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RT8258 1 ds8258-02 march 2011 www.richtek.com ordering information note : richtek products are : ` rohs compliant and compatible with the current require- ments of ipc/jedec j-std-020. ` suitable for use in snpb or pb-free soldering processes. 1.2a, 24v, 700khz step-down converter general description the RT8258 is a high voltage buck converter that can support the input voltage range from 4.5v to 24v and the output current can be up to 1.2a. current mode operation provides fast transient response and eases loop stabilization. the chip also provides protection functions such as cycle- by-cycle current limiting and thermal shutdown protection. the RT8258 is available in a sot-23-6 and tsot-23-6 packages. features z z z z z wide operating input voltage range : 4.5v to 24v z z z z z adjustable output voltage range : 0.8v to 15v z z z z z 1.2a output current z z z z z 0.3 internal power mosfet switch z z z z z high efficiency up to 92% z z z z z 700khz fixed switching frequency z z z z z stable with low esr output ceramic capacitors z z z z z thermal shutdown z z z z z cycle-by-cycle over current protection z z z z z rohs compliant and halogen free applications z distributed power systems z battery charger z pre-regulator for linear regulators z wled drivers typical application circuit pin configurations (top view) marking information for marking information, contact our sales representative directly or through a richtek distributor located in your area. boot gnd fb phase vin en 4 23 5 6 sot-23-6/tsot-23-6 vin en gnd boot fb phase 4 2 3 5 6 1 l 10h c boot 10nf c out 22f r1 100k r2 32.4k v out 3.3v c in 10f chip enable v in 4.5v to 24v RT8258 d1 b230a RT8258 package type e : sot-23-6 j6 : tsot-23-6 lead plating system g : green (halogen free and pb free)
RT8258 2 ds8258-02 march 2011 www.richtek.com function block diagram table 1. recommended component selection functional pin description pin no. pin name pin function 1 boot gate driver bootstrap input pin. connect a 10nf or greater capacitor between phase and boot pins to supply the mosfet driver. 2 gnd ground pin. this pin should be connected to the (-) terminal of the output capacitor and it should be kept away from the d1 and input capacitor for noise prevention. 3 fb output voltage feedback input pin. an external resistor divider from the output to gnd tapped to the fb pin sets the output voltage. the value of the divider resistors also set loop bandwidth. 4 en chip enable (active high). if the en pin is open, it will be pulled to high by internal circuit. 5 vin power supply input pin. bypass vin to gnd with a suitable large capacitor to prevent large voltage spikes from appear ing at the input. 6 phase power switching output pin. connect this pin to the output inductor. v out 1.2v 1.5v 1.8v 2.5v 3.3v 5v 8v 10v 15v r1 (k ) 100 91 91 100 100 91 91 91 120 r2 (k ) 200 100 75 47 32.4 17.4 10 7.87 6.8 l ( h) 3.6 3.6 4.7 6.8 10 15 22 22 33 c ou t ( f) 22 22 22 22 22 22 22 22 22 note : the value of r1 is related to the loop bandwidth of the RT8258. it is strongly recommended to follow the parameters in above table for the specific output voltage. driver r q s bootstrap control + - ramp generator oscillator 700khz + - pwm comparator ea reference regulator + - 1.1v 1a + - 400k 30pf 1pf shutdown comparator oc limit clamp current sense amp x20 boot gnd fb en vin phase 10k 3v 25m
RT8258 3 ds8258-02 march 2011 www.richtek.com electrical characteristics parameter symbol test conditions min typ max unit feedback reference voltage v fb 4.5v v in 24v 0.784 0.8 0.816 v feedback current i fb v fb = 0.8v -- 0.1 0.3 a switch on resistance r ds(on) -- 0.3 -- switch leakage v en = 0v, v phase = 0v -- -- 10 a current limit i lim v boot ? v p has e = 4.8v 1.6 2.1 -- a oscillator frequency f sw 600 700 800 khz maximum duty cycle -- 90 -- % minimum on-time t on -- 100 -- ns under voltage lock out threshold voltage rising 3.9 4.2 4.5 v under voltage lock out threshold hysteresis -- 270 -- mv logic high 1.4 -- -- en input voltage logic low -- -- 0.4 v en pull up current v en = 0v -- 1 -- a shutdown current i shdn v en = 0v -- 25 -- a quiescent current i q v en = 2v, v fb = 1v (no switching) -- 0.55 1 ma thermal shutdown t sd -- 150 -- c (v in = 12v, t a = 25 c unless otherwise specified) absolute maximum ratings (note 1) z supply voltage, v in ------------------------------------------------------------------------------------------------ 26v z phase v oltage ----------------------------------------------------------------------------------------------------- ? 0.3v to (v in + 0.3v) z boot v oltage ------------------------------------------------------------------------------------------------------- v phase + 6v z all other pins -------------------------------------------------------------------------------------------------------- 0.3v to 6 v z power dissipation, p d @ t a = 25 c t/sot-23-6 ----------------------------------------------------------------------------------------------------------- 0.4w z package thermal resistance (note 2) t/sot-23-6, ja ------------------------------------------------------------------------------------------------------ 250 c/w z junction temperature ---------------------------------------------------------------------------------------------- 150 c z lead temperature (soldering, 10 sec.) ------------------------------------------------------------------------ 260 c z storage temperature range -------------------------------------------------------------------------------------- ? 