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MP8726 21v, 6a peak current, 500khz synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 1 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. the future of analog ic technology description the MP8726 is a high frequency synchronous rectified step-down switch mode converter with built in internal power mosfets. it offers a very compact solution to achieve 6a peak output current over a wide input supply range with excellent load and line regulation. the MP8726 operates at high efficiency over a wide output current load range. current mode operation provides fast transient response and eases loop stabilization. full protection features include ocp and thermal shut down. the MP8726 requires a minimum number of readily available standard external components and is available in a space saving 3mm x 4mm 14-pin qfn package. features ? wide 4.5v to 21v operating input range ? 6a peak output current ? low r ds (on) internal power mosfets ? proprietary switching loss reduction technique ? fixed 500khz switching frequency ? sync from 300khz to 2mhz external clock ? internal compensation ? ocp protection and thermal shutdown ? output adjustable from 0.8v ? available in 14-pin qfn3x4 package applications ? notebook systems and i/o power ? networking systems ? digital set top boxes ? personal video recorders ? flat panel television and monitors ? distributed power systems ?mps? and ?the future of analog ic technology? are registered trademarks of monolithic power systems, inc. typical application for notebook efficiency v out =1.2v efficiency (%) output current (a) 0 10 20 30 40 50 60 70 80 90 100 0123456 v in =21v v in =12v v in =5v
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 2 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. ordering information part number* package top marking free air temperature ( t a ) MP8726el 3x4 qfn14 8726 -20 c to +85 c * for tape & reel, add suffix ?z (e.g. MP8726el?z). for rohs compliant packaging, add suffix ?lf (e.g. MP8726el?lf?z) package reference pin 1 id 14 13 12 11 10 9 8 agnd gnd gnd vcc nc pg fb in sw sw sw sw bst en/sync 1 2 3 4 5 6 7 top view exposed pad on backside absolute maxi mum ratings (1) supply voltage v in ....................................... 22v v sw ..........................-0.3v (-5v for<10ns) to 23v v bs ....................................................... v sw + 6v all other pins ..................................-0.3v to +6v operating temperature.............. -20 c to +85 c continuous power dissipation (t a = +25c) (2) ............................................................. 2.6w junction temperature ...............................150 c lead temperature ....................................260 c storage temperature............... -65 c to +150 c recommended operating conditions (3) supply voltage v in ...........................4.5v to 21v operating junct.temp (t j )....... -20 c to +125 c thermal resistance (4) ja jc 3x4 qfn14 ............................. 48 ...... 11... c/w notes: 1) exceeding these ratings may damage the device. 2) the maximum allowable power dissipation is a function of the maximum junction temperature t j (max), the junction-to- ambient thermal resistance ja , and the ambient temperature t a . the maximum allowable continuous power dissipation at any ambient temperature is calculated by p d (max)=(t j (max)- t a )/ ja . exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. internal thermal shutdown circuitry protects the device from permanent damage. 3) the device is not guaranteed to function outside of its operating conditions. 4) measured on jesd51-7, 4-layer pcb.
