1 si-core cor p . rev. 1.02 CX4060 pwm c ontrol 2a s tep-down c onverte r features ? 2a output current ? up to 95% efficiency ? 4.75v to 18v input range ? 20a shutdown supply current ? 360khz switching frequency ? adjustable output voltage from 1.22v to 0.85v in ? cycle-by-cycle current limit protection ? thermal shutdown protection ? frequency fold back at short circuit ? stability with wide range of 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 CX4060 is a current-mode step-down dc-dc converter that generates up to 2a output current at 360khz switching frequency. the device utilizes advanced bcd process fo r operation with input voltage up to 18 v c onsuming only 20a in shutdown mode, the CX4060 is highly efficient with peak efficienc y at 95% when in operation. protection features include cycle-by-cycle current limit, thermal shutdown, and frequenc y fold back at short circuit. the CX4060 is available in sop-8 package and requires very few external devices for operation. CX4060 g act4060 fb comp sw en 4.75v to 18v enable in bs 2.5v/2a en CX4060 sw fb comp gnd 23 1 75 46 figure 1. typical application circuit www..net
2 si-core cor p . rev. 1.02 CX4060 ordering information part number temperature range package pins CX4060 -40 c to 85 c sop-8 7,1 n/c pin configuration pin no. 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 1.22v. 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 2.0v, this pin turns the ic on. when lower than 1.8v, this pin turns the ic off. output voltage is discharged 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. 3 4 2 5 6 7 8 1 bs in sw gnd com en n/c CX4060 fb
3 si-core cor p . rev. 1.02 CX4060 absolute maximum ratings (note: do not exceed these limits to prevent damage to the device. exposure to absolute maximum rating conditions for long peri ods may affect device reliability.) parameter value unit in supply voltage -0.3 to 20 v sw voltage -1 to vin + 1 v bs voltage vsw - 0.3 to vsw + 6 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 charcxeristics (vin = 12v, tj = 25 c unless otherwise specified) parameter symbol test conditions min typ max unit feedback voltage vfb 4.75v ? vin ? 18v, vcomp = 1.5v 1.184 1.222 1.258 v high-side switch on resistance ronh 0.22 ? low-side switch on resistance ronl 4.7 ? sw leakage ven = 0 1 10 a current limit ilim 2.6 3.3 a comp to current limit transconductance gcomp 1.8 a/v error amplifier transconductance gea ? icomp = 10a 550 a/v error amplifier dc gain avea 3200 v/v switching frequency f sw 300 360 420 khz short circuit switching frequency vfb = 0 50 khz maximum duty cycle dmax vfb = 1.1v 90 % minimum duty cycle vfb = 1.4v 0% enable threshold voltage hysteresis = 0.1v 2.0 2.2 v enable pull up current pin pulled up to 4.5v typically when left unconnected 2.5 a supply current in shutdown ven = 0 20 50 a ic supply current in operation ven = 3v, vfb = 1.4v 1.0 1.5 ma thermal shutdown temperature hysteresis = 10c 168 c
4 si-core cor p . rev. 1.02 CX4060 figure 2 . functional block diagram functional description as seen in figure 2, functional block diagram , the CX4060 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 1.22v 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 clam value of 2.55v. the oscillator normally switches at 360khz. 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 CX4060 has an enable input en for turning the ic on or off. when en is less than 1.8v, the ic is in 20a low current shutdown mode and output is discharged through the low-side power switch. when en is higher than 2.0v, the ic is in normal opera tion mode. en is internally pulled up with a 2.5a 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 CX4060 automatically turns off when its junction temperature exceeds 168c. - - + 1.22v
5 si-core cor p . rev. 1.02 CX4060 application information output voltage setting fb vout r fb1 r fb2 figure 3. 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: 12 1 1.22 out fb fb v rr v ?? =? ?? ?? (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 o ff 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: () out in out in sw outmax ripple vvv l vf i k ?? = (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 vout 1.5v 1.8v 2.5v 3.3v 5v 12v l(h) 6.8 6.8 10 15 22 47 input capacitor the input capacitor needs to be carefully selected to maintain sufficiently 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 10f. the best choice is the ceramic type; however, low esr tantalum or electrolytic ty pes 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 0.1f 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: v ripple = i outmax k ripple r esr 2 28 in s w out v flc + i ( 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 electr olytic 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 22f. 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.