65 c to 150 c z esd susceptibility (note 3) hbm (human body mode) ---------------------------------------------------------------------------------------- 2kv mm (machine mode) ----------------------------------------------------------------------------------------------- 200v recommended operating conditions (note 4) z supply voltage, v in ------------------------------------------------------------------------------------------------ 4.5v to 24v z output voltage, v out ---------------------------------------------------------------------------------------------- 0.8v to 15v z en voltage, v en ----------------------------------------------------------------------------------------------------- 0v to 5.5v z junction temperature range ------------------------------------------------------------------------------------- ? 40 c to 125 c z ambient temperature range ------------------------------------------------------------------------------------- ? 40 c to 85 c
RT8258 4 ds8258-02 march 2011 www.richtek.com note 1. stresses listed as the above "absolute maximum ratings" may cause permanent damage to the device. these are for stress ratings. functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliability. note 2. ja is measured in the natural convection at t a = 25 c on a low effective single layer thermal conductivity test board of jedec 51-3 thermal measurement standard. note 3. devices are esd sensitive. handling precaution is recommended. note 4. the device is not guaranteed to function outside its operating conditions.
RT8258 5 ds8258-02 march 2011 www.richtek.com typical operating characteristics efficiency vs. output current 0 10 20 30 40 50 60 70 80 90 100 0 0.2 0.4 0.6 0.8 1 1.2 load current (a) efficiency (%) output voltage vs. output current 3.234 3.256 3.278 3.300 3.322 3.344 3.366 0 0.2 0.4 0.6 0.8 1 1.2 output current (a) output voltage (v) output voltage vs. temperature 3.234 3.256 3.278 3.300 3.322 3.344 3.366 -50 -25 0 25 50 75 100 125 temperature (c) output voltage (v) frequency vs. temperature 550 600 650 700 750 -50-25 0 25 50 75100125 temperature (c) frequency (khz) 1 v out = 3.3v v in = 12v v in = 24v v in = 12v v in = 24v i out = 0a v in = 12v v in = 24v v in = 12v, v out = 3.3v, i out = 0a quiescent current vs. temperature 400 425 450 475 500 525 550 575 600 -50 -25 0 25 50 75 100 125 temperature (c) quiescent current ( a ) v in = 12v v in = 24v v en = 2v, v fb = 1v efficiency vs. output current 0 10 20 30 40 50 60 70 80 90 100 0 0.2 0.4 0.6 0.8 1 1.2 output current (a) efficiency (%) v in = 12v v in = 24v v out = 5v
RT8258 6 ds8258-02 march 2011 www.richtek.com load transient response time (250 s/div) v out (0.1v/div) v in = 12v, v out = 3.3v, i out = 0a to 1.2a i out (0.5a/div) load transient response time (50 s/div) i out (0.5a/div) v out (0.1v/div) v in = 12v, v out = 3.3v, i out = 0.6a to 1.2a output ripple voltage time (500ns/div) i l (1a/div) v phase (10v/div) v out (10mv/div) v in = 12v, v out = 3.3v, i out = 1.2a output ripple voltage time (500ns/div) i l (1a/div) v phase (10v/div) v out (10mv/div) v in = 24v, v out = 3.3v, i out = 1.2a power off from en time (50 s/div) v en (2v/div) v out (1v/div) v in = 12v, v out = 3.3v, i out = 2a power on from en time (100 s/div) v en (2v/div) v out (1v/div) v in = 12v, v out = 3.3v, i out = 1.2a
RT8258 7 ds8258-02 march 2011 www.richtek.com application information the RT8258 is an asynchronous high voltage buck converter that can support the input voltage range from 4.5v to 24v and the output current can be up to 1.2a. output voltage setting the resistive voltage divider allows the fb pin to sense a fraction of the output voltage as shown in figure 1. figure 1. output voltage setting for adjustable voltage mode, the output voltage is set by an external resistive voltage divider according to the following equation : ?? + ?? ?? out fb r1 v = v1 r2 out out l in vv i = 1 fl v ??? ? ?? ??? ? ??? ? having a lower ripple current reduces not only the esr losses in the output capacitors but also the output voltage ripple. high frequency with small ripple current can achieve highest efficiency operation. however, it requires a large inductor to achieve this goal. for the ripple current selection, the value of i l = 0.34(i max ) will be a reasonable starting point. the largest ripple current occurs at the highest v in . to guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : out out l(max) in(max) vv l = 1 fi v ??? ? ? ??? ? ??? ? inductor core selection the inductor type must be selected once the value for l is known. generally speaking, high efficiency converters can not afford the core loss found in low cost powdered iron cores. so, the more expensive ferrite or mollypermalloy cores will be a better choice. the selected inductance rather than the core size for a fixed inductor value is the key for actual core loss. as the inductance increases, core losses decrease. unfortunately, increase of the inductance requires more turns of wire and therefore the copper losses will increase. ferrite