ACT4060 active-semi, inc. - 1 - datasheet rev 2, 6/2005 ACT4060 wide input 2a step down converter features ? 2a output current ? up to 95% efficiency ? 4.75v to 20v input range ? 8a shutdown supply current ? 410khz switching frequency ? adjustable output voltage ? cycle-by-cycle current limit protection ? thermal shutdown protection ? frequency fold back at short circuit ? stability with wide range of capacitors, including low esr ceramic capacitors ? sop-8 package applications ? tft lcd monitors ? portable dvds ? car-powered or battery-powered equipments ? set-top boxes ? telecom power supplies ? dsl and cable modems and routers ? termination supplies general description the ACT4060 is a current-mode step-down dc-dc converter that gen erates up to 2a output current at 410khz switching frequency. the device utilizes active-semi?s proprietary isobcd20 process for operation with input voltage up to 20v. consuming only 8a in shutdown mode, the ACT4060 is highly efficient with peak efficiency at 95% when in operation. protection features include cycle-by-cycle current limit, thermal shutdown, and frequency fold back at short circuit. the ACT4060 is available in sop-8 package and requires very few external devices for operation. figure 1. typical application circuit
ACT4060 active-semi, inc. - 2 - ordering information part number temperature range package pins ACT4060sh -40 c to 85 c sop-8 8 pin configuration pin description pin number pin name pin description 1 bs bootstrap. this pin acts as the positive rail for the high-side switch?s gate driver. connect a 10nf between this pin and sw. 2 in input supply. bypass this pin to g with a low esr capacitor. see input capacitor in application information section. 3 sw switch output. connect this pin to the switching end of the inductor. 4 g ground. 5 fb feedback input. the voltage at this pin is regulated to 1.293v. connect to the resistor divider between output and ground to set output voltage. 6 comp compensation pin. see compensation technique in application information section. 7 en enable input. when higher than 1.3v, this pin tu rns the ic on. when lower than 0.7v, this pin turns the ic off. output voltage is dischar ged when the ic is off. this pin has a small internal pull up current to a high level voltage when pin is not connected. 8 n/c not connected. 1 bs sop-8 2 in 3 sw 4 g 8 7 6 5 n/c en comp fb a ct4060
ACT4060 active-semi, inc. - 3 - absolute maximum ratings (note: do not exceed these limits to prevent damage to the dev ice. exposure to absolute max imum rating condi tions for long periods may affect device reliability.) parameter value unit in supply voltage -0.3 to 25 v sw voltage -1 to v in + 1 v bs voltage v sw ? 0.3 to v sw + 8 v en, fb, comp voltage -0.3 to 6 v continuous sw current internally limited a junction to ambient thermal resistance ( ja ) 105 c/w operating junction temperature -40 to 150 c storage temperature -55 to 150 c lead temperature (soldering, 10 sec) 300 c electrical characteristics (v in = 12v, t j = 25 c unless otherwise specified) parameter symbol test conditions min typ max unit feedback voltage v fb 4.75v v in 20v, v comp = 1.5v 1.267 1.293 1.319 v high-side switch on resistance r onh 0.20 ? low-side switch on resistance r onl 4.7 ? sw leakage v en = 0 0 10 a current limit i lim 2.4 2.85 a comp to current limit transconductance g comp 1.8 a/v error amplifier transconductance g ea ? i comp = 10 a 550 a/v error amplifier dc gain a vea 4000 v/v switching frequency f sw 350 410 470 khz short circuit switching frequency v fb = 0 50 khz maximum duty cycle d max v fb = 1.1v 90 % minimum duty cycle v fb = 1.4v 0 % enable threshold voltage hysteresis = 0.1v 0.7 1 1.3 v enable pull up current pin pulled up to 4.5v typically when left unconnected 1 a supply current in shutdown v en = 0 8 20 a ic supply current in operation v en = 3v, v fb = 1.4v 0.7 ma thermal shutdown temperature hysteresis = 10c 160 c
