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rev. 0.3 april 2012 www.aosmd.com page 1 of 12 aoz1084 1.2a buck led driver general description the aoz1084 is a high efficiency, simple to use, 1.2a buck hb led driver optimized for general lighting applications. the aoz1084 works from a 4.5v to 36v input voltage range, and provides up to 1.2a of continuous led current. the 160mv led current feedback voltage minimizes the power dissipation of the external sense resistor. the fixed switching frequency of 450khz pwm operation reduces inductor and capacitor sizes. the aoz1084 is available in a tiny dfn2x2-8l package. features ? up to 36v operating input voltage range ? 420m ? internal nmos ? up to 95% efficiency ? internal compensation ? 1.2a continuous output current ? fixed 450khz pwm operation ? internal soft start ? 160mv led current feedback voltage with 8% accuracy ? cycle-by-cycle current limit ? short-circuit protection ? thermal shutdown ? small size dfn2x2-8l applications ? general led lighting ? architectura l lighting ? signage lighting typical application figure 1. 1.2a buck hb led driver lx vin bs vin vout fb dim gnd c2 10f c3 c1 4.7f l1 2.2h aoz1084 r s led1
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 2 of 12 ordering information aos green products use reduced levels of halogens, and are also rohs compliant. please visit www.aosmd.com/media/aosgreenpolicy.pdf for additional information. pin configuration pin description part number ambient temperature range package environmental AOZ1084DI -40 c to +85 c dfn2x2-8l green product pin number pin name pin function 1 lx pwm output connection to inductor. 2vin supply voltage input. input range from 4.5v to 36v. when vin rises above the uvlo threshold the device starts up. 3vin supply voltage input. input range from 4.5v to 36v. when vin rises above the uvlo threshold the device starts up. 4 dim pwm dimming pin. this pin is active high. 5fb led current feedback. the fb pin regulatio n voltage is 160mv. connect an external sense resistor between the cathode of the led string and gnd to set led current. 6 gnd ground. 7 gnd ground. 8 bst bootstrap voltage input. high side driver supply. connected to 10nf capacitor between bst and lx. exposed pad epad thermal exposed pad. pad cam be connected to gnd if necessary for improved thermal performance. bst gnd gnd fb lx vin vin dim dfn2x2-8 (top view) 1 2 3 4 8 7 6 5
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 3 of 12 absolute maximum ratings exceeding the absolute maxi mum ratings may damage the device. note: 1. devices are inherently esd s ensitive, handling precautions are required. human body model rating: 1.5k ? in series with 100pf. recommended operating conditions the device is not guaranteed to operate beyond the recommended operating conditions. electrical characteristics t a = 25c, v vin = 12v, v en = 12v, unless otherwise spec ified. specifications in bold indicate a temperature range of -40c to +85c. these specifications are guaranteed by design. parameter rating supply voltage (v vin ) 40v lx to gnd -0.7v to v vin + 2v dim to gnd -0.3v to 36v fb to gnd -0.3v to 6v bst to gnd v lx + 6v junction temperature (t j ) +150c storage temperature (t s ) -65c to +150c esd rating (1) 2kv parameter rating supply voltage (v vin ) 4.5v to 36v output voltage range up to 0.85 * v vin ambient temperature (t a ) -40c to +85c package thermal resistance ( ? ja ) dfn2x2-8l 55c/w symbol parameter conditions min. typ. max. units v vin supply voltage 4.5 36 v v uvlo input under-voltage lockout threshold v vin rising v vin falling 2.2 2.9 v v uvlo hysteresis 200 mv i vin supply current (quiescent) i out = 0, v fb = 1v, v en > 1.2v 11.5 ma i off shutdown supply current v en = 0v 8 ? a v fb feedback voltage t a = 25oc 147 160 173 mv v fb_load load regulation 120ma < load < 1.08a 0.5 % v fb_line line regulation load = 600ma 0.03 %/v i fb feedback voltage input current v fb = 160mv 100 na pwm dimming v dim_off v dim_on dimming input threshold off threshold on threshold 1.2 0.4 v v v dim_hys dimming input hysteresis 200 mv i en dimming input current 3 ? a modulator f o frequency 360 450 540 khz d max maximum duty cycle 87 % t on_min minimum on time 150 ns i lim current limit 1.5 1.9 2.3 a over-temperature shutdown limit t j rising t j falling 150 110 c c t ss soft start interval 600 ? s power state output r ds(on) nmos on-resistance v in = 12v 420 m ? i leakage nmos leakage v en = 0v, v lx = 0v 10 ? a
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 4 of 12 block diagram gnd vin dim fb low voltage regulator dim detection oc detect short detect otp detect softstart pwm logic error amplifier ? + 0.25v + ? pwm comparator osc clk current sense driver bst ldo lx bst
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 5 of 12 typical performance characteristics v in = 12 v, load = 1 white led unless otherwise specified. led short test (36v/1 led) led short recovery (36v/1 led) led open to normal (36v/3 led) normal to led open (36v/3 led) 500 s/div 500 s/div 50 s/div 500 s/div vo 5v/div vlx 10v/div ilx 500ma/div vo 5v/div vlx 10v/div dim 5v/div ilx 500ma/div vo 5v/div dim 5v/div vlx 10v/div ilx 500ma/div vlx 10v/div vo 5v/div ilx 500ma/div 200hz dimming test (12v/3 led) 2ms/div dim 5v/div vo 10v/div vlx 10v/div ilx 500ma/div
