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power management 1 SC198A dual dc-dc buck converter with high current capability features input voltage range 2.7v to 5.5v dual buck converters with independent control high effi ciency over 90% peak up to 800ma load programmable output voltages per channel 8 fixed-frequency operation 1mhz optional sync clock input optional power-save under light-load conditions shutdown current <1a soft-start to limit in-rush current 4mm x 4mm mlpq package applications mobile phones cordless phones mp3 players digital cameras pdas single li-ion cell or (3) nimh/nicd cell devices digital multimedia broadcast (dmb) devices portable gaming systems ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description the SC198A is a dual high-effi ciency synchronous buck converter designed for use in low power applications. each converter is capable of supplying up to 800ma of average current at one of eight programmable output voltages used as microprocessor supply voltages. the SC198A uses a single control logic block for the two converters, but their controls can be set independently. the device uses a fl exible clocking method allowing the user to set the fi xed internal 1mhz oscillator or synchro- nize the clock to an external source. the device also has an optional power-save mode that optimizes effi ciency for light loads. the SC198A provides output voltage fl exibility while con- serving board space by using control pins to select the output voltage. this eliminates at least four parts from the traditional dual buck converter bill of materials. typical application circuit september 26, 2007 4.7 h 4.7 h vina vinr vinb ena ctla0 ctla1 ctla2 enb ctlb0 ctlb1 ctlb2 lxa vouta gnda lxb voutb gndb mode/sync gndr c in c outa c outb SC198A vouta voutb vbatt
SC198A 2 pin confi guration marking information ordering information device package SC198Amltrt (1) (2) mlpq-20 SC198Aevb evaluation board notes: (1) available in tape and reel only. a reel contains 3,000 devices. (2) lead-free package only. device is weee and rohs compliant. top view 1 2 3 4 12 11 10 9 16 15 14 13 5 678 17 18 19 20 t vina lxa gnda vouta ctla0 vinb lxb gndb voutb ctlb0 ena nc vinr mode/sync enb ctla1 ctla2 gndr ctlb2 ctlb1 198a yyww xxxxx xxxxx mlpq: 4mm x 4mm 20 lead ja = 31c/w yyww = date code xxxx = semtech lot no. xxxx = semtech lot no.
SC198A 3 exceeding the above specifi cations may result in permanent damage to the devic e or device malfunction. operation outside of the parameters specifi ed in the electrical characteristics section is not recommended. notes: (1) tested according to jedec standard jesd22-a114-b. (2) calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer fr4 pcb with thermal vias under the exposed pad per jesd51 standards. unless otherwise noted: v in = 3.6v, v mode/sync = v in , l = 4.7h, c = 10f, t a = -40c to +85c. typical values are at t a = 25c for both outputs. electrical characteristics parameter symbol conditions min typ max units input voltage range v in v out = 1.8v 2.7 5.5 v v out = 3.3v 4.0 5.5 v out accuracy (1) v out i out = 10ma to 800ma , v in 0.7v + v out -3 3 % line regulation v line v in = 2.7 to 5.5v, v out = 1.8v, i out = 300ma 0.2 %/v v in = 4.0 to 5.5v, v out = 3.3v, i out = 300ma load regulation v load i out = 10ma to 800ma, v out = 1.8v 0.0008 %/ma i out = 10ma to 800ma, v out = 3.3v, v in = 5.0v minimum duty cycle dc min 18 % lxa, lxb current limit i lim 1.2 a quiescent current i q i out = 0ma, sync = 0, ena, enb = v in 50 a shutdown current i sd v ena , v enb = 0, lxa, lxb = open 0.1 1 a lxa and lxb leakage current i lxa , i lxb v in = 5.5v, v lx = 0v, pmos 0.1 1 a v in = 5.5v, v lx = v in , nmos -2 0.1 a p-channel on resistance r dsp i lxa , i lxb = 100ma 0.35 absolute maximum ratings v in (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +7 v out (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to v in +0.3 v lxa and v lxb (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-1 to v in +1 pin voltage - all other pins (v) . . . . . . . . . -0.3 to (v in + 0.3) output short circuit to gnd duration . . . . . con tinuous lxa and lxb currents (a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 esd protection level (1) (kv) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 recommended operating conditions ambient temperature range (c) . . . . . . . . . . . . . -40 to +85 v in (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 to 5.5 thermal information thermal resistance, junction to ambient (2) (c/w) . . . . 31 maximum junction temperature (c) . . . . . . . . . . . . . . +150 storage temperature range (c) . . . . . . . . . . . . -65 to +150 peak ir reflow temperature (10s to 30s) (c) . . . . . . . +260
