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mp2308 18v, 4a synchronous step-down coverter mp2308 rev. 1.0 www.monolithicpower.com 1 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. the future of analog ic technology description the mp2308 is a 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 mp2308 has synchronous mode operation for higher efficiency over 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 mp2308 requires a minimum number of readily available standard external components and is available in a space saving 2mm x 3mm 14-pin qfn package. features ? wide 4.75v to 18v operating input range ? 4a output current ? low rds(on) internal power mosfets ? 300khz-2mhz programmable switching frequency ? en on/off control ? light load mode selectable ? external soft start ? ocp and thermal shutdown ? available in 14-pin qfn2x3 package applications ? dsl modems ? cable modems ? set top boxes all mps parts are lead-free and adhere to the rohs directive. for mps green status, please visit mps website under products, quality assurance page. ?mps? and ?the future of analog ic technology? are registered trademarks of monolithic power systems, inc. typical application mp2308 aam freq gnd ss fb sw bst in vin u1 3.3v vout vcc en c ss 22nf r1 31.6k r2 10k r freq 18.7k 4.75v to 18v vcc en freq r5 10 0.001 0.01 0.1 1 10 70 75 80 85 90 95 100
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 2 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. ordering information part number package top marking MP2308GD* qfn14 (2x3mm) aee * for tape & reel, add suffix ?z (eg. MP2308GD?z); package reference top view gnd sw bst vcc freq ss fb gnd sw in en aam gnd 1 2 3 4 5 67 14 13 12 11 10 9 8 gnd in 2x3 qfn14 absolute maxi mum ratings (1) v in .................................................. -0.3v to 21v v sw .................................................. -0.3v to 22v v bst ...................................................... v sw + 6v all other pins ................................ -0.3v to 6.5 v junction temperature ............................... 150 ? c lead temperature .................................... 260 ? c continuous power dissipation (t a = +25c) (2) 2x3 qfn14 ................................................ 1.8w junction temperature ............................... 150 ? c operating temperature.............. -40 ? c to +85 ? c recommended operating conditions (3) supply voltage v in ......................... 4.75v to 18v output voltage v out ............... 0.815v - v in x90% operating junction temp. (t j ). -40c to +125c thermal resistance (4) ja jc qfn14 (2x3mm) ..................... 70 ...... 15 ... ? 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 powe r 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.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 3 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 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 10 a supply current (quiescent) i in v en = 2v, v fb = 1v 1 ma hs switch-on resistance hs rds-on 60 m ? ls switch-on resistance ls rds-on 30 m ? switch leakage sw lkg v en = 0v, v sw = 0v or 12v 0 10 a current limit i limit duty=40% 5.6 a oscillator frequency f sw r set =30k 400 500 600 khz maximum duty cycle d max v fb = 700mv 90 95 % feedback voltage v fb 800 815 830 mv feedback current i fb v fb = 800mv 25 50 na en input low voltage vil en 0.98 1.12 1.32 v en input high voltage vih en 1.32 1.50 1.78 v en pin pull-up current i en 1.2 2.3 3.5 a v in under-voltage lockout threshold rising inuv vth 4.0 4.25 4.5 v v in under-voltage lockout threshold hysteresis inuv hys 870 mv vcc regulator v cc 5 v vcc load regulation i cc =5ma 1 % thermal shutdown t sd 150 c soft start current i ss 8 a thermal shutdown hysteresis t sd-hys 30 c
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 4 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. pin functions qfn 2x3mm pin # name description 1, 12, 13, 14 gnd ground. connect these pins with larger copper areas to the negative terminals of the input and output capacitors. connect expos ed pad to gnd plane for proper thermal performance. 2, 11 sw switch output. use wide pcb traces and multiple vias to make the connection. 3, center pad in supply voltage. the mp2308 operates from a +4.75v to +18v input rail. cin is needed to decouple the input rail. use wide pcb traces and multiple vias to make the connection. 4 en en=1 to enable the mp2308. and en=0 disables the mp2308. 5 aam advanced asynchronous modulation input. an active-high signal enables low-power mode operation. connect aam pin to gnd to disable it and make the converter always operate in ccm (continuous current mode). 