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 6 of 12 detailed description the aoz1084 is a high efficiency, simple to use, 1.2a buck hb led driver optimized for general lighting applications. features include enable control, under voltage lock-out, internal soft-start, output over-voltage protection, over-current protection and thermal shut down. the aoz1084 is available in a dfn2x2-8l package. soft start and pwm dimming the aoz1084 has internal soft start feature to limit in-rush current and ensure the output voltage ramps up smoothly to regulation voltage. a soft start process begins when the input voltage rises to the voltage higher than uvlo and voltage on dim pin is high. in soft start process, the output voltage is ramped to regulation voltage in typically 600 ? s. the 600 ? s soft start time is set internally. the dim pin of the aoz1084 is active high. connect the dim pin to vin if enable function is not used. pull it to ground will disable the aoz1 084. do not le ave it open. the voltage on dim pin must be above 1.2v to enable the aoz1084. when voltage on dim pin falls below 0.4v, the aoz1084 is disabled. steady-state operation under steady-state conditions, the converter operates in fixed frequency and continuous-conduction mode (ccm). the aoz1084 integrates an internal nmos as the high- side switch. inductor current is sensed by amplifying the voltage drop across the drain to source of the high side power mosfet. output voltage is divided down by the external voltage divider at the fb pin. the difference of the fb pin voltage and reference is amplified by the internal transconductance error amplifier. the error voltage, is compared against the current signal, which is sum of inductor current signal and ramp compensation signal, at pwm comparator input. if the current signal is less than the error voltage, the internal high-side switch is on. the inductor current flows from the input through the inductor to the output. when the current signal exceeds the error voltage, the high-side switch is off. the inductor current is freewheeling through the external schottky diode to output. switching frequency the aoz1084 switching frequency is fixed and set by an internal oscillator. the sw itching frequency is set internally 450khz. led current programming led current can be set by feeding back the output to the fb pin with the sense resistor r s shown in figure 1. the led current can be programmed as: protection features the aoz1084 has multiple protection features to prevent system circuit damage under abnormal conditions. over current protection (ocp) the sensed inductor current signal is also used for over current protection. the cycle by cycle current li mit threshold is set normal value of 2a. when the load current reaches the current limit threshold, the cycle by cyc le current limit circuit turns off the high side switch immediately to terminate the current duty cycle. the inductor current stop rising. the cycle by cycle current limit protection directly limits inductor peak current. the average inductor current is also limited due to the limitation on peak inductor current. when cycle by cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreasing. the aoz1084 has internal short circuit protection to protect itself from catastrophic failure under output short circuit conditions. as a result , the converter is shut down and hiccups. the converter w ill start up via a soft start once the short circuit condition disappears. in short circuit protection mode, the inductor average current is greatly reduced. uvlo an uvlo circuit monitors the input voltage. when the input voltage exceeds 2.9v, the converter starts operation. when input voltage falls below 2.2v, the converter will stop switching. thermal protection an internal temperature sensor monitors the junction temperature. it shuts down the internal control circuit and high side nmos if the junction temperature exceeds 150oc. the regulator will rest art automatica lly under the control of soft-start circuit when the junction temperature decreases to 110c. i led 0.16 r s ----------- =
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 7 of 12 application information the basic aoz1084 application circuit is shown in figure 1. component selection is explained below. input capacitor the input capacitor must be connected to the vin pin and pgnd pin of the aoz1084 to maintain steady input voltage and filter out the pulsing input current. the voltage rating of input capacitor must be greater than maximum input voltage plus ripple voltage. the input ripple voltage can be approximated by equation below:: since the input current is discontinuous in a buck converter, the current stress on the input capacitor is another concern when selecting the capacitor. for a buck circuit, the rms value of i nput capacitor current can be calculated by: if we let m equal the conversion ratio: the relationship between the input capacitor rms current and voltage