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 3 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. electrical characteristics v in = 12v, t a = +25 c, unless otherwise noted. parameters symbol condition min typ max units supply current (shutdown) i in v en = 0v 0 a supply current (quiescent) i q v en = 2v, v fb = 1v 0.7 ma hs switch on resistance hs rds-on 120 m ? ls switch on resistance ls rds-on 20 m ? switch leakage sw lkg v en = 0v, v sw = 0v or 12v 0 10 a current limit i limit 7 a oscillator frequency f sw v fb = 0.75v 425 500 575 khz fold-back frequency f fb v fb = 300mv 0.25 f sw maximum duty cycle d max v fb = 700mv 85 90 % sync frequency range f sync 0.3 2 mhz feedback voltage v fb 789 805 821 mv feedback current i fb v fb = 800mv 10 50 na en rising threshold v en_rising 1.1 1.3 1.6 v en threshold hysteresis v en_hys 0.4 v v en = 2v 2 a en input current i en v en = 0v 0 en turn off delay en td-off 5 s power good rising threshold pg vth-hi 0.9 v fb power good falling threshold pg vth-lo 0.7 v fb power good delay pg td 20 s power good sink current capability v pg sink 4ma 0.4 v power good leakage current i pg_leak v pg = 3.3v 10 na v in under voltage lockout threshold rising inuv vth 3.8 4.0 4.2 v v in under voltage lockout threshold hysteresis inuv hys 880 mv vcc regulator v cc 5 v vcc load regulation icc=5ma 5 % soft-start period 2 4 6.5 ms thermal shutdown t sd 150 c
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 4 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. pin functions pin # name description 1 in supply voltage. the MP8726 operates from a +4.5v to +21v input rail. c1 is needed to decouple the input rail. use wide pcb traces and multiple vias to make the connection. 2,3,4,5 sw switch output. use wide pcb traces and multiple vias to make the connection. 6 bst bootstrap. a capacitor connected between sw and bs pins is required to form a floating supply across the high-side switch driver. 7 en/sync en=1 to enable the chip. external clock can be applied to en pin for changing switching frequency. for automatic start-up, connect en pin to vin by proper en resistor divider as figure 2 shows. 8 fb feedback. an external resistor divider fr om the output to gnd, tapped to the fb pin, sets the output voltage. to prevent current limit r un away during a short circuit fault condition the frequency fold-back comp arator lowers the oscillator frequency when the fb voltage is below 500mv. 9 pg power good output, the output of this pi n is open drain. power good threshold is 90% low to high and 70% high to low of regulation value. 10, exposed pad nc no internal connection. connect exposed pad to ground plane for optional performance. 11 vcc bias supply. decouple with 0.1f~0.22 f cap. and the capacitance should be no more than 0.22f. 12,13 gnd system ground. this pin is the referenc e ground of the regulated output voltage. for this reason care must be taken in pcb layout. 14 agnd signal ground. agnd is not internally co nnected to system ground, make sure agnd connected to system ground in pcb layout.
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 5 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. typical performanc e characteristics vin = 12v, vout = 1.2v, l = 1.8h, ta = +25oc, unless otherwise noted. dmax limit minimum on t ime vcc regulator line regulation output current (a) output current (a) load regulation input voltage (v) input voltage (v) operating range enable supply current (ma) input voltage (v) input voltage (v) input voltage (v) enable supply current vs input voltage disable supply current vs input voltage 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 vcc(v) 0.1 1 10 100 0 5 10 15 20 25 output voltage (v) normalized output voltage (%) line regulation case temperature rise vs output current 0 5 10 15 20 25 line regulation(%) case temperature rise ( ) o c 550 600 650 700 750 800 850 900 950 1000 v fb =1v -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 v en =0v 3.5 4 4.5 5 5.5 6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0123456 v in =12v v in =21v v in =5v 0 10 20 30 40 50 60 01234567 -0.5 -0.3 -0.1 0.1 0.3 0.5 i o =0a i o =3a i o =6a
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 6 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. typical performanc e characteristics (continued) vin = 12v, vout = 1.2v, l = 1.8h, ta = +25oc, unless otherwise noted. 10 20 30 40 50 60 70 80 90 100 output current (a) efficiency v out =1.2v efficiency v out =2.5v efficiency v out =3.3v efficiency v out =1.8v efficiency (%) efficiency (%) efficiency (%) efficiency (%) output current (a) output current (a) output current (a) 0 10 20 30 40 50 60 70 80 90 100 0 0123456 v in =21v 0 10 20 30 40 50 60 70 80 90 100 01234 56 0 10 20 30 40 50 60 70 80 90 100 0123456 0123456 v in =12v v in =5v v in =12v v in =21v v in =5v v in =12v v in =21v v in =5v v in =12v v in =5v v in =21v