6 si-core cor p . rev. 1.02 CX4060 stabilty compensation c comp2 in needed only for high esr output capacitor figure 4. stability c ompensation v out cout rcomp ccomp ccomp2 2.5v 22f ceramic 8.2k ? 2.2nf none 3.3v 22f ceramic 12k ? 1.5nf none 5v 22f ceramic 15k ? 1.5nf none 12v 22f ceramic 15k ? 3.3nf none 2.5v 22f sp cap 15k ? 1.5nf none 3.3v 22f sp cap 15k ? 1.8nf none 5v 22f sp cap 15k ? 2.7nf none 12v 22f sp cap 15k ? 6.8nf none 2.5v 470f/6.3v/30m ? 15k ? 15nf 1nf 3.3v 470f/6.3v/30m ? 15k ? 22nf 1nf 5v 470f/6.3v/30m ? 15k ? 27nf none 12v 220f/25v/30m ? 15k ? 33nf none 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: a vdc = 1.3 out v i a vea g comp (4) the dominant pole p1 is due to c comp : 1 2 ea p vba comp g ac f = i (5) the second pole p2 is the output pole: 2 out esrcout comp comp cr r c = (6) the first zero z1 is due to r comp and c comp : 1 1 2 z comp comp rc f = i (7) and finally, the third pole is due to r comp and c comp2 (if c comp2 is used): 3 2 1 2 p comp comp rc f = i (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 : 2 10 1.3 out out sw comp ea comp vcf gg v r = i = 1.7 i 10 8 out c out ( ? ) (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: 1.8 i 10 -5 c comp = ( f ) (10) r comp if r comp is limited to 15k ? , then the actual cross over frequency is 3.4 / (v out c out ). therefore: 5 1.2 10 comp out out cvc ? = ( f ) (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: r esrcout ( ) 6 1.1 10 0.012 out out min v c ? ? (?) (12) and the proper value for c comp2 is: 2 out esrcout comp comp cr r 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 comp CX4060 c comp r comp c comp2
7 si-core cor p . rev. 1.02 CX4060 typical application 3.3v/2a output typical application 5.0v/2a output c1 10uf/35v c2 22nf 4.75v to 18v act4060 in en 5v/2a bs fb comp sw enable g r3 15k r2 10k d1 r1 31k CX4060 c3 10nf l1 22uh/3a 22uf/10v c eramic,or 47uf/6.3 sp c ap c4 c5 (optional) in 46 7 2 1 5 3 c1 10uf/35v c2 22nf 4.75v to 18v act4060 in en 3.3v/2a bs fb comp sw enable g r3 15k r2 10k d1 r1 16.9k CX4060 c3 10nf l1 15uh/3a 22uf/10v c eramic,or 47uf/6.3 sp c ap c4 c5 (optional) in 7 2 46 5 3 1 figure5b: CX4060 5v/2a output application figure5a: CX4060 3.3v/2a output application
8 si-core cor p . rev. 1.02 CX4060 typical application 2.5v/2a output 8: 6 : 9: 7 : c1 10uf/35v c2 22nf 4.75v to 18v act4060 in en 2.5v/2a bs fb comp sw enable g r3 15k r2 10k d1 r1 10.5k ic1 CX4060 c3 10nf l1 10uh/3a 22uf/10v c eramic,or 47uf/6.3 sp c ap c4 c5 (optional) in figure5c: CX4060 2.5v/2a output application c1 10uf/35v c2 22nf 4.75v to 18v in en 2.5v/2a bs fb comp sw enable g r3 15k r2 10k d1 r1 10.5k CX4060 c3 10nf l1 10uh/3a 22uf/10v c eramic,or 47uf/6.3 sp c ap c4 c5 (optional) in 7 2 4 6 5 3 1 figure5c: CX4060 2.5v/2a output application typecial performance and characteristics:
9 si-core cor p . rev. 1.02 CX4060 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 figure10 : figure11 :
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