ACT4060 active-semi, inc. - 4 - functional description as seen in figure 2, functional block diagram , the ACT4060 is a current mode pulse width modulation (pwm) converter. the converter operates as follows: a switching cycle starts when the rising edge of the oscillator clock output causes the high-side power switch to turn on and the low-side power switch to turn off. with the sw side of the inductor now connected to in, the inductor current ramps up to store energy in the its magnetic field. the inductor current level is measured by the current sense amplifier and added to the oscillator ramp signal. if the resulting summation is higher than the comp voltage, the output of the pwm comparator goes high. when this happens or when oscillator clock output goes low, the high-side power switch turns off and the low-side power switch turns on. at this point, the sw side of the inductor swings to a diode voltage below ground, causing the inductor current to decrease and magnetic energy to be transferred to output. this state continues until the cycle starts again. the high-side power switch is driven by logic using bs bootstrap pin as the positive rail. this pin is charged to v sw + 6v when the low-side power switch turns on. the comp voltage is the integration of the error between fb input and the internal 1.293v reference. if fb is lower than the reference voltage, comp tends to go higher to increase current to the output. current limit happens when comp reaches its maximum clamp value of 2.55v. the oscillator normally switches at 410khz. however, if fb voltage is less than 0.7v, then the switching frequency decreases until it reaches a minimum of 50khz at v fb = 0.5v. shutdown control the ACT4060 has an enable input en for turning the ic on or off. when en is less than 0.7v, the ic is in 8a low current shutdown mode and output is discharged through the low-side power switch. when en is higher than 1.3v, the ic is in normal operation mode. en is internally pulled up with a 1a current source and can be left unconnected for always-on operation. note that en is a low voltage input with a maximum voltage of 6v; it should never be directly connected to in. thermal shutdown the ACT4060 automatically turns off when its junction temperature exceeds 160c. logic sw comp figure 2. functional block diagram oscillator & ramp bs ? + ? + + ? pwm comparator current sense a mplifier regulator & reference in fb + ? en error a mplifier 1a 1.293v enable foldback control g thermal shutdown 0.2 ? high-side power switch 4.7 ? low-side power switch
ACT4060 active-semi, inc. - 5 - application information output voltage setting figure 3 shows the connections for setting the output voltage. select the proper ratio of the two feedback resistors r fb1 and r fb2 based on the output voltage. typically, use r fb2 10k ? and determine r fb1 from the output voltage: ? ? ? ? ? ? ? ? ? = 1 v 293 . 1 v r r out 2 fb 1 fb (1) inductor selection the inductor maintains a continuous current to the output load. this 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 the reduction in current handling capability. in general, select an inductance value l based on ripple current requirement: ripple outmax sw in out in out k i f v ) v v ( v l ? ? = (2) where v in is the input voltage, v out is the output voltage, f sw is the switching frequency, i outmax is the maximum output current, and k ripple is the ripple factor. typically, choose k ripple = 30% to correspond to the peak-to-peak ripple current being 30% of the maximum output current. with this inductor value (table 1), the peak inductor current is i out ? (1 + k ripple / 2). make sure that this peak inductor current is less that the 3a current limit. finally, select the inductor core size so that it does not saturate at 3a. table 1. typical inductor values v out 1.5v 1.8v 2.5v 3.3v 5v l 6.8h 6.8h 10h 15h 22h input capacitor the input capacitor needs to be carefully selected to maintain suffic iently low ripple at the supply input of the converter. a low esr capacitor is highly recommended. since large current flows in and out of this capacitor during switching, its esr also affects efficiency. the input capacitance needs to be higher than 10 f. the best choice is the ceramic type; 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. the input capacitor should be placed close to the in and g pins of the ic, with shortest traces possible. in the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1 f ceramic capacitor is placed right next to the ic. output capacitor the output capacitor also needs to have low esr to keep low output voltage ripple. the output ripple voltage is: esr ripple outmax ripple r k i v = out 2 sw in lc f 28 v ? + (3) where i outmax is the maximum output current, k