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 7 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. typical performanc e characteristics (continued) vin = 12v, vout = 1.2v, l = 1.8h, ta = +25oc, unless otherwise noted. enable startup with 6a load enable startup without load power up without load power up with 6a load v out 1v/div v en 5v/div v sw 5v/div i inductor 5a/div i inductor 5a/div v out 1v/div v en 5v/div v sw 5v/div i inductor 5a/div v out 1v/div v in 10v/div v sw 5v/div i inductor 5a/div 2ms/div 400us/div 1us/div 1us/div 2ms/div 2ms/div output ripple voltage i out =6a input ripple voltage i out =6a v out 1v/div v in 10v/div v sw 5v/div i inductor 5a/div 2ms/div v sw 5v/div v sw 5v/div v out /ac 20mv/div v in /ac 100mv/div load transient response i out =3a-6a v out /ac 200mv/div i out 2a/div
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 8 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. block diagram figure 1?function block diagram
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 9 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. operation the MP8726 is a high frequency synchronous rectified step-down switch mode converter with built in internal power mosfets. it offers a very compact solution to achieve 6a peak output current over a wide input supply range with excellent load and line regulation. the MP8726 operates in a fixed frequency, peak current control mode to regulate the output voltage. a pwm cycle is initiated by the internal clock. the integrated high-side power mosfet is turned on and remains on until its current reaches the value set by the comp voltage. when the power switch is off, it remains off until the next clock cycle starts. if, in 90% of one pwm period, the current in the power mosfet does not reach the comp set current value, the power mosfet will be forced to turn off power good indicator when the fb is below 0.70v fb , the pg pin will be internally pulled low. when the fb is above 0.9v fb , the pg becomes an open-drain output. internal regulator most of the internal circuitries are powered from the 5v internal regulator. this regulator takes the vin input and operates in the full vin range. when vin is greater than 5.0v, the output of the regulator is in full regulation. when vin is lower than 5.0v, the output decreases, 0.1uf ceramic capacitor for decoupling purpose is required. error amplifier the error amplifier compares the fb pin voltage with the internal fb reference (v fb ) and outputs a current proportional to the difference between the two. this output current is then used to charge or discharge the internal compensation network to form the comp voltage, which is used to control the power mosfet current. the optimized internal compensation network minimizes the external component counts and simplifies the control loop design. enable/sync control en/sync is a digital control pin that turns the regulator on and off. drive en high to turn on the regulator, drive it low to turn it off. there is an internal 1meg resistor from en/sync to gnd thus en/sync can be floated to shut down the chip. 1) enabled by external logic h/l signal the chip starts up once the enable signal goes higher than en/sync input high voltage (2v), and is shut down when the signal is lower than en/sync input low voltage (0.4v). to disable the chip, en must be pulled low for at least 5 s. the input is compatible with both cmos and ttl. 2) enabled by vin through voltage divider. connect en with vin through a resistive voltage divider for automatic startup as the figure 2 shows. en v in r en1 r en2 figure 2?enable divider circuit choose the value of the pull-up resistor r en1 and pull-down resistor r en2 to reset the automatic start-up voltage: en1 en2 in_start en_rising en2 (r r || 1m ) vv r||1m +? =? ? en1 en2 in_stop en-falling en2 (r r || 1m ) vv r||1m = +? ? ? vin en/sync vcc vout v en_rising 5us v en_falling v in_start 1ms v cc_rising turn on delay turn off delay v in_stop figure 3?startup sequence using en divider