SC198A 4 electrical characteristics (continued) parameter symbol conditions min typ max units n-channel on resistance r dsn i lxa , i lxb = 100ma 0.25 start-up time t start 5 ms oscillator frequency f osc 0.95 1.1 1.25 mhz sync frequency (upper) f syncu 1.25 mhz sync frequency (lower) f syncl 0.75 mhz thermal shutdown t hi 145 c thermal shutdown hysteresis t hysr 10 c uvlo threshold v uvl decreasing v in 2.4 2.5 2.6 v uvlo hysteresis v uvlhys 50 mv logic input high (2) v ih 1.6 v logic input low (2) v il 0.6 v notes: (1) see output voltage options table in the application information section. (2) for ena, enb, sync, ctla0, ctla1, ctla2, ctlb0, ctlb1, ctlb2.
SC198A 5 pin descriptions pin # pin name pin function 1 vina input power supply voltage for converter a 2 lxa connection point for the inductor on converter a 3 gnda ground reference to converter a 4 vouta feedback control input for converter a 5 ctla0 voltage select control pin 0 for output a (see table 1 output voltage options). 6 ctla1 voltage select control pin 1 for output a (see table 1 output voltage options). 7 ctla2 voltage select control pin 2 for output a (see table 1 output voltage options). 8 gndr ground for reference supply 9 ctlb2 voltage select control pin 2 for output b (see table 1 output voltage options). 10 ctlb1 voltage select control pin 1 for output b (see table 1 output voltage options). 11 ctlb0 voltage select control pin 0 for output b (see table 1 output voltage options). 12 voutb feedback control input for converter b 13 gndb ground reference for converter b 14 lxb connection point for the inductor on converter b 15 vinb input power supply voltage for converter b 16 enb enable pin for output b 17 mode/sync oscillator synchronization input connect to vin for forced pwm mode, ground for power-save mode or connect to an external clock for frequency synchronization. 18 vinr reference supply input 19 nc not connected leave open 20 ena enable pin for output a t thermal pad thermal pad for heatsinking purposes connect to ground plane using multiple vias not con- nected internally
SC198A 6 block diagram current amp vinb lxb gndb p limit amp switching control logic n limit amp oscillator and slope generator error amp a voltage select error amp b voutb voltage select ctlb0 ctlb1 ctlb2 error amp a error amp b current amp p limit amp pwm comp pwm comp n limit amp vina lxa gnda vouta ctla0 ctla1 ctla2 17 sync voltage references vref3 vref3 vref2 vref2 vref1 vref1 vref1 vref2 vref3 gndr ena enb vinr 1 2 3 20 4 5 6 7 18 8 9 10 11 12 16 13 14 15
SC198A 7 typical characteristics effi ciency vs. load at v in = 2.7v, side a 0 10 20 30 40 50 60 70 80 90 100 0.001 0.01 0.1 1 load (a) efficiency (%) v out = 1.8v psave v out = 1.0v psave v out = 1.8v pwm v out = 1.0 pwm effi ciency vs. load at v in = 5.5v, side a 0 10 20 30 40 50 60 70 80 90 100 0.001 0.01 0.1 1 load (a) efficiecny (%) v out = 1.8v psave v out = 1.0v psave v out = 1.8v pwm v out = 1.0 pwm effi ciency vs. load at v in = 5.5v, side b 0 10 20 30 40 50 60 70 80 90 100 0.001 0.01 0.1 1 load (a) efficiecny (%) v out = 1.8v psave v out = 1.0v psave v out = 1.8v pwm v out = 1.0 pwm load regulation v out =1.8v, v in =3.6v -3 -2 -1 0 1 2 3 0.001 0.01 0.1 1 load (a) output (%) v out = 1.8v pwm v out = 1.0 pwm line regulation at 300ma -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 2.7 3.263.824.384.94 5.5 input (v) output (%) v out = 1.8v pwm v out = 1.0 pwm effi ciency vs. load at v in = 3.6v, side a 0 10 20 30 40 50 60 70 80 90 100 0.001 0.01 0.1 1 load (a) efficiency (%) v out = 1.8v psave v out = 1.0v psave v out = 1.8v pwm v out = 1.0 pwm
