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innovative power tm - 1 - www.active-semi.com copyright ? 2012 active-semi, inc. typical application circuit act4088 rev 2, 15-nov-12 28v, 1.5a, 1.4mhz step-down dc/dc converter in sot23-6 features ? wide 4.5v to 28v input voltage range ? 1.5a output current (12v in to 5v out ) ? output adjustable down to 0.81v ? 0.3 ? internal power mosfet ? up to 92% efficiency ? stable with low esr ceramic output capacitors ? fixed 1.4mhz operating frequency ? internal soft-start function ? over current protection with hiccup-mode ? thermal shutdown ? available in a sot23-6 package applications ? tft lcd monitors ? portable dvds, headphones, mp3 players, etc. ? car-powered or battery-powered equipment ? set-top boxes ? telecom power supplies ? dsl and cable modems and routers general description the act4088 is a current-mode step-down dc/dc converter that supplies up to 1.5a into 5v from a 12v input. 1.4mhz switching frequency allows the use of tiny external components, and internal loop compensation provides simple, stable power sup- plies with a minimum of external components. opti- mized for use with ceramic input and output capaci- tors, the act4088 provides a very compact 1.5a power supply for space constrained mobile and consumer applications. the act4088 operates over a wide input voltage range and utilizes current-mode operation to pro- vide excellent line and load transient response while requiring no external compensation compo- nents. fault protection includes cycle-by-cycle cur- rent limiting, frequency fo ld-back, hiccup mode, and thermal shutdown. internal soft-start provides a controlled startup with no overshoot, even at light loads. the act4088 is available in a tiny sot23-6 pack- age and requires very few external components. act4088 in en bst sw fb v out g v in 4.5v to 28v off on
act4088 rev 2, 15-nov-12 innovative power tm - 2 - www.active-semi.com copyright ? 2012 active-semi, inc. ordering information pin configuration pin descriptions part number temperature range package pins packing act4088us-t -40c to 85c sot23-6 6 tape & reel top mark frwj ACT4088US-T8082 -40c to 85c sot23-6 6 tape & reel frwj sot23-6 pin number pin name pin description 1 sw switch output. connect this pin to the switching end of the inductor. 2 in power supply input. bypass this pin with a 10f ceramic capacitor to g, placed as close to the ic as possible. 3 en enable input. en is pulled up to 5v with a 2a current, and contains a precise 1.24v logic threshold. drive this pin to a logic-high or leave unconnected to enable the ic. drive to a logic-low to disable the ic and enter micro-power shutdown mode. 4 fb feedback input. the voltage at this pin is regulated to 0.81v. connect to the center point of a resistive voltage-divider bet ween out and g to set the output voltage. 5 g ground and heat sink. connect this pin to a large, uncovered pcb copper area for best heat dissipation. 6 bst bootstrap. this pin acts as the power suppl y for the high-side switch?s gate driver. connect a 22nf capacitor between this pin and sw. 6 5 4 1 2 3 act4088 bst g fb sw in en
act4088 rev 2, 15-nov-12 innovative power tm - 3 - www.active-semi.com copyright ? 2012 active-semi, inc. absolute maximum ratings c parameter value unit in supply voltage -0.3 to 32 v sw voltage -1 to v in + 1 v bst voltage v sw - 0.3 to v sw + 7 v en, fb voltage -0.3 to 6 v continuous sw current internally limited a junction to ambient thermal resistance ( ja ) 220 c/w maximum power dissipation 0.5 w operating junction temperature -40 to 150 c storage temperature -55 to 150 c lead temperature (soldering, 10 sec) 300 c parameter symbol test cond itions min typ max unit input voltage v in v out = 3.3v, i load = 0a to 1.5a 4.5 28 v under voltage lockout voltage v uvlo input voltage rising 4 4.2 4.49 v under voltage lockout hysteresis 250 mv feedback voltage v fb 4.75v v in 20v, v comp = 1.5v 0.79 0.81 0.83 v frequency foldback threshold 250 mv high-side switch on resistance r onh 0.300 ? low-side switch on resistance r onh 15 ? sw leakage v en = 0, v sw = 0v 1 10 a current limit i ltm v in = 12v, v out = 5v, or en = g, sw = g 1.8 a switching frequency f sw 1.1 1.4 1.6 mhz foldback switching frequency v fb = 0v, or fb = g 467 khz maximum duty cycle d max v fb = 0.6v 92 % minimum on-time 75 ns en threshold voltage en rising 1.12 1.24 1.36 v en hysteresis en rising 100 mv en internal pull-up current 2 a supply current in shutdown v en = 0v or en = g 15 30 a supply current in operation v en = 2v, v fb = 1.0v 1 2 ma thermal shutdown temperature 160 c thermal shutdown hysteresis 10 c electrical characteristics (v in = 12v, t a = 25c, unless otherwise specified.) c : do not exceed these limits to prevent damage to the device. exposure to absolute maximum rati ng conditions for long periods m ay affect device reliability.