6 fb feedback. an external resistor divider from the output to gnd, tapped to the fb pin, sets the output voltage. to prevent current lim it run away during a short circuit fault condition the frequency fold-back comparator lowers the oscillator frequency when the fb voltage is below 100mv. 7 ss soft start. connect an external capacitor to program the soft start time for the switch mode regulator. 8 freq switching frequency program input. connect a resistor from this pin to ground to set the switching frequency. 9 vcc bias supply. decouple with 1f capacitor. 10 bst bootstrap. a capacitor connected between sw and bst pins is required to form a floating supply across the high-side switch driver.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 5 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. typical performanc e characteristics v in = 12v, v out = 3.3v, l = 4.7h, fs=500khz, t a = +25oc, unless otherwise noted. 815 816 817 818 819 820 821 822 823 824 825 -50 -30 -10 10 30 50 70 90 110130 100 500 900 1300 1700 2100 2500 2900 350 450 550 650 750 850 950 1050 input voltage(v) peak current (a) output voltage(%) output current(a) peak current vs. duty cycle input voltage(v) input voltage(v) output current(a) efficiency(%) output current(a) efficiency vs. output current output voltage(%) 0.001 0.01 0.1 1 10 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 4 6 8 10 12 14 16 18 20 22 2.5 3.5 4.5 5.5 6.5 7.5 8.5 5152535455565 7.8 8 8.2 8.4 8.6 8.8 9 46810121416182022 863 864 865 866 867 868 4 6 8 10121416182022 typical v fb vs. temperature 0 7 14 21 28 35 42 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 70 75 80 85 90 95 100
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 6 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. typical performanc e characteristics (continued) v in = 12v, v out = 3.3v, l = 4.7h, fs=500khz, t a = +25oc, unless otherwise noted.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 7 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. typical performanc e characteristics (continued) v in = 12v, v out = 3.3v, l = 4.7h, fs=500khz, t a = +25oc, unless otherwise noted.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 8 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. function block diagram ss freq gnd sw vcc bst rsen in vcc vcc regulator boost regulator ls driver hs driver en fb aam error amplifier current limit comparator comparator on time control logic control current sense amplifer oscillator reference 1pf 50pf 400k 1meg figure 1?block function diagram
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 9 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. operation the mp2308 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 mp2308 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 1 f ceramic capacitor for decoupling purpose is required. enable control the mp2308 has a dedicated enable control pin (en): pulling it high enables the ic, pulling it low disables it. tie en to vin through a resistor for automatic start up. en must be pulled low to disable the part. the en pin is clamped internally using a 6.7v series-zener-diode as shown in figure 2. connect the en input pin through a pullup resistor to any voltage connected to the vin pin such that the pullup resistor limits the en input current to less than 100a. for example, connecting 12v to vin, r pullup (12v ? 6.7v)/100a = 53k ? . connecting the en pin is directly to a voltage source without any pullup resistor requires limiting the amplitude of the voltage source to below 6v to prevent damage to the zener diode. en en logic zener 6.7v typ. gnd figure 2: zener diode between en and gnd the en pin also features an internal 2.3 a current source. connect a capacitor to the en pin for delayed startup. when vin exceeds the input uvlo, an internal 2.3 a current source charges the external capacitor. the external capacitor connects to the non-inverting input of a comparator. the part is enabled once the capacitor voltage exceeds the 1.5v internal reference voltage. external soft-start the soft start time can be adjusted by connecting a capacitor from this pin to ground. when the soft-start period starts, an internal 8 a current source begins charging the external capacitor. during soft-start, the voltage on the soft-start capacitor is connected to the non-inverting input of the error amplifier. the soft-start period lasts until the voltage on the soft-start capacitor exceeds the reference voltage of 0.815v. at this point the reference voltage takes over at the no inverting error amplifier input. the soft-start time can be calculated as follows: ss ss 0.815v c (nf) t(ms) 8a ? ? ?
