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ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 1 of 13 www.diodes.com september 2011 ? diodes incorporated description the ap6503 is a 340khz switching frequency external compensated synchronous dc/dc buck converter. it has integrated low r dson high and low side mosfets. the ap6503 enables continues load current of up to 3a with efficiency as high as 95%. the ap6503 features current m ode control operation, which enables fast transient response times and easy loop stabilization. the ap6503 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 ap6503 is available in a standard green so-8ep package with exposed pad for improved thermal performance and is rohs compliant. pin assignments comp sw gnd bs en fb ss ( top view ) 1 2 3 4 8 7 6 5 in so-8ep figure 1. package pin out features ? v in 4.75v to 23v ? 3a continuous output current, 4a peak ? v out adjustable to 0.925 to 20v ? 340khz switching frequency ? programmable soft-start ? enable pin ? protection ? ocp ? thermal shutdown ? lead free finish/ rohs compliant (note 1) applications ? gaming consoles ? flat screen tv sets and monitors ? set top boxes ? distributed power systems ? home audio ? consumer electronics ? network systems ? fpga, dsp and asic supplies ? green electronics note: 1. eu directive 2002/95/ec (rohs). all applic able rohs exemptions applied. please visit our website at http://www.diodes.c om/products/lead_free.html . typical application circuit figure 2. typical application circuit load current (a) efficiency vs. load current effi c ien c y (%) 0123 v = 12v in v = 5v in v = 3.3v l = 10h out 40 50 60 70 80 90 100
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 2 of 13 www.diodes.com september 2011 ? diodes incorporated pin descriptions pin # name description 1 bs high-side gate drive boost input. bs supplies the drive for the high-side n-channel mosfet switch. connect a 0.01f or greater capacitor from sw to bs to power the high side switch. 2 in power input. in supplies the power to the ic, as well as the step-down converter switches. drive in with a 4.75v to 24v power source. bypa ss in to gnd with a suitably large capacitor to eliminate noise on the input to the ic. see input capacitor. 3 sw power switching output. sw is the switching node that supplies power to the output. connect the output lc filter from sw to the output load. note that a capacitor is required from sw to bs to power the high-side switch. 4 gnd ground (connect the exposed pad to pin 4). 5 fb feedback input. fb senses the output voltage and regulates it. drive fb with a resistive voltage divider connected to it from the output voltage. the f eedback threshold is 0.925v. see setting the output voltage. 6 comp compensation node. comp is used to compens ate the regulation control loop. connect a series rc network from comp to gnd. in some cases, an additional ca pacitor from comp to gnd is required. see compensation components. 7 en enable input. en is a digital input that turns the regulator on or off. drive en high to turn on the regulator; low to turn it off. attach to in with a 100k ? pull up resistor for automatic startup. 8 ss soft-start control input. ss controls the soft-sta rt period. connect a capacitor from ss to gnd to set the soft-start period. a 0.1f capacitor se ts the soft-start period to 15ms. to disable the soft-start feature, leave ss floating. functional block diagram figure 3. functional block diagram
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 3 of 13 www.diodes.com september 2011 ? diodes incorporated absolute maximum ratings (note 2) symbol parameter rating unit v in supply voltage -0.3 to 26 v v sw switch node volt age -1.0 to v in +0.3 v v bs bootstrap voltage v sw -0.3 to v sw + 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 3 kv mm machine model 250 v notes: 2. stresses greater than the 'absolute maximum ratings' specified above, may cause permanent damage to the device. thes e are stress ratings only; functional operation of the device at these or any other conditions exceeding those indicated in this specification is no t implied. device reliability may be affected by exposure to absolute ma ximum rating conditions for extended periods of time. 3. semiconductor devices are esd sens itive and may be damaged by exposure to esd events. suitable esd precautions should be taken when handling and transporting these devices. thermal resistance (note 4) symbol parameter rating unit ja junction to ambient 74 c/w jc junction to case 16 c/w note: 4. test condition for so-8ep: measured on approximately 1? square of 1 oz copper recommended operating conditions (note 5) symbol parameter min max unit v in supply voltage 4.75 23 v t a operating ambient te mperature range -40 +85 c note: 5. the device function is not guarant eed outside of the recommended operating conditions.