conversion ratio is calculated and shown in figure 2. it can be seen that when v o is half of v in , c in is under the worst current stress. the worst current stress on c in is at 0.5 x i o . figure 2. i cin vs. voltage conversion ratio for reliable operation and best performance, the input capacitors must have curr ent rating higher than i cin-rms at worst operating conditions. ceramic capacitors are preferred for input capacitors because of their low esr and high ripple current rating. depending on the application circuits, other low esr tantalum capacitor or aluminum electrolytic capacitor may also be used. when selecting ceramic capacitors, x5r or x7r type dielectric ceramic capacitors are preferred for their better temperature and voltage characteristics. note that the ripple current rating from capacitor manufactures are based on certain amount of life time. further de-rating may be necessary for practical design requirement. inductor the inductor is used to supply constant current to output when it is driven by a switching voltage. for given input and output voltage, inductance and switching frequency together decide the inductor ripple current, which is: the peak inductor current is: high inductance gives low inductor ripple current but requires larger size inductor to avoid saturation. low ripple current reduces inductor core losses. it also reduces rms current through inductor and switches, which results in less conduction loss. when selecting the inductor, make sure it is able to handle the peak current without saturation even at the highest operating temperature. the inductor takes the highest current in a buck circuit. the conduction loss on inductor needs to be checked for thermal and efficiency requirements. surface mount inductors in different shape and styles are available from coilcraft, elytone and murata. shielded inductors are small and radiate less emi noise. but they cost more than unshielded inductors. the choice depends on emi requirement, price and size. output capacitor the output capacitor is selected based on the dc output voltage rating, output ripple voltage specification and ripple current rating. ? v in i o fc in ? ----------------- 1 v o v in -------- - ? ?? ?? ?? v o v in -------- - ? ? = i cin_rms i o v o v in -------- - 1 v o v in -------- - ? ?? ?? ?? ? = v o v in -------- - m = 0 0.1 0.2 0.3 0.4 0.5 0 0.5 1 m i cin_rms (m) i o ? i l v o fl ? ---------- - 1 v o v in -------- - ? ?? ?? ?? ? = i lpeak i o ? i l 2 -------- + =
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 8 of 12 the selected output capacitor must have a higher rated voltage specification than the maximum desired output voltage including ripple. de-rating needs to be considered for long term reliability. output ripple voltage specification is another important factor for selecting the output capacitor. in a buck converter circuit, output ripple voltage is determined by inductor value, switching frequency, output capacitor value and esr. it can be calculated by the equation below: where, co is output capacitor value, and esrco is the equivalent series resistance of the output capacitor. when low esr ceramic capacitor is used as output capacitor, the impedance of the capacitor at the switching frequency dominates. output ripple is mainly caused by capacitor value and inductor ripple current. the output ripple voltage calculation can be simplified to: if the impedance of esr at switching frequency dominates, the output ripple voltage is mainly decided by capacitor esr and inductor ripple current. the output ripple voltage calculation can be further simplified to: for lower output ripple voltage across the entire operating temperature range, x5r or x7r dielectric type of ceramic, or other low esr tantalum capacitor or aluminum electrolytic capacitor may also be used as output capacitors. in a buck converter, output capacitor current is continuous. the rms current of output capacitor is decided by the peak to peak inductor ripple current. it can be calculated by: usually, the ripple current rating of the output capacitor is a smaller issue because of the low current stress. when the buck inductor is selected to be very small and inductor ripple current is high, output capacitor could be overstressed. the external freewheeling diode supplies the current to the inductor when the high side nmos switch is off. to reduce the losses due to the forward voltage drop and recovery of diode, schottky diode is recommended to use. the maximum reverse voltage rating of the chosen schottky diode should be greater than the maximum input voltage, and the current rating should be greater than the maximum load current. thermal management and layout considerations in the aoz1084 buck regulator circuit, high pulsing current flows through two circuit loops. the first loop starts from the input capacitors, to the vin pin, to the lx pins, to the filter inductor, to the output capacitor and load, and then return to the input capacitor through ground. current flows in the first loop when the high side switch is on. the second loop starts from inductor, to the output capacitors and