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nb632 high effeciency 4a, 24v, 500khz synchronous step -down converter nb632 rev. 0.9 www.monolithicpower.com 1 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. the future of analog ic technology description the nb632 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 4a continuous output current over a wide input supply range with excellent load and line regulation. the nb632 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 latch-off ocp and thermal shut down. the nb632 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 24v operating input range ? 4a 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 ? latch-off ocp protection and thermal shutdown ? output adjustable from 0.8v ? available in a 14-pin, qfn 3x4mm 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 output current (a) efficiency(%) 50 55 60 65 70 75 80 85 90 95 100 01 2 3 4 5 v in =7v v in =12v v in =24v
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 2 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. ordering information part number* package top marking free air temperature (t a ) nb632el qfn14 (3mm x 4mm) 632e ?20 ? c to +85 ? c * for tape & reel, add suffix ?z (e.g. nb632el?z); for rohs compliant packaging, add suffix ?lf; (e.g. nb632el?lf?z) package reference top view in sw sw sw sw bst en/sync 1 2 3 4 5 6 7 agnd gnd gnd vcc aam pg fb 14 13 12 11 10 9 8 exposed pad on backside absolute maxi mum ratings (1) supply voltage v in ....................................... 28v v sw .......................... ?0.3v (-5v for 10ns) to 28v v bs ....................................................... v sw + 6v all other pins .................................?0.3v to +6v 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 24v operating junct. temp. (t j ).... ?20 ? c to +125 ? c thermal resistance (4) ja jc qfn14 (3mm x 4mm) .............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.
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 3 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 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 in v en = 2v, v fb = 1v 1 ma hs switch on resistance (5) hs rds-on 120 m ? ls switch on resistance (5) 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 350 500 650 khz fold-back frequency f fb v fb = 100mv 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 785 805 825 mv feedback current i fb v fb = 800mv 10 50 na en/sync input low voltage vil en 0.4 v en/sync input high voltage vih en 2 v i en v en = 2v 2 a en input current v en = 0v 0 en turn off delay en td-off 5 sec 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 250 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 i cc =5ma 5 % thermal shutdown t sd 150 c notes: 5) guaranteed by design.
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 4 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. pin functions pin # name description 1 in supply voltage. the nb632 operates from a +4.5v to +24v 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 trac es and multiple vias to make the connection. 6 bst bootstrap. a capacitor connected between sw and bst pins is required to form a floating supply across the high-side switch driver. 7 en/sync en=1 to enable the nb632. external clock can be applied to en pin for changing switching frequency. for automatic start- up, connect en pin to vin with 100k ? resistor. 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 100mv. 9 pg power good output, the output of this pi n is open drain. power good threshold is 90% low to high with typical 250 s delay and 70% high to low of regulation value. 10 aam connects to a voltage set by 2 resistor dividers from vcc forces nb632 into non- synchronous mode when load is small. 11 vcc bias supply. decouple with 0.1f capacitor. 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 ground. connect these pins with larger copper areas to the negative terminals of the input and output capacitors. connect exposed pad to gnd plane for proper thermal performance.
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 5 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics v in =12v, v out =1.8v, l=1.0h, t a =+25c, unless otherwise noted. 550 555 560 565 570 575 580 585 590 peak current vs. duty cycle duty cycle(%) peak current(a) input voltage (v) output voltage (v) operating range 0 1 2 3 4 5 6 7 8 9 10 020406080100 0.1 1 10 100 0 5 10 15 20 25 load regulation normalized output voltage (%) output current (a) input voltage (v) normalized output voltage (%) output current (a) line regulation case temperature rise vs. output current output current (a) efficiency(%) 0 5 10 15 20 25 dmax limit minimun on time limit enabled supply current vs. input voltage input voltage (v) input voltage (v) disabled supply current vs. input voltage vcc(v) input voltage (v) vcc regulator line regulation 0 5 10 15 20 25 3.5 4.0 4.5 5.0 5.5 0 5 10 15 20 25 0 5 10 15 20 25 v fb =1v -0.1 -0.05 0 0.05 0.1 i out =0a -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 012345 -0.6 -0.4 -0.2 0 0.2 0.4 0 5 10 15 20 25 30 35 012345 50 55 60 65 70 75 80 85 90 95 100 01 2 3 4 5 v in =7v v in =12v v in =24v v in =5v v in =12v v in =24v i out =2a i out =4a
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 6 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics (continued) v in =12v, v out =1.8v, l=1.0h, t a =+25c, unless otherwise noted. output ripple voltage i out =4a v out 1v/div v en 10v/div v out 1v/div v en 10v/div v sw 10v/div v sw 10v/div i inductor 1a/div i inductor 5a/div v out 1v/div v en 10v/div v sw 10v/div i inductor 5a/div v out 1v/div v in 10v/div v out 1v/div v sw 10v/div i inductor 5a/div v out / ac 20mv/div v sw 5v/div i inductor 5a/div i inductor 5a/div v out 1v/div v en 10v/div v sw 10v/div i inductor 500ma/div enable startup i out =0a enable shut down i out =0a enable shut down i out =4a enable startup i out =4a short circuit protection i out 5a/div v out/ac 50mv/div v sw 10v/div pg 5v/div 90% v out
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 7 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. block diagram figure 1?function block diagram
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 8 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. operation the nb632 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 more than 4a continuous output current over a wide input supply range with excellent load and line regulation. the nb632 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. error amplifier the error amplifier compares the fb pin voltage with the internal 0.8v reference (ref) 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. 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, a 0.1uf ceramic capacitor for decoupling purpose is required. enable/sync control the nb632 has a dedicated enable/sync control pin (en/sync). by pulling it high or low, the ic can be enabled and disabled by en. tie en to vin through a resistor for automatic start up. to disable the part, en must be pulled low for at least 5s. the nb632 can be synchronized to an external clock ranging from 300 khz to 2mhz through the en/sync pin. the internal clock rising edge is synchronized to the external clock rising edge. under-voltage lockout (uvlo) under-voltage lockout (uvlo) is implemented to protect the chip from operating at insufficient supply voltage. the nb632 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. over-current-protection and latch off the nb632 has cycle-by-cycle over current limit when the inductor current peak value exceeds the set current limit threshold. when output voltage drops below 70% of the reference, and inductor current exceeds the current limit at the meantime, nb632 will be latched off. this is especially useful to ensure system safety under fault condition. the nb632 clears the latch once the en or input power is recycled. the latch-off function is disabled during soft-start duration. 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.
