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ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 1 of 14 www.diodes.com july 2011 ? diodes incorporated new product description the ap6508 is a 500khz switching frequency internal compensated synchronous dcdc buck converter. it has integrated compensation, and low r dson high and low side mosfets. the ap6508 enables continues load current of up to 3a with efficiency as high as 93%. the ap6508 features current m ode control operation, which enables fast transient response times and easy loop stabilization. the ap6508 has external programmable softstart and a power good indicator enabling sequencing and ramp control. the ap6508 simplifies board layout and reduces space requirements with its high level of integration and minimal need for external components, making it ideal for distributed power architectures. the ap6508 is available in a standard green dfn4030-14 package with exposed pad for improved thermal performance and is rohs compliant. pin assignments sw bst sw pg sw agnd gnd gnd 9 6 5 4 3 2 1 12 13 14 11 10 sw in dfn4030-14 exposed pad (top view) ss vcc en 7 fb 8 features ? v in 4.5v to 21v ? v out adjustable to 0.8v ? 500khz switching frequency ? enable pin ? external softstart ? power good ? protection o ocp o thermal shutdown ? lead free finish/ rohs compliant (note 1) applications ? gaming consoles ? tv sets and monitors ? set top boxes ? distributed power systems ? home audio ? consumer electronics note: 1. eu directive 2002/95/ec (rohs). all applicabl e rohs exemptions applied. please visit our website at http://www.diodes.com/products/lead_free.html . typical application circuit typical application circuit 40 50 60 70 80 90 100 0 2 1 3 load current (a) efficiency vs. load current v = 12v v = 5v in in v = 2.5v out e f f i c ien c y (%)
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 2 of 14 www.diodes.com july 2011 ? diodes incorporated new product pin descriptions pin # name description 1 in supply voltage. the ap6508 operates from a 4. 5v to 21v input rail. c1 is needed to decouple the input rail. use wide pc b trace to make the connection. 2,3,4,5 sw switch output. use wide pcb trace to make the connection. 6 bst bootstrap. a capacitor conn ected between sw and bs pins is required to form a floating supply across the high-side switch driver. 7 en en=1 to enable the chip. for automatic start-up, connect en pin to vin by proper en resistor divider as figure 1 shows. 8 fb feedback. an external resistor divider from the output to gnd, tapped to the fb pin, sets the output voltage. to prevent curr ent limit run away during a s hort circuit fault condition the frequency fold-back comparator lowers the oscillator frequency when the fb voltage is below 500mv. 9 pg power good 10 ss external softstart 11 v cc bias supply. decouple with 0. 1f ? 0.22 f cap. and the capacitance should be no more than 0.22 f 12, 13 gnd system ground. this pin is the reference gro und for the regulated output voltage. for this reason care must be taken in its pcb layout . suggested to be connected to gnd with copper and vias. 14 agnd analog ground exposed pad no internal connection. it is recommended to connect exposed pad to gnd plane for optimal thermal performance functional block diagram
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 3 of 14 www.diodes.com july 2011 ? diodes incorporated new product absolute maximum ratings (note 2) symbol parameter rating unit v in supply voltage 22 v v sw switch node volta ge -0.3 to 23 v v bs bootstrap voltage vsw + 6 v v fb feedback voltage ?0.3v to +6 v v en enable/uvlo voltage ?0.3v to +6 v v comp comp voltage ?0.3v to +6 v t st storage temperature -65 to +150 c t j junction temperature +150 c t l lead temperature +260 c esd susceptibility (note 3) hbm human body model 4 kv mm machine model 400 v cdm charged device model 1 kv thermal resistance (note 4) symbol parameter rating unit ja junction to ambient 48 c/w jc junction to case 11 c/w recommended operating conditions (note 5) symbol parameter min max unit v in supply voltage 4.5 21 v t a operating ambient te mperature range -20 +85 c notes: 2. exceeding these ratings may damage the device. 3. semiconductor devices are esd sensitive and may be damaged by exposure to esd events. suitable esd precautions should be ta ken when handling and transporting these device. 4. test condition for dfn4030-14: measured on approximately 1? square of 1 oz copper. 5. the device function is not guaranteed outside of the recommended operating conditions.