load, to the anode of schottky diode, to the cathode of schottky diode. current flows in the second loop when the low side diode is on. in pcb layout, minimizing the two loops area reduces the noise of this circuit and im proves efficiency. a ground plane is strongly recommended to connect input capacitor, output capacitor, and pgnd pin of the aoz1084. in the aoz1084 buck regulator circuit, the major power dissipating components are the aoz1084, the schottky diode and output inductor. the total power dissipation of converter circuit can be measured by input power minus output power: the power dissipation in the schottky diode can be approximated as: where, vfw_schottky is the schottky diode forward voltage drop. the power dissipation of the inductor can be approximately calculated by output current and dcr of the inductor: ? v o ? i l esr co 1 8 fc o ? ? ------------------------- + ?? ?? ? = ? v o ? i l 1 8 fc o ? ? ------------------------- ? = ? v o ? i l esr co ? = i co_rms ? i l 12 ---------- = p total_loss v in i in ? ?? v o i o ? ?? ? = p diode_loss i o 1 d ? ?? v fw_schottky ? ? = p inductor_loss i o 2 r inductor 1.1 ? ? =
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 9 of 12 the actual junction temperature can be calculated with power dissipation in the aoz1084 and thermal impedance from junction to ambient. the maximum junction temperature of aoz1084 is 150oc, which limits the maximum load current capability. the thermal performance of the aoz1084 is strongly affected by the pcb layout. extra care should be taken by users during design process to ensure that the ic will operate under the recommended environmental conditions. several layout tips are listed below for the best electric and thermal performance. 1. input capacitor should be connected to the vin pin and the gnd pin as close as possible. 2. the inductor should be placed as close as possible the lx pin and the output capacitor. 3. keep the connection of schottky diode between the lx pin and the gnd pin as short and wide as possible. 4. place the feedback resistors and compensation components as close to the chip as possible. 5. keep sensitive signal trace far away from the lx pin. 6. pour a maximized copper area to the vin pin, the lx pin and especially the gnd pin to help thermal dissipation. 7. pour copper plane on all unused board area and connect it to stable dc nodes, like vin,gnd or vout. t junction p total_loss p diode_loss p inductor_loss ? ? ??? ? ja t ambient + =
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 10 of 12 package dimensions, dfn2x2-8l top view bottom view side view recommended land pattern notes: 1. dimensions and tolerances conform to asme y14.5m-1994. 2. controlling dimension is millimeter, converted inch dimensions are not necessarily exact. 3. dimension b applied to metallized terminal and is measured between 0.10mm and 0.30mm from the terminal tip. if the terminal has the optional radius on the other end of the terminal, dimension b should not be measured in that radius area. 4. coplanarity ddd applies to the terminals and all other bottom surface metallization. symbols a a1 b c d d1 e e1 e l r aaa bbb ccc ddd dimensions in millimeters min. 0.70 0.00 0.18 1.35 0.75 0.20 nom. 0.75 0.02 0.25 0.20 ref. 2.00 bsc 1.50 2.00 bsc 0.90 0.50 bsc 0.30 0.20 0.15 0.10 0.10 0.08 max. 0.80 0.05 0.30 1.60 1.00 0.40 symbols a a1 b c d d1 e e1 e l r aaa bbb ccc ddd dimensions in inches min. 0.028 0.000 0.007 0.053 0.030 0.008 nom. 0.030 0.001 0.010 0.008 ref. 0.079 bsc 0.059 0.079 bsc 0.035 0.020 bsc 0.012 0.008 0.006 0.004 0.004 0.003 max. 0.031 0.002 0.012 0.063 0.039 0.016 unit: mm e d b e l d1 chamfer 0.2 x 45 a1 seating plane a 0.50 0.25 1.70 1.50 0.30 0.90 0.85 c e1 r bottom view pin #1 id option 2 pin #1 id option 1
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 11 of 12 tape and reel dimensions, dfn2x2 carrier tape reel leader / trailer & orientation tape size 8mm reel size ?180 m ?180.00 0.50 package dfn 2x2 a0 2.25 0.05 b0 2.25 0.05 k0 1.00 0.05 d0 1.50 +0.10/-0 d1 1.00 +0.25/-0 e 8.00 +0.30/-0.10 e1 1.75 0.10 e2 3.50 0.05 p0 4.00 0.10 p1 4.00 0.10 p2 2.00 0.10 t 0.254 0.02 n 60.0 0.50 w1 r m s k h n trailer tape 300mm min. components tape orientation in pocket leader tape 500mm min. p1 p0 unit: mm unit: mm a0 b0 t k0 p2 e e2 e1 d0 d1 feeding direction w1 8.4 +1.5/-0.0 h 13.0 0.20 s 1.5 min. k 13.5 min. r 3.0 0.50
aoz1084 rev. 0.3 april 2012 www.aosmd.com page 12 of 12 part marking AOZ1084DI (dfn2x2-8) 12 9b assembly lot code part number code option code assembly location code an1a week & year code as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user. 2. a critical component in any component of a life support, device, or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. this data sheet contains preliminary data; supplementary data may be published at a later date. alpha & omega semiconductor reserves the right to make changes at any time without notice. life support policy alpha & omega semiconductor products are not authorized for use as critical components in life support devices or systems.

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