act4088 rev 2, 15-nov-12 innovative power tm - 4 - www.active-semi.com copyright ? 2012 active-semi, inc. typical performanc e characteristics (circuit of figure 2, v in = 12v, l = 4.7h, c1 = 10f, c2 = 22f, t a = +25c, unless otherwise specified.) temperature (c) fb voltage (mv) fb voltage vs. temperature 800 804 808 812 816 820 -40 -20 0 20 40 60 80 100 120 oscillator frequency (mhz) oscillator frequency vs. temperature 1.20 1.30 1.40 1.50 1.60 act4088-001 act4088-002 act4088-003 act4088-004 efficiency vs. load current efficiency (%) 50 75 85 55 65 0.1 1 10 efficiency vs. load current efficiency (%) 95 v in = 12v v out = 5v v out = 3.3v duty cycle peak current limit (a) peak current limit vs. duty cycle 0.0 0.5 1.0 1.5 2.0 2.5 0 20 40 80 100 input voltage (v) quiescent supply current (a) shutdown current vs. input voltage 0 5 10 15 20 30 0 4 8 24 28 act4088-005 act4088-006 load current (a) v in = 24v v in = 18v 0.1 1 10 load current (a) 50 75 85 55 65 95 v in = 24v v in = 18v v in = 12v temperature (c) -40 -20 0 20 40 60 80 100 120 3.0 60 25 s u p p l y c u r r e n t en pull-up current 16 12 20
act4088 rev 2, 15-nov-12 innovative power tm - 5 - www.active-semi.com copyright ? 2012 active-semi, inc. typical performance ch aracteristics cont?d (circuit of figure 2, v in = 12v, l = 4.7h, c1 = 10f, c2 = 22f, t a = +25c, unless otherwise specified.) act4088-008 act4088-009 act4088-010 act4088-011 act4088-012 ch1 ch2 load transient response ch1 ch2 start-up waveforms ch1 ch2 start-up waveforms ch1: v out , 50mv/div, (ac coupled) ch2: v sw , 10v/div ch3: i l , 500ma/div time: 400ns/div ch1 ch2 ch3 steady state switching waveforms ch1: v out , 100mv/div, (ac coupled) ch2: i l , 1a/div time: 1ms/div ch1 ch2 hiccup mode switching waveforms act4088-007 ch1 ch2 load transient response i load = 200ma to 800ma ch1: v out , 2v/div ch2: v sw , 10v/div ch3: i l , 1a/div time: 200s/div ch1: v en , 2v/div ch2: v out , 2v/div ch3: v sw , 10v/div ch4: i l , 1a/div time: 400s/div ch1: v out , 50mv/div ch2: i load , 500ma/div time: 100s/div ch1: v out , 50mv/div ch2: i load , 500ma/div time: 100s/div i load = 200ma to 1.5a i load = 0ma ch3 i load = 1a ch3 ch4 i load = 1a
act4088 rev 2, 15-nov-12 innovative power tm - 6 - www.active-semi.com copyright ? 2012 active-semi, inc. functional block diagram functional description the act4088 is a current-mode step-down dc/dc converter that provides excellent transient response with no extra external compensation components. this device contains an internal, low-resistance, high-voltage power mosfet, and operates at a high 1.4mhz operating frequency to ensure a com- pact, high-efficiency design with excellent ac and dc performance. setting the output voltage an external voltage divider is used to set the output voltage, as well as provide a known impedance from v out to fb for compensation purposes. con- nect a 50k ? resistor from the output to fb to ensure stable compensation, and select the bottom resistor to provide the desired regulation voltage. figure 1: output voltage setting the feedback resistor (r fb1 ) interacts with the inter- nal compensation network, and plays an important in setting the act4088's transient response and ensuring stability. for most applications, choosing r fb1 = 49.9k ? provides good results. for applica- tions with output voltages of 1.8v or lower, use a larger r fb1 value such as 80.6k ? . once r fb1 is chosen, use the following equation to choose r fb2 : selecting the inductor the act4088 was optimized for use with a 4.7h inductor. when choosing an inductor, choose one with a dc resistance of less than 250m ? and a dc current rating that is typically 30% higher than the maximum load current. during typical operation, the inductor maintains a continuous current to out put load. the inductor current has a ripple that is dependent on the inductance value. higher inductance reduces the peak-to-peak ripple current. the trade off for high inductance value is the increase in inductor core size and series resistance, and a reduction in current handling capability. if efficiency at light loads (such as less than 100ma) is critical in the application, a larger inductor is recommended. r fb1 r fb2 v out act4088 fb in en fb g reference & thermal shutdown control soft-start pwm comparator ilim comparator hiccup regulator & uvlo oscillator compensation driver csa q1 bst sw ea ? ? ? ? ? ? ? = 1 v 81 . 0 v r r out 1 fb 2 fb (1)
