by modulating the pulse width or pulse-width modulation (pwm). vin en sw fb mic2297 - 42bm l 0.47f /50v 6.8h-22h 10 ovp pgnd 1f 1-cell li ion 3v to 4.2v brt ref agnd comp 1f 0.1f figure 2. 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: out in v v d ? = 1 however, at light loads t he inductor will completely discharge before the end of a sw itching cycle. the current in the inductor reaches 0a bef ore the end of the switching cycle. this is known as discontinuous conduction mode (dcm). dcm occurs when: 2 peak out in out i v v i ? < where ( ) ? ? ? ? ? ? ? ? ? ? ? = out in in out peak v v f l v v i in dcm, the duty cycle is smaller than in continuous conduction mode. in dcm t he duty cycle is given by: in in out out v v v il f d ) ( 2 ? ? ?? ? = 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 overshoo t 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. downloaded from: http:///
micrel mic2297 mach 2008 9 m9999-032708 over-voltage protection the mic2297 has an over-voltage protection function. if an led is disconnected from the circuit or the feedback pin is shorted to ground, the feedb ack pin will fall to ground potential. this will cause the mic2297 to switch at full duty cycle in an attempt to maintain the feedback voltage. as a result, the output voltage will c limb 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 mic2297 ovp pin will shut the switch off when an over- voltage condition is detected, saving itself and the output capacitor. brightness control in the mic2297, the reference to the voltage error amplifier is pinned out. the brt pin and ref pin form a voltage divider off the internal 1.245v reference. the voltage is such that with nothing connected to the brt pin, the ref voltage is 0.2v and the brt voltage is 1v. the ref voltage is 1/5 the brt voltage. the minimum ref voltage with brt pulled to ground is typically 10mv. with a 10 ? sense resistor, the led current is typically 1ma with the brt pin pulled to ground. an analog dc voltage can be connected to the brt pin. the mic2297 will create an led current proportional to the brt voltage according to the following equation: sense led r brt i ? = 5 where brt is the voltage applied to the brt pin, and rsense is the sense resistor used in the led string. its important to use a 1uf ceramic capacitor on the ref pin to filter any noise. an external pwm signal can be applied to the brt for dimming. the 1uf ref capacitor and internal brt 124k ? resistor form an rc that filter s the voltage to the ref pin. the led current is propor tional the pwm duty cycle according to the following equation: sense peak led r d v i ? ? = 5 where vpeak is the peak pwm voltage and d is the duty cycle of the pwm signal. component selection inductor inductor selection is a balance between efficiency, stability, cost, size, and rated current. for most applications a 22 h 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 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 (m inus the mic2297 operating current) is passed through the inductor, higher dcr inductors will reduce efficiency. to maintain stability, increasing inducto r size will have to be met with an increase in output capacitance. this is due to the unavoidable right half plane ze ro effect for the continuous current boost converter topology. the frequency at which the right half plane zero occurs can be calculated as follows: 2 2 ? ? ? = out out in rhpz i l v v f 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 (i ncreasing rhpz frequency) or increasing the output capacitor value (decreasing loop gain). output capacitor output capacitor selection is also a trade-off between performance, size, and cost. increasing output capacitance will lead to an im proved transient response, but also an increase in size and cost. x5r or x7r dielectric ceramic capacitors are recommended for designs with the mic2297. the output capacitor sets the frequency of the pole and zero in the power stage. the zero is given by: 2 1 ? ? = esr z r c f for ceramic capacitors, the esr is very small. this puts the zero at a very high freq uency where it can be ignored. the frequency of the pole caus ed by the output capacitor is given by. ? ? = out out p v c i f downloaded from: http:///
micrel mic2297 mach 2008 10 m9999-032708 reference capacitor a 1uf ceramic should be used on the reference pin to prevent noise from getting into this node. a 1uf ceramic is needed when a pwm signal is connected to the brt pin. diode selection the mic2297 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. input capacitor a minimum 1f ceramic capacitor with an x5r or x7r dielectric is recommended for designing with the mic2297. increasing input capa citance will improve performance and greater noise immunity on the source. the input capacitor should be as close as possible to the inductor and the mic2297, with short traces for good noise performance. the mic2297 utilizes a feedback pin to compare the led current to an internal refe rence. the led current is adjusted by selecting the appr opriate feedback resistor value. the desired output current can be calculated as follows: r v i led 2.0 = compensation the comp pin is connected to the output of the voltage error amplifier. the voltage error amplifier is a transconductance amplifier. addi ng a series rc to ground adds a zero at: 11 2 1 c r f zero = the resistor typically ranges from 10kohm to 50kohm. the capacitor typically ranges from 1nf to 100nf. adding a capacitor from comp to ground adds a pole at 21 2 1 c r f pole = 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 comp pin. the capacitor should be a ceramic with a x5r, x7r, or cog dielectric. grounding both the agnd and pgnd must be connected to the exposed backside pad. the ex posed backside pad also improves thermal performance. a large ground plane decreases thermal resistance to ambient air. downloaded from: http:///
micrel mic2297 mach 2008 11 m9999-032708 package information 10-pin package 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 belie ved to be accurate and reliable. however, no responsibility is a ssumed 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 authori zed for use as components in life support appliances, devices or systems where malfu nction of a product can reasonably be expected to result in personal injury. life suppo rt devices or systems are devices or systems that (a) are in tended 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 significan t injury to the user. a purchasers use or sale of micrel produc ts for use in life support app liances, devices or systems is a purchasers own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 2005 micrel, incorporated. downloaded from: http:///

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