designs are preferred at high switching frequency due to the characteristics of very low core losses. so, design goals can focus on the reduction of copper loss and the saturation prevention. ferrite core material saturates ? hard ? , which means that inductance collapses abruptly when the peak design current is exceeded. the previous situation results in an abrupt increase in inductor ripple current and consequent output voltage ripple. inductor selection the inductor value and operating frequency determine the ripple current according to a specific input and output voltage. the ripple current i l increases with higher v in and decreases with higher inductance. RT8258 gnd fb r1 r2 v out external bootstrap diode connect a 10nf low esr ceramic capacitor between the boot pin and phase pin. this capacitor provides the gate driver voltage for the high side mosfet. it is recommended to add an external bootstrap diode between an external 5v and the boot pin for efficiency improvement when input voltage is lower than 5.5v or duty ratio is higher than 65%. the bootstrap diode can be a low cost one such as 1n4148 or bat54. the external 5v can be a 5v fixed input from system or a 5v output of the rt8268. where v fb is the feedback reference voltage (0.8v typ.). figure 2. external bootstrap diode phase boot RT8258 10nf 5v
RT8258 8 ds8258-02 march 2011 www.richtek.com c in and c out selection the input capacitance, c in, is needed to filter the trapezoidal current at the source of the top mosfet. to prevent large ripple current, a low esr input capacitor sized for the maximum rms current should be used. the rms current is given by : out in rms out(max) in out v v i = i 1 vv ? this formula has a maximum at v in = 2v out , where i rms = i out /2. this simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. choose a capacitor rated at a higher temperature than required. several capacitors may also be paralleled to meet size or height requirements in the design. the selection of c out is determined by the required effective series resistance (esr) to minimize voltage ripple. moreover, the amount of bulk capacitance is also a key for c out selection to ensure that the control loop is stable. loop stability can be checked by viewing the load transient response as described in a later section. the output ripple, v out , is determined by : out l out 1 viesr 8fc ?? ?? + ?? ?? the output ripple will be highest at the maximum input voltage since i l increases with input voltage. multiple capacitors placed in parallel may be needed to meet the esr and rms current handling requirement. dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. special polymer capacitors offer very low esr value. however, it provides lower capacitance density than other types. although tantalum capacitors have the highest capacitance density, it is important to only use types that pass the surge test for use in switching power supplies. aluminum electrolytic capacitors have significantly higher esr. however, it can be used in cost-sensitive applications for ripple current rating and long term reliability considerations. ceramic capacitors have excellent low esr characteristics but can have a high voltage coefficient and audible piezoelectric effects. the high q of ceramic capacitors with trace inductance can also lead to significant ringing. higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. their high ripple current, high voltage rating and low esr make them ideal for switching regulator applications. however, care must be taken when these capacitors are used at input and output. when a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, v in . at best, this ringing can couple to the output and be mistaken as loop instability. at worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at v in large enough to damage the part. do not allow the core to saturate! different core materials and shapes will change the size/ current and price/current relationship of an inductor. toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate energy. however, they are usually more expensive than the similar powdered iron inductors. the rule for inductor choice mainly depends on the price vs. size requirement and any radiated field/emi requirements. diode selection when the power switch turns off, the path for the current is through the diode connected between the switch output and ground. this forward biased diode must have a minimum voltage drop and recovery times. schottky diode is recommended and it should be able to handle those current. the reverse voltage rating of the diode should be greater than the maximum input voltage, and current rating should be greater than the maximum load current. for more detail, please refer to table 3. checking transient response the regulator loop response can be checked by looking at the load transient response. switching regulators take several cycles to respond to a step in load current. when a load step occurs, v out immediately shifts by an amount equal to i load (esr) and also begins to charge or discharge c out generating a feedback error signal for the regulator to return v out to its steady-state value. during this recovery time, v out can be monitored for overshoot or ringing that would indicate a stability problem.