act4088 rev 2, 15-nov-12 innovative power tm - 7 - www.active-semi.com copyright ? 2012 active-semi, inc. out 2 sw in esr ripple outmax ripple lc f 8 v r k i v + = (2) rectifier diode use a schottky diode as the rectifier to conduct cur- rent when the high-side power switch is off. the schottky diode must have current rating higher than the maximum output current and the reverse volt- age rating higher than the maximum input voltage (see figure 2). selecting the input capacitor for best performance choose a ceramic type ca- pacitor with x5r or x7r dielectrics due to their low esr and small temperature coefficients. however, low esr tantalum or electrolytic types may also be used, provided that the rms ripple current rating is higher than 50% of the output current. for most applications, a 10f capacitor is sufficient. the input capacitor should be placed close to the in and g pins of the ic, with shor test possible traces. in the case of tantalum or electrolytic types, connect a small parallel 0.1f ceramic capacitor right next to the ic. selecting the output capacitor a 22f ceramic capacitor with x5r or x7r dielec- tric provides the best re sults over a wide range of applications. the output capacitor also needs to have low esr to keep low output voltage ripple. the output ripple voltage is: where i outmax is the maximum output current, k ripple is the ripple factor (typically 20% to 30%), r esr resistance is the esr of the output capacitor, f sw is the switching frequency, l is the inductor value, and c out is the output capacitance. in the case of ceramic output capacitors, r esr is very small and does not cont ribute to the ripple. in the case of tantalum or elec trolytic type, the ripple is dominated by r esr multiplied by the ripple current. in that case, the output capacitor is chosen to have sufficiently low due to esr, typically choose a ca- pacitor with less than 50m ? esr. external bootstrap diode an external bootstrap diode (d2 in figure 2) is rec- ommended if the input voltage is less than 5.5v or if there is a 5v system rail available. this diode helps strengthen gate drive at lower input voltages, result- ing in lower on-resistance and higher efficiency. low cost diodes, such as 1n4148 or bat54, are suitable for this application. shutdown control the act4088 enable pin provides several features for adjusting and sequencing the power supply. an internal 2a current source pull-up, and a precision 1.24v comparator with hysteresis. with these com- ponents, a user has the flexibility of using the en pin as: 1) a digital on/off control by pulling down the en current source with an external open-drain tran- sistor. the voltage at en is internally clamped to 6v. 2) a sequenced power supply by tying the en pin through a resistor to the output of another power supply. the ic will be enabled when the voltage at en exceeds 1.24v, or a resistor divider can be used to adjust the turn-on threshold. 3) an always-on converter by floating the en pin or pulling en to a desired voltage with a high value (1m ? ) external resistor. en is internally clamped at 6v and will dissipate power if an external re- sistor attempts to pull en above the 6v clamp voltage. 4) line uvlo. if desired, to achieve a uvlo volt- age that is higher than the internal uvlo, an external resistor divider from vin to en to gnd can be used to disable the act4088 until a higher input voltage is achi eved. for example, it is not useful for a converter with 9v output to start up with a 4.2v input voltage, as the output cannot reach regulation. to enable the act4088 when the input voltage reaches 12v, a 9k ? /1k ? resistor divider from in to gnd can be con- nected to the en pin. both the precision 1.2v threshold and 80mv hysteresis are multiplied by the resistor ratio, providing a proportional 6.67% hysteresis for any startup threshold. for the ex- ample of a 12v enable threshold, the turn off threshold would be 11.2v. 5) power supply sequencing. by connecting a small capacitor from en to gnd, the 2a current source and 1.24v threshold can provide a stable and predictable delay betw een startup of multiple power supplies. for exampl e, a startup delay of roughly 10ms is provided using 150nf, and roughly 20ms by using 330nf. the en current source is active anytime an input supply is ap- plied, so disabling the ic or resetting the delay requires an external open-drain pull-down device to reset the capacitor and hold the en pin low for shutdown.