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 4 of 14 www.diodes.com july 2011 ? diodes incorporated new product electrical characteristics (v in = 12v, t a = +25c, unless otherwise noted) symbol parameter test conditions min typ. max unit i in shutdown supply current v en = 0v 0 a i in supply current (quiescent) v en = 2.0v, v fb = 1.0v 1.2 ma r ds(on)1 high-side switch on-resistance (note 6) 120 m ? r ds(on)2 low-side switch on-resistance (note 6) 20 m ? sw lkg switch leakage current v en = 0v, v sw = 0v 0 10 a i limit current limit 5.8 a f sw oscillator frequency v fb = 0.75v 350 500 650 khz f fb fold-back frequency v fb = 300mv 0.3 f sw d max maximum duty cycle v fb = 700mv 80 85 % v fb feedback voltage t a = -20c to +85c 791 807 823 mv i fb feedback current v fb = 800mv 10 50 na v en_rising en rising threshold 1.1 1.3 1.5 v v en_hys en threshold hysteresis 0.4 v i en en input current v en = 2v 2 a v en = 0v 0 en td-off en turn off delay (note 6) 5 s pg vth-hi power good rising threshold 0.9 v fb pg vth-lo power good falling threshold 0.7 v fb pg td power good delay 20 s v pg power good sink current capability 0.4 v i pg_leak power good leakage current 10 na i ss soft-start current 10.5 a inuv vth v in under voltage threshold rising 4.0 4.2 4.4 v inuv hys v in under voltage threshold hysteresis 200 mv v cc v cc regulator 5 v v cc load regulation icc=5ma 5 % t sd thermal shutdown 140 c note: 6. guaranteed by design
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 5 of 14 www.diodes.com july 2011 ? diodes incorporated new product typical performance characteristics 1 1.05 1.1 1.15 1.2 1.25 1.3 0 5 10 15 20 25 input voltage quiescent supply current vs. input voltage quiescent supply current (ma) 0 2 4 6 8 10 12 14 16 0 5 10 15 20 25 input voltage (v) shutdown supply vs. input voltage shutd o wn suppl y c u r r ent ( a ) 5.02 5.025 5.03 5.035 5.04 5.045 5.05 0 5 10 15 20 25 input voltage (v) v regulator line regulation cc v (v) cc -10 -20 0 20 10 30 50 40 60 8070 90 temperature (c) current limit vs. temperature 7 6.6 6 5 6.8 6.2 6.4 5.6 5.8 5.2 5.4 c u r r e n t li m i t ( a ) v = 12v v = 1.2v in out 1.22 1.2205 1.221 1.2215 1.222 1.2225 1.223 1.2235 1.224 1.2245 1.225 0 5 10 15 20 25 input voltage (v) line regulation vs. output current o u t p u t v o lt a g e (v) 11.5 00.5 2.53 2 output current (a) load regulation vs. output current v = 5v in v = 12v in 1.208 1.21 1.204 1.206 1.2 1.202 1.196 1.198 1.192 1.194 o u t p u t vol t a g e (v)
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 6 of 14 www.diodes.com july 2011 ? diodes incorporated new product typical performace characteristics (cont.) 40 50 60 70 80 90 100 0 2 1 3 load current (a) efficiency vs. load current v = 12v v = 5v in in v = 1.2v out e f f i c i e n c y ( % ) 40 50 60 70 80 90 100 0 2 1 3 load current (a) efficiency vs. load current v = 12v v = 5v in in v = 1.8v out e f f i c i e n c y ( % ) 40 50 60 70 80 90 100 0 2 1 3 load current (a) efficiency vs. load current v = 12v v = 5v in in v = 2.5v out e f f i c i e n c y ( % ) 40 50 60 70 80 90 100 0 2 1 3 load current (a) efficiency vs. load current efficiency (%) 40 50 60 70 80 90 100 0 2 1 3 load current (a) efficiency vs. load current v = 12v v = 5v in out e f f i c ie n c y (%)
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 7 of 14 www.diodes.com july 2011 ? diodes incorporated new product typcal performance characteristics v in = 12v, v out = 1.2v, l = 3.3h, c1 = 22f, c2 = 47f, t a = +25 c, unless otherwise noted. time- 2s/div steady state test i out = 3a time- 200s/div load transient test i out = 1.5a to 3a. step at 0.8a/s time- 500s/div start-up through enable (no load) time- 2ms/div start-up through v in (no load) time- 50s/div shutdown through enable (no load) time- 50s/div shutdown through enable (iout =1a) time- 50s/div short circuit entry time- 100s/div short circuit recovery
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 8 of 14 www.diodes.com july 2011 ? diodes incorporated new product typcal performance characteristics v in = 12v, v out = 1.2v, l = 3.3h, c1 = 22f, c2 = 47f, t a = +25 c, unless otherwise noted. time- 1s/div input voltage ripple time- 2s/div output voltage ripple time- 1us/div powergood rising threshold time- 1s/div powergood fallingthreshold 90% of vfb 72% of vfb