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 4 of 13 www.diodes.com september 2011 ? diodes incorporated 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 0.3 3.0 a i in supply current (quiescent) v en = 2.0v, v fb = 1.0v 0.6 1.5 ma r ds(on)1 high-side switch on-resistance (note 6) 100 m ? r ds(on)2 low-side switch on-resistance (note 6) 100 m ? i limit hs current limit minimum duty cycle 5.5 a i limit ls current limit from drain to source 0.9 a high-side switch leakage current v en = 0v, v sw = 0v, v sw =12v 0 10 a avea error amplifier voltage gain (note 5) 800 v/v gea error amplifier transconductance ic = 10 a 1000 ua/v gcs comp to current sense transconductance 2.8 a/v f sw oscillator frequency v fb = 0.75v 300 340 380 khz f fb fold-back frequency v fb = 0v 0.30 f sw d max maximum duty cycle v fb = 800mv 90 % t on minimum on time 200 ns v fb feedback voltage t a = -40c to +85c 900 925 950 mv feedback overvoltage threshold 1.1 v v en_rising en rising threshold 0.7 0.8 0.9 v en lockout threshold voltage 2.2 2.5 2.7 v en lockout hysteresis 220 mv inuv vth v in under voltage threshold rising 3.80 4.05 4.40 v inuv hys v in under voltage threshold hysteresis 250 mv soft-start current v ss = 0v 6 a soft-start period c ss = 0.1f 15 ms t sd thermal shutdown 150 c note: 6. guaranteed by design
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 5 of 13 www.diodes.com september 2011 ? diodes incorporated typical performance graphs (v in = 12v, v out =3.3v ,t a = +25c, unless otherwise noted) quiescent supply current (ma) 0.48 0.5 0.52 0.54 0.56 0.58 0.6 0 5 10 15 20 25 input voltage (v) quiescent supply current vs. input voltage shutdown supply current (a) 0 5 10 15 20 25 input voltage (v) shutdown supply current vs. input voltage 0.004 0.014 0.024 0.034 0.044 0.054 0.064 0.074 current limit (a) 4.8 5 5.2 5.4 5.6 5.8 6 6.2 -60 -40 -20 0 20 40 60 80 100 temperature (c) current limit vs. temperature output voltage (v) 3.32 3.321 3.322 3.323 3.324 3.325 3.326 3.327 3.328 3.329 3.33 4.75 9.75 14.75 19.75 24.75 input voltage (v) line regulation -60 -40 -20 0 20 40 60 80 100 temperature (c) feedback voltage vs. temperature feedba c k v o l t age (v) 0.9 0.902 0.904 0.906 0.908 0.91 0.912 0.914 0.916 0.918 0.92 350 355 360 365 370 375 -60 -40 -20 0 20 40 60 80 100 temperature (c) oscillator frequency vs. temperature oscillator frequency (khz) 40 45 50 55 60 65 70 75 80 85 90 load current (a) efficiency vs. load current effi c ien c y ( % ) 0123 v = 12v in v = 5v in v = 1.2v l = 3.3h out 40 45 50 55 60 65 70 75 80 85 9 0 load current (a) efficiency vs. load current effi c ien c y ( % ) 0123 v = 12v in v = 5v in v = 1.8v l = 3.3h out 40 50 60 70 80 90 100 load current (a) efficiency vs. load current effi c ien c y ( % ) 0123 v = 12v l = 10h in v = 5v out
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 6 of 13 www.diodes.com september 2011 ? diodes incorporated typical performance characteristics (v in = 12v, v out =3.3v ,l=3.3h, c1=22uf, c2=47uf, t a = +25c, unless otherwise noted) steady state test no load time-2us/div steady state test 3a time-2us/div startup through enable_no load time-10ms/div startup through enable 3a time-10ms/div shutdown through enable_no load time-10ms/div shutdown through enable 3a time-5ms/div load transient test 1.5 to 3a time-100us/div short circuit test time-20us/div short circuit recovery time-20us/div
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 7 of 13 www.diodes.com september 2011 ? diodes incorporated applications information theory of operation the ap6503 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 3 depicts the functional block diagram of ap6503. 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 340khz 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 0.925v. 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. w hen the hs mosfet turns off, the synchronous ls mosfet tu rns on until the next clock cycle begins. there is a ?dead time? between the hs turn off and ls turn on that prevents the switches from ?shooting through? from the input supply to ground. the voltage loop is compensated through an internal transconductance amplifier and can be adjusted through the external compensation components. enable the enable (en) input allows t he 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). external soft start soft start is traditionally implemented to prevent the excess inrush current. this in turn prevents the converter output voltage from overs hooting when it reaches regulation. the ap6503 has an internal current source with a soft start capacitor to ramp the reference voltage from 0v to 0.925v. the soft start current is 6ua. 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; i ss = soft start current c = external capacitor dv=change in feedback voltage from 0v to maximum voltage dt = soft start time current limit protection in order to reduce the total power dissipation and to protect the application, ap6503 has cycle-by-cycle current limiting implementation. th e 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 340khz to a fold-back frequency of 102khz. the current limit is reduced to 70% of nom inal current limit when the part is operating at 102khz. 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 ap6503 has a uvlo comparator that monitors the input voltage and the internal bandgap reference. if the input voltage falls below 4.0v, the ap6503 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. over voltage protection when the ap6503 fb pin exceeds 20% of the nominal regulation voltage of 0.925v, the over voltage comparator is tripped and the comp pin and the ss pin are discharged to gnd, forci ng the high-side switch off.