act4088 rev 2, 15-nov-12 innovative power tm - 8 - www.active-semi.com copyright ? 2012 active-semi, inc. soft-start the act4088 provides an internal soft-start fea- ture, which ramps the output voltage and output current are from 0 to the full value over 0.5 millisec- onds. this feature prevents output voltage over- shoot at light loads as well as to prevent large in- rush currents upon startup. the soft-start circuitry is internally reset anytime the ic is disabled using the en pin, as well as if the ic reaches hiccup mode or thermal shutdown. in all of these cases, soft-start provides a smooth, controll ed restart after the fault is removed. frequency foldback the voltage at fb is monitored by a comparator to detect an extreme output overload condition. if the voltage at the fb pin falls to below 0.3v, the inter- nal oscillator slows to a decreased frequency of 467khz, 33% of the nominal value. this prevents the inductor current from rising excessively during a dead-short condition, potent ially resulting in induc- tor saturation. figure 2: act4088 typical 5v/1.5a output application
act4088 rev 2, 15-nov-12 innovative power tm - 9 - www.active-semi.com copyright ? 2012 active-semi, inc. act4088-013 ch1: i load , 500ma/div ch2: v out , 100mv/div (ac coupled) time: 200s/div act4088-014 ch1 ch2 circuit of figure 3 i load = 150ma to 850ma ch1: v sw , 10v/div ch2: v out , 20mv/div (ac coupled) time: 400ns/div circuit of figure 3 i load = 1a figure 3: act4088 optimized for minimal external components the act4088 with provides excellent ac and dc results across a wide range of external component combinations. the circuit of figure 3 can be used to generate a 5v output from a 12v input ut ilizing a smaller (i.e. lower-cost) output capacitor while main taining good performance. ch1 ch2 figure 4: circuit of figure 3 (i load = 150ma to 850ma) figure 5: circuit of figure 3 (i load = 1a)
act4088 rev 2, 15-nov-12 innovative power tm - 10 - www.active-semi.com copyright ? 2012 active-semi, inc. hiccup mode if the act4088 transitions from normal operation to a severe overload condition (the voltage at fb falls below 0.3v), the controll er automatically enters "hiccup mode" to provide maximum protection to the system. in hiccup mode, the ic stops switching, clears the soft-start circuitry, then attempts to re- start. if the overload condition has been removed, the ic will start up normally and continue regulating. in the case of a sustai ned overload, however, the ic will attempt to regulate for a period of time equal to 3x the soft-start period (1.5ms). if the overload condition persists until the end of this period, the ic will begin another hiccup cycle. this hiccup-mode control scheme minimizes power dissipation during severe overload conditions, and ensures that the act4088 responds quickly to instantaneous severe overload conditions while providing immunity to false hiccups that may occur with a heavily loaded output. thermal shutdown the act4088 automatically turns off when the ic junction temperature exceeds 160c, and re- enables when the ic junction temperature drops by 10c (typ). pc board layout the high current paths at g, 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 g pins. the external feedback resistors should be placed next to the fb pin. keep the switch node traces short and away from the feedback network and use shielded inductors.
act4088 rev 2, 15-nov-12 innovative power tm - 11 - www.active-semi.com copyright ? 2012 active-semi, inc. package outline sot23-6 package outline and dimensions d b e1 e e e1 a1 a2 a c ll1 0.2 symbol dimension in millimeters dimension in inches min max min max a - 1.450 - 0.057 a1 0.000 0.150 0.000 0.006 a2 0.900 1.300 0.035 0.051 b 0.300 0.500 0.012 0.020 c 0.080 0.220 0.003 0.009 d 2.900 bsc 0.114 bsc e 1.600 bsc 0.063 bsc e1 2.800 bsc 0.110 bsc e 0.950 bsc 0.037 bsc e1 1.900 bsc 0.075 bsc l 0.300 0.600 0.012 0.024 0 8 0 8 active-semi, inc. reserves the right to modify the circuitry or specifications without notice. user s should evaluate each product to make sure that it is suitable for their applicat ions. active-semi products are not intended or authorized for use as critical components in life-support dev ices or systems. active-semi, inc. does not assume any liability arising out of the use of any product or circuit described in this datasheet, nor does it convey any patent license. active-semi and its logo are trademarks of active-semi, inc. for more information on this and other products, contact sales@active-semi.com or visit http://www.active-semi.com . is a registered trademark of active-semi.

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