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 9 of 14 www.diodes.com july 2011 ? diodes incorporated new product application information theory of operation the ap6508 is a 3a current mode control, synchronous buck regulator with built in power mosfets. current mode control assures excellent line and load regulation and a wide loop bandwidth for fast response to load transients. figure. 2 depicts the functional block diagram of ap6508. the operation of one switching cycle can be explained as follows. at the beginning of each cycle, hs (high-side) mosfet is off. the ea outpu t voltage is higher than the current sense amplifier output, and the current comparator?s output is low. the rising edge of the 500khz oscillator clock signal sets the rs flip-flop. its output turns on hs mosfet. the current sense amplifier is reset for every switching cycle. when the hs mosfet is on, in ductor current starts to increase. the current sense amplifier senses and amplifies the inductor curren t. since the current mode control is subject to sub-harm onic oscillations that peak at half the switching frequency, ramp slope compensation is utilized. this will help to stabilize the power supply. this ramp compensation is summed to the current sense amplifier output and compared to the error amplifier output by the pwm comparator. when the sum of the current sense amplifie r output and the slope compensation signal exceeds the ea output voltage, the rs flip-flop is reset and hs mosfet is turned off. for one whole cycle, if the sum of the current sense amplifier output and the slope compensation signal does not exceed the ea output, then the falling edge of the oscillator clock rese ts the flip-flop. the output of the error amplifier increases when feedback voltage (vfb) is lower than the reference voltage of 0.807v. this also increases the inductor current as it is proportional to the ea voltage. if in one cycle the current in the power mosfet does not reach the comp set current value, the power mosfet will be forced to turn off. when the hs mosfet turns off, the synchronous ls mosfet turns on until the next clock cycle begins. there is a 520ns ?dead time? between the hs turn off and ls turn on that prevents the switc hes from ?shooting through? from the input supply to ground. the voltage loop is internally compensated with the 50pf and 200k ? rc network. the maximum eamp voltage output is precisely clamped at 2.1v. internal regulator most of the internal circuitries including the bottom driver are powered from the 5v internal regulator. this regulator uses the vin input to regulat e at 5v. when vin is less than 5v, this internal regulato r cannot maintain the 5v regulation and hence the output voltage would also drop from regulation. enable the enable (en) input allows the user to control turning on or off the regulator. to enable the regulator en must be pulled above the ?en rising threshold? and to disable the regulator en must be pulled below ?en falling threshold? (en rising threshold ? en threshold hysteresis). few conditions on en function: 1) en must be pulled low for atleast 5us to disable the regulator. 2) the voltage on en cannot exceed 5v. 3) ap6508 can be enabled by vin through a voltage divider as shown in the figure 1 below. figure 1. en divider network 1m ? ||r 1m ? ||r(r v v bot bot top riseen risein + = ? ? where riseen v ? = 1.3v(typ) 1m ? ||r 1m ? ||r(r v v bot bot top fallen fallin + = ? ? where fallen v ? = 0.9v(typ) power good power good is an open drain and active high output. this output can be pulled up high to the appropriate level with an external resistor. the pgood is flagged low when vfb=0.7v and is an open drain output when vfb=0.9v. the pgood output can deliv er a max of 4 ma sink current at 0.4 v when de- as serted. the pgood pin is held low during soft-start. once output voltage reaches 90% of its final value, pgood goes high if there are no faults.