SC198A 8 typical characteristics (continued) v out vs. v in 3.3v programmed output 0 0.5 1 1.5 2 2.5 3 3.5 4 2.7 3.26 3.82 4.38 4.94 5.5 input (v) output (v) i out = 300ma i out = 800ma v out vs. v in 1.8v programmed output 1.746 1.764 1.782 1.8 1.818 1.836 1.854 2.7 3.26 3.82 4.38 4.94 5.5 input (v) output (v) i out = 300ma i out = 800ma crosstalk in psave 10s crosstalk side a (psave) side b (pwm) 10s (50mv/div) (50mv/div) (100mv/div) (100mv/div) v in =5.0v, 25c, v outa = 1.0v, i outa = 80ma, v outb = 3.3v, i outb = 0ma v in =5.0v, 25c, v outa = 1.0v, i outa = 80ma, v outb = 3.3v, i outb = 800ma transient in psave time (200s/div) v out =1.0v, v in =3.6v, 25c, i out = 0 to 800ma (5.0mv/div) (200mv/div) (200mv/div) (1.0a/div) transient in pwm time (200s/div) v out =1.8v, v in =3.6v, 25c, i out = 0 to 800ma (5.0mv/div) (200mv/div) (200mv/div) (1.0a/div)
SC198A 9 typical characteristics (continued) switching waveforms v out =1.0v time (400ns/div) v in =3.6v, 25c, i out = 800ma (10mv/div) (2v/div) switching waveforms v out =1.8v v in =3.6v, 25c, i out = 800ma time (400ns/div) (10mv/div) (2v/div) switching waveforms in psave v out =1.0v, v in =3.6v, 25c, i out = 0ma (10mv/div) (2v/div) time (20ms/div) switching waveforms in psave v out =1.8v, v in =3.6v, 25c, i out = 80ma (50mv/div) (2v/div) time (4s/div)
SC198A 10 detailed description the SC198A is a dual step-down, pulse-width modulated dc-dc converter with programmable output voltage and power-save mode. this device has a 1mhz internal oscil- lator that can be used during forced pwm mode or power-save mode (psave). operation during normal operation, the pmos switch is activated on each rising edge of the internal oscillator. current feed- back for the switching regulator is through the pmos current path, and it is amplifi ed and summed with the internal slope compensation network. the voltage feed- back loop uses an internal feedback divider. the on-time is determined by comparing the summed current feed- back and the output of the error amplifi er. the period is set by the onboard oscillator or by an external clock attached to the mode/sync pin. when synchronizing to an external frequency, the SC198A operates in pwm mode only. the device has an internal synchronous nmos recti- fi er and does not require a schottky diode on the lx pin. the device is designed to operate as a buck converter in pwm mode with a fi xed frequency of 1mhz. at light loads, the part can enter psave mode to maximize effi ciency. output filter the SC198A converter also has internal loop compensa- tion. the internal compensation is designed to work with a specifi c output fi lter corner frequency defi ned by the equation out c c l 2 1 f u s when selecting output lter components, the lc product should not vary over a wide range. the practical lower limit for the inductor value is 4.7h to ensure system sta- bility. selection of smaller inductor and capacitor values will shift the corner frequency higher, which may impact stability. the internal compensation is designed to operate with a single-pole output lter where l = 4.7h and cout = 10f. programmable output voltage the SC198A has three control pins per output to allow selection of a voltage level from eight predetermined output voltage levels (see table 1). caution all ctl pins much be pulled high or low for proper operation (they must not be allowed to fl oat). the output voltage can be selected while the device is enabled and loaded. table 1 output voltage options programmable output voltage ctla2/b2 ctla1/b1 ctla0/b0 nominal output voltage (v) ab 0 0 0 1.0 1.8 0 0 1 1.1 2.2 0 1 0 1.2 2.5 0 1 1 1.3 2.7 1 0 0 1.4 2.8 1 0 1 1.5 2.9 1 1 0 1.6 3.0 1 1 1 1.8 3.3 continuous conduction and oscillator synchronization the SC198A is designed to operate in continuous con- duction mode, where it maintains a fixed frequency. when the mode/sync pin is tied high, the part runs under control of the internal oscillator in pwm mode. the part can be synchronized to an external clock by con- necting a clock signal to the mode/sync pin. the part synchronizes to the rising edge of the clock. power-save mode operation the SC198A has a power-save mode (psave) which may be selected by tying the mode/sync pin to gnd. selecting psave mode enables automatic activation of psave mode at light loads, thus maximizing effi ciency across the full load range. the device will automatically detect the load current at which it should enter psave mode. the device is optimized to track maximum effi ciency. application information