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 10 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. 3) synchronized by external sync clock signal the chip can be synchronized to external clock range from 300khz up to 2mhz through this pin 2ms right after output voltage is set, with the internal clock rising edge synchronized to the external clock rising edge. vin en/sync vcc vout clk 5us foldback 500khz external clk 0.625*vout_set 2ms 1ms v cc_rising vout_set figure 4?startup sequence using external sync clock signal under-voltage lockout (uvlo) under-voltage lockout (uvlo) is implemented to protect the chip from operating at insufficient supply voltage. the MP8726 uvlo comparator monitors the output voltage of the internal regulator, vcc. the uvlo rising threshold is about 4.0v while its falling threshold is a consistent 3.2v. internal soft-start the soft-start is implemented to prevent the converter output voltage from overshooting during startup. when the chip starts, the internal circuitry generates a soft-start voltage (ss) ramping up from 0v to 1.2v. when it is lower than the internal reference (ref), ss overrides ref so the error amplifier uses ss as the reference. when ss is higher than ref, ref regains control. the ss time is internally fixed to 4ms. over-current-protection and latch off the MP8726 has cycle-by-cycle over current limit when the inductor current peak value exceeds the set current limit threshold. meanwhile, output voltage starts to drop until fb is below the under- voltage (uv) threshold, typically 30% below the reference. once a uv is triggered, the MP8726 is latched off until en or in is recycled. this protection mode is especially useful when the output is dead-short to ground. thermal shutdown thermal shutdown is implemented to prevent the chip from operating at exceedingly high temperatures. when the silicon die temperature is higher than 150 c, it shuts down the whole chip. when the temperature is lower than its lower threshold, typically 140 c, the chip is enabled again. floating driver and bootstrap charging the floating power mosfet driver is powered by an external bootstrap capacitor. this floating driver has its own uvlo protection. this uvlo?s rising threshold is 2.2v with a hysteresis of 150mv. the bootstrap capacitor voltage is regulated internally by vin through d1, m3, c4, l1 and c2 (figure 5). if (vin-vsw) is more than 5v, u2 will regulate m3 to maintain a 5v bst voltage across c4. sw figure 5?internal bootstrap charging circuit startup and shutdown if both vin and en are higher than their appropriate thresholds, the chip starts. the reference block starts first, generating stable reference voltage and currents, and then the internal regulator is enabled. the regulator provides stable supply for the remaining circuitries. three events can shut down the chip: en low, vin low and thermal shutdown. in the shutdown procedure, the signaling path is first blocked to avoid any fault triggering. the comp voltage and the internal supply rail are then pulled down. the floating driver is not subject to this shutdown command.
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 11 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. application information setting the output voltage the external resistor divider is used to set the output voltage (see typical application on page 1). the feedback resistor r1 also sets the feedback loop bandwidth with the internal compensation capacitor (see typical application on page 1). choose r1 to be around 40.2k ? for optimal transient response. r2 is then given by: out fb r1 r2 v 1 v = ? the t-type network is highly recommended when vo is low, as figure 6 shows. fb 1 r2 r1 rt vout figure 6? t-type network table 1 lists the recommended t-type resistors value for common output voltages. table 1?resistor selection for common output voltages v out (v) r1 (k ? ) r2 (k ? ) rt (k ? ) l (h) c out (f, ceramic) 1.05 4.99 16.5 24.9 1-4.7 47 1.2 4.99 10.2 24.9 1-4.7 47 1.5 4.99 5.76 24.9 1-4.7 47 1.8 4.99 4.02 24.9 1-4.7 47 2.5 40.2 19.1 0 1-4.7 47 3.3 40.2 13 0 1-4.7 47 5 40.2 7.68 0 1-4.7 47 note: the above feedback resistor table applies to a specific load capacitor condition as shown in t he table 1. other capacitive loading conditions will require different values. selecting the inductor a 1 h to 10 h 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 dc resistance should be less than 15m ? . for most designs, the inductance value can be derived from the following equation. osc l in out in out f i v ) v v ( v l ? ? = where ? i l is the inductor ripple current. choose inductor ripple current to be approximately 30% of the maximum load current, 6a. the maximum inductor peak current is: 2 i i i l load ) max ( l ? + = under light load conditions below 100ma, larger inductance is recommended for improved efficiency. selecting the input capacitor the input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the ac current to the step-down converter while maintaining the dc input voltage. use low esr capacitors for the best performance. ceramic capacitors with x5r or x7r dielectrics are highly recommended because of their low esr and small temperature coefficients. for most applications, a 22 f capacitor is sufficient. since the input capacitor (c1) absorbs the input switching current it requires an adequate ripple current rating. the rms current in the input capacitor can be estimated by: ? ? ? ? ? ? ? ? ? = in out in out load 1 c v v 1 v v i i the worse case condition occurs at v in = 2v out , where: 2 i i load 1 c = for simplification, choose the input capacitor whose rms current rating greater than half of the maximum load current. the input capacitor can be electrolytic, tantalum or ceramic. when using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1f, should be placed as close to the ic as possible. when using ceramic capacitors, make sure that they have enough