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 10 of 14 www.diodes.com july 2011 ? diodes incorporated new product application information (cont.) external soft start soft start is traditionally implemented to prevent the excess inrush current. this in turn prevents the converter output voltage from overshooting when it reaches regulation. the ap6508 has an internal current source with a soft start capacitor to ramp the reference voltage from 0v to 0.807v. the soft start time is internally fixed at 2ms (typ). the soft start time can be extended > 2ms by adding a soft start capacitor externally. the soft start sequence is reset when there is a thermal shutdown, under voltage lockout (uvlo) or when the part is disabled using the en pin. external soft start can be calculated from the formula below: dt dv *c ss i = where; iss = soft start current c = external capacitor dv=change in output voltage from 0v to maximum voltage dt = soft start time current limit protection the ap6508 has cycle-by-cycle current limiting implementation. the voltage drop across the internal high- side mosfet is sensed and compared with the internally set current limit threshold. this voltage drop is sensed at about 30ns after the hs turns on. when the peak inductor current exceeds the set current limit threshold, current limit protection is activated. during this time the feedback voltage (vfb) drops down. when the voltage at the fb pin reaches 0.3v, the internal oscillator shifts the frequency from the normal operating frequency of 500khz to a fold- back frequency of 150khz. the current limit is reduced to 70% of nominal current limit when the part is operating at 150khz. this low fold-back frequency prevents runaway current. under voltage lockout (uvlo) under voltage lockout is implemented to prevent the ic from insufficient input voltages. the ap6508 has a uvlo comparator that monitors the inout voltage and internal bandgap reference. if the input voltage falls below 3.8v, the ap6508 will latch an under voltage fault. in this event the output will be pulled low and power has to be re- cycled to reset the uvlo fault. thermal shutdown the ap6508 has on-chip thermal protection that prevents damage to the ic when the die temperature exceeds safe margins. it implements a thermal sensing to monitor the operating junction temperature of the ic. once the die temperature rises to approximately 140c, the thermal protection feature gets activated .the internal thermal sense circuitry turns the ic off thus preventing the power switch from damage. a hysteresis in the thermal sense circuit allows the device to cool down to approximately 120c before the ic is enabled again through soft start. this thermal hysteresis feature prevents undesirable oscillations of the thermal protection circuit. setting the output voltage the output voltage can be adjusted from 0.81v to 15v using an external resistor divider. table 1 shows a list of resistor selection for common output voltages. resistor r1 is selected based on a design tradeoff between efficiency and output voltage accuracy. for high values of r1 there is less current consumption in the feedback network. however the trade off is output voltage accuracy due to the bias current in the error amplifier. r2 can be determined by the following equation: ? ? ? ? ? ? ? ? ??= 1 0.81 out v 2 r 1 r figure 2. feedback divider network when output voltage is low, t-type network as shown in figure 2 is recommended. v out (v) r 1 (k ? ) r 2 (k ? ) r t (k ? ) 1.2 4.99 10.2 24.9 1.8 4.99 (1%) 4.02 (1%) 24.9 2.5 40.2 (1%) 19.1 (1%) 0 3.3 40.2 (1%) 13 (1%) 10 5 40.2 (1%) 7.68 (1%) 35.7 table 1.resistor selection for common output voltages inductor calculating the inductor value is a critical factor in designing a buck converter. for most designs, the following equation can be used to calculate the inductor value; swlin out in out f ? iv )v(vv l ?? ?? = where l i is the inductor ripple current. and sw f is the buck converter switching frequency. choose the inductor ripple current to be 30% of the maximum load current. the maximum inductor peak current is calculated from: 2 ?i ii l load l(max) +=
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 11 of 14 www.diodes.com july 2011 ? diodes incorporated new product application information (cont.) inductor (cont.) peak current determines the required saturation current rating, which influences the size of the inductor. saturating the inductor decreases the converter efficiency while increasing the temperatures of the inductor and the internal mosfets. hence choosing an inductor with appropriate saturation current rating is important. a 1h to 10h inductor with a dc current rating of at least 25% percent higher than the maximum load current is recommended for most applications. for highest efficiency, the inductor?s dc resistance should be less than 200m ? . use a larger inductance for improved efficiency under light load conditions. input capacitor the input capacitor reduces the surge current drawn from the input supply and the switching noise from the device. the input capacitor has to sustain the ripple current produced during the on time on the upper mosfet. it must hence have a low esr to minimize the losses. the rms current rating of the input capacitor is a critical parameter that must be higher than the rms input current. as a rule of thumb, select an input capacitor which has an rms rating that is greater than half of the maximum load current. due