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 10 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. if the output of the mp2308 is pre-biased to a certain voltage during startup, the ic will disable the switching of both high-side and low-side switches until the voltage on the internal soft- start capacitor exceeds the sensed output voltage at the fb pin. over-current-protection the mp2308 has a hiccup 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 the inductor current exceeds the current limit at the meantime, mp2308 will hiccup. this is especially useful to ensure system safety under fault conditions. light load mode the mp2308 can operate in discontinuous mode (dcm) and pulse skip mode (psm) under light load to improve efficiency. to enable light load mode, connect aam pin to vcc pin or to a voltage divider from vcc. to disable light load mode, connect aam pin to ground and the converter will always operate in ccm. when the aam pin connects to vcc, the inductor peak current in set internally which is about 240ma for vin=12v, vout=3.3v and l=4.7 h. peak inductor current also can be set by connecting aam pin to a voltage divider from vcc with 2 external resistors. by program aam voltage, efficiency and output ripple can be traded off and optimize. higher aam gives higher peak inductor current thus more efficient but more output ripple. to help select the aam voltage, the curve of inductor peak current vs. low power mode voltage is shown in page 5, typical performance characteristics section. under-voltage lockout (uvlo) under-voltage lockout (uvlo) protects the chip from operating at insufficient supply voltage. the mp2308?s uvlo comparator monitors the output voltage of the internal regulator, vcc. the uvlo rising threshold is 4.25v while its falling threshold is 3.38v. 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, m1, c4, l1 and c2 (figure 3). if (v in -v sw ) 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 3?internal bootstrap charging circuit startup and shutdown if both v in 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, v in 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.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 11 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. application information component selection 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 v out is low, as figure 4 shows. fb 8 rt r2 r1 vout figure 4? 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 the 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 switching frequency the mp2308 oscillating frequency is set by an external resistor, rfreq from the freq pin to ground. the programmable switching frequency range is from 300khz to 2mhz. the value of rfreq can be calculated from: freq s 15000 r(k) f(khz) ?? 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 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 worst case condition occurs at v in = 2v out , where: 2 i i load 1 c ? to simplify, choose the input capacitor whose rms current rating is greater than half of the maximum load current.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 12 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. 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 mp2308 can be optimized for a wide range of capacitance and esr values. 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 5. figure 5?add optional external bootstrap diode to enhance efficiency the recommended external bst diode is in4148, and the bst cap is 0.1~1 f. pc board layout this pcb board layout is referring to the schematic in figure 6. place the high-current paths (gnd, in and sw) very close to the device with short, direct and wide traces. the input decoupling capacitor needs to be placed as close as possible to the in and gnd pins. the vcc decoupling capacitor needs to be placed as close as possible to the vcc pin and multiple vias should be used on both the ground side of the vcc decoupling capacitor and the gnd pins to connect to the inner and bottom ground plane. place the external feedback resistors next to the fb pin. keep the switching node sw short and away from the feedback network.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 13 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved.
mp2308 ? 18v, 4a synchronous step-down converter mp2308 rev. 1.0 www.monolithicpower.com 14 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. typical application circuits vin 9 4 5 1 ,12 13 ,14 8 10 3 2 11 6 7 6-18v gnd vcc en in bs sw 3.3v/4a vout gnd sw sw fb gnd ss aam freq en r4 23.2k r9 37.5k r3 10k r11 4.99k r10 18.7k mp2308 r5 10 c6 18pf r8 25.5k r1 30.1k r2 10k r7 51 r6 0 u1 figure 6? 800khz, 3.3v output at 4a step-down converter
mp2308 ? 18v, 4a 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. mp2308 rev. 1.0 www.monolithicpower.com 15 7/9/2012 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. package information

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