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 12 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. capacitance to provide sufficient charge to prevent excessive voltage ripple at input. the input voltage ripple caused by capacitance can be estimated by: ? ? ? ? ? ? ? ? ? = ? in out in out s load in v v 1 v v 1 c f i v selecting the output capacitor the output capacitor (c2) is required to maintain the dc output voltage. ceramic, tantalum, or low esr electrolytic capacitors are recommended. low esr capacitors are preferred to keep the output voltage ripple low. the output voltage ripple can be estimated by: ? ? ? ? ? ? ? ? + ? ? ? ? ? ? ? ? ? = ? 2 c f 8 1 r v v 1 l f v v s esr in out s out out where l is the inductor value and resr is the equivalent series resistance (esr) value of the output capacitor. in the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. the output voltage ripple is mainly caused by the capacitance. for simplification, the output voltage ripple can be estimated by: ? ? ? ? ? ? ? ? ? = in out 2 s out out v v 1 2 c l f 8 v ? v in the case of tantalum or electrolytic capacitors, the esr dominates the impedance at the switching frequency. for simplification, the output ripple can be approximated to: esr in out s out out r v v 1 l f v ? v ? ? ? ? ? ? ? ? ? = the characteristics of the output capacitor also affect the stability of the regulation system. the MP8726 can be optimized for a wide range of capacitance and esr values. pcb layout pcb layout is very important to achieve stable operation. please follow these guidelines and take figure 7 for references. 1) keep the connection of input ground and gnd pin as short and wide as possible. 2) keep the connection of input capacitor and in pin as short and wide as possible. 3) ensure all feedback connections are short and direct. place the feedback resistors and compensation components as close to the chip as possible. 4) route sw away from sensitive analog areas such as fb. 5) connect in, sw, and especially gnd respectively to a large copper area to cool the chip to improve thermal performance and long-term reliability. 6) adding rc snubber circuit from in pin to sw pin can reduce sw spikes. sw sw sw sw c4 c2 c2 l1 c4 c1 2 14 13 12 11 10 9 8 1 2 3 4 5 6 7 fb pg gnd gnd gnd vcc c3 vin in r3 r2 r1 rt gnd n/c bst a en vout top layer gnd bottom layer figure 7?pcb layout
MP8726 ? 6a peak current, 21v, synchronous step-down converter MP8726 rev. 0.92 www.monolithicpower.com 13 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. external bootstrap diode an external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external bst diode is: z duty cycle is high: d= in out v v >65% in this case, an external bst diode is recommended from the vcc pin to bst pin, as shown in figure 8 figure 8?add optional external bootstrap diode to enhance efficiency the recommended external bst diode is in4148, and the bst cap is 0.1~1 f.
MP8726 ? 6a peak current, 21v, synchronous step-down converter notice: the information in this document is subject to change wi thout notice. users should warra nt and guarantee that third party intellectual property rights are not infringed upon w hen integrating mps products into any application. mps will not assume any legal responsibility for any said applications. MP8726 rev. 0.92 www.monolithicpower.com 14 6/15/2010 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2010 mps. all rights reserved. package information 3mm x 4mm qfn14 side view top view 1 14 87 bottom view 2. 90 3. 10 1.60 1.80 3.90 4.10 3.20 3.40 0.50 bsc 0.18 0.30 0.80 1.00 0.00 0.05 0.20 ref pin 1 id marking 1.70 0.50 0.25 recommended land pattern 2.90 note: 1) all dimensions are in millimeters . 2) exposed paddle size does not include mold flash . 3) lead coplanarity shall be 0.10 millimeter max. 4) jedec reference is mo-229, variation vged-3. 5) drawing is not to scale. pin 1 id see detail a 3.30 0.70 pin 1 id option b r0.20 typ. pin 1 id option a 0.30x45 o typ. detail a 0.30 0.50 pin 1 id index area

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