to large di/dt through the input capacitors, electrolytic or ceramics should be used. if a tantalum must be used, it must be surge protected. otherwise, capacitor failure could occur. for most applications, a 4.7f ceramic capacitor is sufficient. output capacitor the output capacitor keeps the output voltage ripple small, ensures feedback loop stability and reduces the overshoot of the output voltage. the output capacitor is a basic component for the fast response of the power supply. in fact, during load transient, for the first few microseconds it supplies the current to the load. the converter recognizes the load transient and sets the duty cycle to maximum, but the current slope is limited by the inductor value. maximum capacitance required can be calculated from the following equation: 2 out v 2 ) out v v( 2 ) 2 inductor i out l(i o c ?+ + = where ?v is the maximum output voltage overshoot. esr of the output capacitor dominates the output voltage ripple. the amount of ripple can be calculated from the equation below: esr* inductor i capacitor vout = an output capacitor with ample capacitance and low esr is the best option. for most applications, a 22f ceramic capacitor will be sufficient. pc board layout this is a high switching frequency converter. hence attention must be paid to the switching currents interference in the layout. switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces. these interconnecting impedances should be minimized by using wide, short printed circuit traces. ap6508 is exposed at the bottom of the package and must be soldered directly to a well designed thermal pad on the pcb. this will help to increase the power dissipation. external bootstrap diode it is recommended that an external bootstrap diode be added when the input voltage is no greater than 5v or the 5v rail is available in the system. this helps to improve the efficiency of the regulator. this solution is also applicable for d > 65%. the bootstrap diode can be a low cost one such as bat54 or a schottky that has a low vf. ap6508 bst sw 10nf boost diode 5v 6 2,3,4,5 figure 3. external bootstrap diode input capacitor c1 must be placed as close as possible to the ic and to l1. 34mm 52mm external feedback resistor dividers must be placed close to the fb pin
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ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 13 of 14 www.diodes.com july 2011 ? diodes incorporated new product tape orientation (note 8) note: 8. the taping orientation of the other package type c an be found on our website at http://www.diodes.com/datasheets/ap 02007.pdf
ap6508 500khz 21v 3a synchronous dc/dc buck converter ap6508 document number: ds33437 rev. 4 - 2 14 of 14 www.diodes.com july 2011 ? diodes incorporated new product important notice diodes incorporated makes no warranty of any kind, express or implied, with regards to this document, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose (and their equivalents under the laws of any jurisdiction). diodes incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or ot her changes without further notice to this document and any product described herein. diodes incorporated does not assume any liabi lity arising out of the application or use of this document or any product described herein; neither does diodes incorporated convey any license under its patent or trademark rights, nor the rights of others. any customer or user of this document or products desc ribed herein in such applications shall assume all risks of such use and will agree to hold diodes incorporated and all the companies whose products are represented on diodes incorporated website, harmless against all damages. diodes incorporated does not warrant or accept any liability whatsoever in respect of any products purchased through unauthoriz ed sales channel. should customers purchase or use diodes incorporated products for any unintended or unauthorized application, customers shall indemnify and hold diodes incorporated and its representatives harmless against all claims, damages, expenses, and attorney fee s arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized a pplication. products described herein may be covered by one or more united states, international or foreign patents pending. product names and markings noted herein may also be covered by one or more united states, international or foreign trademarks. life support diodes incorporated products are specifically not authorized for use as critical components in life support devices or systems without the express written approval of the chief executive officer of diodes incorporated. as used herein: a. life support devices or systems are devices or systems which: 1. are intended to implant into the body, or 2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in significant injury to the user. b. a critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support dev ices or systems, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of diodes incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systems-related information or support that may be provided by diodes incorporated. further, customers must fully indemnify diodes incorporated and its representatives against any damages arising out of the use of diodes incorporated products in such safety-critical, life support devices or systems. copyright ? 2011, diodes incorporated www.diodes.com

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