ripple is the ripple factor, r esr is the esr resistance of the output capacitor, f sw is the switching frequency, l in the inductor value, c out is the output capacitance. in the case of ceramic output capacitors, r esr is very small and does not contribute to the ripple. therefore, a lower capacitance value can be used for ceramic type. in the case of tantalum or electrolytic 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 esr. for ceramic output type, typically choose a capacitance of about 22 f. for tantalum or electrolytic type, choose a capacitor with less than 50m ? esr. rectifier diode use a schottky diode as the rectifier to conduct current when the high-side power switch is off. the schottky diode must have current rating higher than the maximum output current and the reverse voltage rating higher than the maximum input voltage. figure 3. output voltage setting fb r fb1 r fb2 v out ACT4060
ACT4060 active-semi, inc. - 6 - stability compensation the feedback system of the ic is stabilized by the components at comp pin, as shown in figure 4. the dc loop gain of the system is determined by the following equation: comp vea out vdc g a i v 3 . 1 a = (4) the dominant pole p1 is due to c comp : comp vea ea 1 p c a 2 g f = (5) the second pole p2 is the output pole: out out out 2 p c v 2 i f = (6) the first zero z1 is due to r comp and c comp : comp comp 1 z c r 2 1 f = (7) and finally, the third pole is due to r comp and c comp2 (if c comp2 is used): 2 comp comp 3 p c r 2 1 f = (8) follow the following steps to compensate the ic: step 1. set the cross over frequency at 1/10 of the switching frequency via r comp : v 3 . 1 g g 10 f c v 2 r comp ea sw out out comp ? = ) ( c v 10 7 . 1 out out 8 ? = (9) but limit r comp to 15k
maximum. step 2. set the zero f z1 at 1/4 of the cross over frequency. if r comp is less than 15k
, the equation for c comp is: ) f ( r 10 8 . 1 c comp 5 comp ? = (10) if r comp is limited to 15k
, then the actual cross over frequency is 3.4 / (v out c out ). therefore: ) f ( c v 10 2 . 1 c out out 5 comp ? = (11) step 3. if the output capacitor?s esr is high enough to cause a zero at lower than 4 times the cross over frequency, an additional compensation capacitor c comp2 is required. the condition for using c comp2 is: esrcout r ) ( v 012 . 0 , c 10 1 . 1 min out out 6 ? ? ? ? ? ? ? ? ? ? ? (12) and the proper value for c comp2 is: comp esrcout out 2 comp r r c c = (13) though c comp2 is unnecessary when the output capacitor has sufficiently low esr, a small value c comp2 such as 100pf may improve stability against pcb layout parasitic effects. table 2 shows some calculated results based on the compensation method above. table 2. typical compensation for different output voltages and output capacitors v out c out r comp c comp c comp2 2.5v 22f ceramic 8.2k
2.2nf none 3.3v 22f ceramic 12k
1.5nf none 5v 22f ceramic 15k
1.5nf none 2.5v 47f sp cap 15k
1.5nf none 3.3v 47f sp cap 15k
1.8nf none 5v 47f sp cap 15k
2.7nf none 2.5v 470f/6.3v/30m
15k
15nf 1nf 3.3v 470f/6.3v/30m
15k
22nf 1nf 5v 470f/10v/30m
15k
27nf none figure 5 shows a sample ACT4060 application circuit generating 2.5v/2a output. figure 4. stability compensation comp r comp ACT4060 c comp c comp2 ? ? c comp2 is needed only for high esr output capacitor
ACT4060 active-semi, inc. - 7 - typical performance characteristics figure 5. ACT4060 2.5v/2a output application
ACT4060 active-semi, inc. - 8 - package outline sop-8 package outline and dimensions dimension in milimeters dimension in inches symbol min max min max a 1.350 1.750 0.053 0.069 a1 0.100 0.250 0.004 0.010 a2 1.350 1.550 0.053 0.061 b 0.330 0.510 0.013 0.020 c 0.190 0.250 0.007 0.010 d 4.780 5.000 0.188 0.197 e 3.800 4.000 0.150 0.157 e1 5.800 6.300 0.228 0.248 e 1.270 typ 0.050 typ l 0.400 1.270 0.016 0.050 0 8 0 8 a ctive-semi, inc. reserves the right to m odify the circuitry or spec ifications without notice. users should evaluate each produc t to make sure that it is suitable for their applications. active -semi products are not intended or authorized for use as critica l components in life-support devices or systems. active-semi, inc. does not assume any liability arising out of the use of an y p roduct or circuit described in this datasheet, nor does it convey any patent license. 44081 old warm springs blvd, fremont, california 94538, usa
|