SC198A 11 in psave mode (see figure 1), vout regulation is con- trolled by bursts in switching. while the output voltage is between the low and high voltage thresholds, switching is stopped to reduce loss. when the voltage drops to the lower threshold, the switch bursts at a fi xed on time until the upper threshold is reached. at this point the output voltage is allowed to ramp down to the lower threshold without switching to conserve power. in the psave mode, less switching is used. this results in less power loss which can be seen in the diff erence of effi ciency between pwm mode and psave mode at light loads. lower thershold limit upper thershold limit psave mode at moderate load pwm mode at high load psave exit threshold burst off higher load applied 0a inductor current time v out burst off burst pwm mode psave mode at high load figure 1 psave operation the psave switching burst frequency is controlled such that the inductor current ripple during the burst is similar to that in pwm mode. the SC198A automatically detects when to exit psave mode. the device sets a maximum peak current in psave mode, and then detects when vout falls due to the load increasing above the level that the peak current will support. for the SC198A to exit psave mode, the load must be increased, which causes vout to decrease until the psave exit threshold is reached. psave levels are set high to minimize the undershoot when the SC198A exits psave. figure 1 shows psave mode operation and exiting into pwm mode at increased load. protection features the SC198A provides the following protection features: thermal shutdown current limit over-voltage protection soft-start thermal shutdown the SC198A has a thermal shutdown feature to protect against damage if the junction temperature exceeds 145c. in thermal shutdown, the on-chip power devices are disabled, tri-stating the lx output. switching resumes when the temperature drops by 10c. during this time, if the output voltage decreases by more than 60% of its programmed value, a soft-start will be invoked. current limit the internal power devices are protected by current limit functions. in the event of a short to ground on the output, the part enters frequency foldback mode. this causes the switching frequency to decrease by a factor determined by the output voltage. this prevents the inductor current from staircasing. over-voltage protection in the event of an over-voltage on the output in switcher mode, the pwm drive is disabled, eff ectively tri-stating the lx output. the part will not resume switching until the output voltage has fallen below 2% of the regulation voltage. soft-start the soft-start mode operates by limiting the current through the inductor and controlling the switching fre- quency. the current limit is increased over several milliseconds in discrete steps. this has the desired eff ect of limiting in-rush current from the input supply. the soft- start mode is cancelled once the output reaches regulation. soft-start is only re-enabled by power cycling, toggling enable, a uvlo event, or shutdown. ? ? ? ? application information (continued)