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 9 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. 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, m1, c4, l1 and c2 (figure 2). if (vin-vsw) is more than 5v, u1 will regulate m1 to maintain a 5v bst voltage across c4. v in v out d1 m1 bst c4 l1 sw u1 5v c2 figure 2?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.
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 10 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 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 10k ? . r2 is then given by: out r1 r2 v 1 0.8v ? ? the t-type network is highly recommended when vo is low, as figure 3 shows. fb 8 rt r2 r1 vout figure 3? 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 ? ) 1.05 3.09(1%) 10(1%) 24.9(1%) 1.2 4.99(1%) 10(1%) 24.9(1%) 1.8 10(1%) 8.06(1%) 24.9(1%) 2.5 10(1%) 4.75(1%) 24.9(1%) 3.3 10(1%) 3.16(1%) 24.9(1%) 5 10(1%) 1.91(1%) 24.9(1%) selecting the inductor 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 dc resistance should be less than 15m ? . for most designs, the inductance value can be derived from the following equation. out in out 1 in l osc v(vv) l vif ?? ? ?? ? where i l is the inductor ripple current. choose inductor current to be approximately 30% of the maximum load current. 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. setting the aam voltage the aam voltage is used for setting the transition point from aam to ccm. it should be chosen to provide the best combination of efficiency, stability, ripple, and transient. if the aam voltage is set lower, then stability and ripple improve, but efficiency during aam mode and transient degrades. likewise, if the aam voltage is set higher, then the efficiency during aam and transient improves, but stability and ripple degrade. therefore the optimal balance point of aam voltage for good efficiency, stability, ripple and transient should be selected. as figure 4 shows, aam voltage can be set by using a resistor divider. r3 r4 vcc(5v) aam figure 4? aam network the optimized aam voltage can be set according to figure 5. generally, choose r4 to be around 10 k ? , r3 is then determined by the following equation: vcc r3 r4 1 aam ?? ? ? ?? ??
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 11 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 aam (v) v o =1.05v v o =1.8v v o =3.3v v o =5v figure 5? aam selection for common output voltages (v in =5v-24v) 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 coe fficients. for most applications, a 22f 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 vin = 2vout, 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. 0.1 f, should be placed as close to the ic as possible. when using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. the input voltage ripple caused by capacitance can be estimated by: load out out in in sin iv v v1 fc1v 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: out out out esr s1 in s vv 1 v1r fl v 8fc2 ?? ?? ?? ?? ? ? ?? ?? ??? ?? ?? where l 1 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: out out out 2 in s1 vv v1 v 8f l c2 ?? ??? ?? ??? ?? 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: out out out esr in s1 vv v1r fl v ?? ???? ?? ? ?? the characteristics of the output capacitor also affect the stability of the regulation system. the nb632 can be optimized for a wide range of capacitance and esr values.
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets nb632 rev. 0.9 www.monolithicpower.com 12 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 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 are: ? v out is 5v or 3.3v; and ? duty cycle is high: d= in out v v >65% in these cases, an external bst diode is recommended from the vcc pin to bst pin, as shown in figure 6. sw bst nb632 c l bst c out external bst diode vcc in4148 figure 6?add optional external bootstrap diode to enhance efficiency the recommended external bst diode is in4148, and the bst cap is 0.1~1uf. pc board layout the high current paths (gnd, in and sw) should be placed very close to the device with short, direct and wide traces. the input capacitor needs to be as close as possible to the in and gnd pins. the external feedback resistors should be placed next to the fb pin. keep the switching node sw short and away from the feedback network. en agnd vcc 14 12 11 10 9 8 1 2 3 4 5 6 7 pg in fb sw sw sw sw bst aam gnd c2 l1 r1 rt c1 gnd r3 c3 r2 r4 r5 c4 13 gnd top layer bottom layer figure 7?pcb layout
nb632 ? 4a, 24v, synchronous step-down converter with internal mosfets 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. nb632 rev. 0.9 www.monolithicpower.com 13 12/25/2013 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2013 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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