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 9 of 13 www.diodes.com september 2011 ? diodes incorporated applications information (cont.) thermal shutdown the ap6503 has on-chip therma l 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 approxim ately 150c, the thermal protection feature gets acti vated .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 o scillations of the thermal protection circuit. setting the output voltage the output voltage can be adjusted from 0.925v to 18v using an external resistor divider. table 1 shows a list of resistor selection for comm on 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 0.925 v rr out 21 figure 4. feedback divider network when output voltage is low, network as shown in figure 4 is recommended. v out(v) r1(k ? ) r2(k ? ) 5 45.3 10 3.3 26.1 10 2.5 16.9 10 1.8 9.53 10 1.2 3 10 table 1?resistor selection for common output voltages compensation components the ap6503 has an external comp pin through which system stability and transient response can be controlled. comp pin is the output of the internal trans-conductance error amplifier. a series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. the dc gain of the voltage feedback loop is given by: out v fb v vea a cs g load r vdc a = where v fb is the feedback voltage (0.925v), r load is the load resistor value, g cs is the current sense trans- conductance and a vea is the error amplifier voltage gain. the control loop transfer function incorporates two poles one is due to the compensation capacitor (c3) and the output resistor of error ampl ifier, and the other is due to the output capacitor and the load resistor. these poles are located at: vea a3c2 ea g p1 f = load r2c2 1 p2 f = where g ea is the error amplifier trans-conductance. one zero is present due to the compensation capacitor (c3) and the compensation resistor (r3). this zero is located at: 3r3c2 1 z1 f = the goal of compensation design is to shape the converter transfer function to get a desired loop gain. the system crossover frequency where the feedback loop has the unity gain is crucial. a rule of thumb is to set t he crossover frequency to below one-tenth of the switching frequency. use the following procedure to optimize the compensation components: 1. choose the compensation resistor (r3) to set the desired crossover frequency. determine the r3 value by the following equation: fb v out v cs g ea g fs1.02c2 fb v out v cs g ea g fc2c2 3r < = where f c is the crossover frequency, which is typically less than one tenth of t he switching frequency.
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 9 of 13 www.diodes.com september 2011 ? diodes incorporated applications information (cont.) compensation components (cont.) 2. choose the compensation capacitor (c3) to achieve the desired phase margin set the compensation zero, f z1 , to below one forth of the crossover frequency to provide sufficient phase margin. determine the c3 value by the following equation: fc3r 2 3c > where r3 is the compensa tion resistor value. v out (v) cin/c1 (f) cout/c2 (f) rc/r3 (k ? ) cc/c3 (nf) l1 (h) 1.2 22 47 3.24 6.8 3.3 1.8 22 47 6.8 6.8 3.3 2.5 22 47 6.8 6.8 10 3.3 22 47 6.8 6.8 10 5 22 47 6.8 6.8 10 12 22 47 6.8 6.8 15 table 2?recommended component selection 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; sw f l i in v ) out v in (v out v 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 l i load i l(max) i += 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. ot herwise, 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 voltag e. 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: esr of the output capacitor do minates 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. 2 out v 2 ) out v v( 2 ) 2 inductor i out l(i o c ?+ + = where v is the maximum output voltage overshoot.
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 10 of 13 www.diodes.com september 2011 ? diodes incorporated applications information (cont.) 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. ap6503 is exposed at the bottom of the package and must be soldered directly to a well designed thermal pad on the pcb. th is 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 syst em. 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. ap6503 bst sw 10nf boost diode 5v 4 3 figure 7?external bootstrap compensation components recommended diodes: part numbe r v oltage/current rating v endo r b130 30v, 1a diodes inc sk13 30v, 1a diodes inc 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.
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 11 of 13 www.diodes.com september 2011 ? diodes incorporated ordering information ap6503 sp - 13 packing package sp : so-8ep 13 : tape & reel device package code packaging (note 7) 13? tape and reel quantity part number suffix AP6503SP-13 sp so-8ep 2500/tape & reel -13 note: 7. pad layout as shown on diodes inc. suggested pad layout document ap02001, which c an be found on our website at http://www.diodes.c om/datasheets/ap02001.pdf. marking information
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 12 of 13 www.diodes.com september 2011 ? diodes incorporated package outline dimensions (all dimensions in mm) 1.27typ 0.3/0.5 7~9 4.85/4.95 3.85/3.95 5.90/6.10 0.15/0.25 7~9 3.70/4.10 0.62/0.82 0/0.13 gauge plane 0.254 seating plane 0.35max. 45 detail "a" detail "a" 1.30/1.50 1.75max. bottom view 2.4ref. 3.3ref. exposed pad 8x-0.60 land pattem recommendation (unit:mm) exposed pad 1 1 1 5.4 8x-1.55 6x-1.27
ap6503 340khz 23v 3a synchronous dc/dc buck converter ap6503 document number: ds35077 rev. 1 - 2 13 of 13 www.diodes.com september 2011 ? diodes incorporated 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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