SC198A 12 inductor selection the SC198A was designed for optimum performance when using a 4.7h inductor. the magnitude of the inductor current ripple is dependent on the inductor value and is determined by the equation ? 1 ? u in out osc out l v v 1 f l v ? i this equation demonstrates the relationship between input voltage, output voltage, and inductor ripple current. the inductor should also have a low dcr to minimize the conduction losses and maximize e ciency. the minimum dc current rating of the inductor should be equal to the maximum load current plus half of the inductor current ripple as shown by the equation 2 ? i i i l out(max) l(peak) final inductor selection will depend on various design considerations such as e ciency, emi, size, and cost. table 2 lists some manufacturers of practical inductor options. table 2 recommended inductors manufacturer/ part no. value (h) dcr () rate current (a) tolerance (+%) dimensions lxwxh (mm) coilcraft lpo6610-472ml 4.7 0.20 1.1 20 6.60 5.50 1.10 coilcraft 1812ps-472klb 4.7 0.11 2.10 20 5.87 4.89 3.81 coilcraft d01608c-472ml 4.7 0.09 1.5 20 6.60 4.50 13.0 capacitor selection input capacitor the source input current to a buck converter is non-con- tinuous. to prevent large input voltage ripple a low esr ceramic capacitor is required. a minimum value of 4.7f should be used for suffi cient input voltage fi ltering and a 22f mlcc may be used for optimum input voltage fi ltering. applications information (continued) output capacitor output voltage ripple is dominated by the fi lter capaci- tance esr as shown in the equation v out(p-p) = di l esr cout capacitors with x7r or x5r ceramic dielectric are strongly recommended for their low esr and superior temperature and voltage characteristics. y5v capacitors should not be used as their temperature coeffi cients make them unsuit- able for this application. when selecting capacitors for the application, fi rst con- sider the dc voltage characteristic of the capacitor. the capacitance value at the dc operating voltage may be considerably lower than the rated value. table 3 lists rec- ommended capacitor values which have been chosen to minimize the impact of this limitation. a 10f ceramic capacitor is the minimum recommended value for the output fi lter capacitor. it is important to ensure the capaci- tor value does not drop below 9f for the operating range of the application. table 3 recommended capacitors manufacturer/ part no. value (f) rated voltage (vdc) type case size murata grm21br60j226me39 22 6.3 x5r 0805 murata grm188r60j106me47 10 6.3 x5r 0603 murata grm21br60j106ke01b 10 6.3 x5r 0805
SC198A 13 pcb layout considerations poor layout can degrade the performance of the dc-dc converter and can be a contributory factor in emi prob- lems, ground bounce, thermal issues, and resistive voltage losses. poor regulation and instability can result. the following design rules are recommended: place the inductor and fi lter capacitors as close to the device as possible and use short wide traces between the power components. route the output voltage feedback path away from inductor and lx node to minimize noise and magnetic interference. use a ground plane to further reduce noise interference on sensitive circuit nodes. 1. 2. 3. applications information (continued) vin gnd gnd gnd gnd lxa lxb cin a cin b cin r cout a cout b l a l b vouta voutb
SC198A 14 outline drawing C mlpq-20 4x4 . 001 max . 002 - . 040 nom 0.80 0.02 (0.20) 0.90 controlling dimensions are in millimeters (angles in degrees). coplanarity applies to the exposed pad as well as the terminals notes : 2. 1. 1 2 n e1 . 100 . 106 . 110 2. 55 2.70 2.80 pin 1 indicator 4.10 3.90 4. 00 4. 10 3.90 . 157 . 153 . 161 . 153 . 161 aaa c a c (laser mark ) d e b a1 a a2 seating plane lxn e/2 bxn bbb c a b e d/2 d1 e1 inches . 020 bsc b . 007 bbb aaa n d1 e l e d . 011 . 100 dim a1 a2 a min . 000 - . 031 0.30 0.18 . 012 0.25 . 010 0.50 2.80 0.30 2.55 . 004 . 004 20 . 016 . 157 . 106 . 020 . 110 0.10 0.10 20 0.40 4. 00 2.70 0. 50 bsc millimeters max 0.05 - 1.00 dimensions min 0.00 - nom (.008) .035
semtech corporation power mangement products division 200 flynn road, camarillo, ca 93012 phone: (805) 498-2111 fax: (805) 498-3804 www.semtech.com contact information SC198A 15 land pattern C mlpq-20 4x4 dimensions k h g z x p (c) company?s manufacturing guidelines are met 4.80 . 189 z y this land pattern is for reference purposes only . consult your manufacturing group to ensure your notes: 1. dim x y h k p c g millimeters inches (3.95) . 010 . 033 . 122 . 021 . 106 . 106 (. 155) 0.25 0.85 2.70 0.50 2.70 3.10

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