RT8258 9 ds8258-02 march 2011 www.richtek.com layout consideration follow the pcb layout guidelines for optimal performance of RT8258. ` keep the traces of the main current paths as short and wide as possible. ` put the input capacitor as close as possible to the device pins (vin and gnd). ` phase node is with high frequency voltage swing and should be kept at small area. keep sensitive components away from the phase node to prevent stray capacitive noise pick-up. ` place the feedback components to the fb pin as close as possible. ` connect the gnd to a ground plane for noise reduction and thermal dissipation. thermal considerations for continuous operation, do not exceed the maximum operation junction temperature 125 c. the maximum power dissipation depends on the thermal resistance of ic package, pcb layout, the rate of surroundings airflow and temperature difference between junction to ambient. the maximum power dissipation can be calculated by following formula : p d(max) = (t j(max) ? t a ) / ja where t j(max) is the maximum operation junction temperature, t a is the ambient temperature and the ja is the junction to ambient thermal resistance. for recommended operating conditions specification of the RT8258, the maximum junction temperature of the die is 125 c. the junction to ambient thermal resistance ja is layout dependent. for t/sot-23-6 package, the thermal resistance ja is 250 c/w on standard jedec 51-3 single layer thermal test board. the maximum power dissipation at t a = 25 c can be calculated by following formula : p d(max) = (125 c ? 25 c) / (250 c/w) = 0.4w for t/sot-23-6 package the maximum power dissipation depends on operating ambient temperature for fixed t j(max) and thermal resistance ja . for RT8258 package, the figure 3 of derating curve allows the designer to see the effect of rising ambient temperature on the maximum power dissipation allowed. figure 3. derating curve for RT8258 package 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0255075100125 ambient temperature (c) maximum power dissipation (w) t/sot-23-6 single layer pc b figure 4. pcb layout guide boot gnd fb en vin phase 4 2 3 5 6 1 v out c b r2 r1 c in v out l c out gnd d1
RT8258 10 ds8258-02 march 2011 www.richtek.com table 3. suggested capacitors for c in and c out component supplier series dimensions (mm) tdk slf12555t 12.5x12.5x5.5 taiyo yuden nr8040 8x8x4 tdk slf12565t 12.5x12.5x6.5 table 2. suggested inductors for l component supplier series v rrm (v) i out (a) package diodes b230a 30 2 do-214ac diodes b330a 30 3 do-214ac panjit sk23 30 2 do-214ac panjit sk33 30 3 do-214ab table 4. suggested diode for d1 location component supplier part no. capacitance ( f) case size c in murata grm31cr61e106k 10 1206 c in tdk c3225x5r1e106k 10 1206 c in taiyo yuden tmk316bj106ml 10 1206 c out murata grm31cr61c226m 22 1206 c out tdk c3225x5r1c226m 22 1206 c out taiyo yuden emk316bj226ml 22 1206
RT8258 11 ds8258-02 march 2011 www.richtek.com a a1 e b b d c h l sot-23-6 surface mount package dimensions in millimeters dimensions in inches symbol min max min max a 0.889 1.295 0.031 0.051 a1 0.000 0.152 0.000 0.006 b 1.397 1.803 0.055 0.071 b 0.250 0.560 0.010 0.022 c 2.591 2.997 0.102 0.118 d 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 h 0.080 0.254 0.003 0.010 l 0.300 0.610 0.012 0.024 outline dimension
RT8258 12 ds8258-02 march 2011 www.richtek.com information that is provided by richtek technology corporation is believed to be accurate and reliable. richtek reserves the ri ght to make any change in circuit design, specification or other related things if necessary without notice at any time. no third party intellectual property infringemen t of the applications should be guaranteed by users when integrating richtek products into any application. no legal responsibility for any said applications is assumed b y richtek. richtek technology corporation headquarter 5f, no. 20, taiyuen street, chupei city hsinchu, taiwan, r.o.c. tel: (8863)5526789 fax: (8863)5526611 richtek technology corporation taipei office (marketing) 5f, no. 95, minchiuan road, hsintien city taipei county, taiwan, r.o.c. tel: (8862)86672399 fax: (8862)86672377 email: marketing@richtek.com tsot-23-6 surface mount package dimensions in millimeters dimensions in inches symbol min max min max a 0.700 1.000 0.028 0.039 a1 0.000 0.100 0.000 0.004 b 1.397 1.803 0.055 0.071 b 0.300 0.559 0.012 0.022 c 2.591 3.000 0.102 0.118 d 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 h 0.080 0.254 0.003 0.010 l 0.300 0.610 0.012 0.024 a a1 e b b d c h l

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