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high performance current mode controllers order this document by uc3842b/d d suffix plastic package case 751a (so14) n suffix plastic package case 626 d1 suffix plastic package case 751 (so8) 8 1 8 1 14 1 device operating temperature range package ordering information uc384xbd uc384xbd1 t a = 0 to +70 c t a = 25 to +85 c so14 uc384xbn uc284xbd uc284xbd1 uc284xbn uc384xbvd uc384xbvd1 uc384xbvn t a = 40 to +105 c so8 plastic so14 so8 plastic so14 so8 plastic pin connections compensation nc voltage feedback nc current sense nc r t /c t compensation voltage feedback current sense r t /c t v ref v ref nc v cc v c output gnd power ground v cc output gnd (top view) 8 7 6 5 1 2 3 4 1 2 3 4 14 13 12 11 5 6 7 10 9 8 (top view) x indicates either a 2 or 3 to define specific device part numbers. 1 motorola analog ic device data
the uc3842b, UC3843B series are high performance fixed frequency current mode controllers. they are specifically designed for offline and dctodc converter applications offering the designer a costeffective solution with minimal external components. these integrated circuits feature a trimmed oscillator for precise duty cycle control, a temperature compensated reference, high gain error amplifier, current sensing comparator, and a high current totem pole output ideally suited for driving a power mosfet. also included are protective features consisting of input and reference undervoltage lockouts each with hysteresis, cyclebycycle current limiting, programmable output deadtime, and a latch for single pulse metering. these devices are available in an 8pin dualinline and surface mount (so8) plastic package as well as the 14pin plastic surface mount (so14). the so14 package has separate power and ground pins for the totem pole output stage. the ucx842b has uvlo thresholds of 16 v (on) and 10 v (off), ideally suited for offline converters. the ucx843b is tailored for lower voltage applications having uvlo thresholds of 8.5 v (on) and 7.6 v (off). ? trimmed oscillator for precise frequency control ? oscillator frequency guaranteed at 250 khz ? current mode operation to 500 khz ? automatic feed forward compensation ? latching pwm for cyclebycycle current limiting ? internally trimmed reference with undervoltage lockout ? high current totem pole output ? undervoltage lockout with hysteresis ? low startup and operating current simplified block diagram 5.0v reference latching pwm v cc undervoltage lockout oscillator error amplifier 7(12) v c 7(11) output 6(10) power ground 5(8) 3(5) current sense input v ref 8(14) 4(7) 2(3) 1(1) gnd 5(9) r t /c t voltage feedback input r r + v ref undervoltage lockout output compensation pin numbers in parenthesis are for the d suffix so14 package. v cc ? motorola, inc. 1996 rev 1
uc3842b, 43b uc2842b, 43b 2 motorola analog ic device data maximum ratings rating symbol value unit total power supply and zener current (i cc + i z ) 30 ma output current, source or sink (note 1) i o 1.0 a output energy (capacitive load per cycle) w 5.0 m j current sense and voltage feedback inputs v in 0.3 to + 5.5 v error amp output sink current i o 10 ma power dissipation and thermal characteristics d suffix, plastic package, so14 case 751a maximum power dissipation @ t a = 25 c thermal resistance, junctiontoair d1 suffix, plastic package, so8 case 751 maximum power dissipation @ t a = 25 c thermal resistance, junctiontoair n suffix, plastic package, case 626 maximum power dissipation @ t a = 25 c thermal resistance, junctiontoair p d r q ja p d r q ja p d r q ja 862 145 702 178 1.25 100 mw c/w mw c/w w c/w operating junction temperature t j +150 c operating ambient temperature uc3842b, UC3843B uc2842b, uc2843b uc3842bv, UC3843Bv t a 0 to + 70 25 to + 85 40 to +105 c storage temperature range t stg 65 to +150 c electrical characteristics (v cc = 15 v [note 2], r t = 10 k, c t = 3.3 nf. for typical values t a = 25 c, for min/max values t a is the operating ambient temperature range that applies [note 3], unless otherwise noted.) uc284xb uc384xb, xbv characteristics symbol min typ max min typ max unit reference section reference output voltage (i o = 1.0 ma, t j = 25 c) v ref 4.95 5.0 5.05 4.9 5.0 5.1 v line regulation (v cc = 12 v to 25 v) reg line 2.0 20 2.0 20 mv load regulation (i o = 1.0 ma to 20 ma) reg load 3.0 25 3.0 25 mv temperature stability t s 0.2 0.2 mv/ c total output variation over line, load, and temperature v ref 4.9 5.1 4.82 5.18 v output noise voltage (f = 10 hz to 10 khz, t j = 25 c) v n 50 50 m v long term stability (t a = 125 c for 1000 hours) s 5.0 5.0 mv output short circuit current i sc 30 85 180 30 85 180 ma oscillator section frequency t j = 25 c t a = t low to t high t j = 25 c (r t = 6.2 k, c t = 1.0 nf) f osc 49 48 225 52 250 55 56 275 49 48 225 52 250 55 56 275 khz frequency change with voltage (v cc = 12 v to 25 v) d f osc / d v 0.2 1.0 0.2 1.0 % frequency change with temperature t a = t low to t high d f osc / d t 1.0 0.5 % oscillator voltage swing (peaktopeak) v osc 1.6 1.6 v discharge current (v osc = 2.0 v) t j = 25 c t a = t low to t high (uc284xb, uc384xb) t a = t low to t high (uc384xbv) i dischg 7.8 7.5 8.3 8.8 8.8 7.8 7.6 7.2 8.3 8.8 8.8 8.8 ma notes: 1. maximum package power dissipation limits must be observed. 2. adjust v cc above the startup threshold before setting to 15 v. 3. low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible. t low =0 c for uc3842b, UC3843B t high = +70 c for uc3842b, UC3843B t low = 25 c for uc2842b, uc2843b t high = +85 c for uc2842b, uc2843b t low = 40 c for uc3842bv, UC3843Bv t high = +105 c for uc3842bv, UC3843Bv
uc3842b, 43b uc2842b, 43b 3 motorola analog ic device data electrical characteristics (v cc = 15 v [note 2], r t = 10 k, c t = 3.3 nf. for typical values t a = 25 c, for min/max values t a is the operating ambient temperature range that applies [note 3], unless otherwise noted.) uc284xb uc384xb, xbv characteristics symbol min typ max min typ max unit error amplifier section voltage feedback input (v o = 2.5 v) v fb 2.45 2.5 2.55 2.42 2.5 2.58 v input bias current (v fb = 5.0 v) i ib 0.1 1.0 0.1 2.0 m a open loop voltage gain (v o = 2.0 v to 4.0 v) a vol 65 90 65 90 db unity gain bandwidth (t j = 25 c) bw 0.7 1.0 0.7 1.0 mhz power supply rejection ratio (v cc = 12 v to 25 v) psrr 60 70 60 70 db output current sink (v o = 1.1 v, v fb = 2.7 v) source (v o = 5.0 v, v fb = 2.3 v) i sink i source 2.0 0.5 12 1.0 2.0 0.5 12 1.0 ma output voltage swing high state (r l = 15 k to ground, v fb = 2.3 v) low state (r l = 15 k to v ref , v fb = 2.7 v) (uc284xb, uc384xb) (uc384xbv) v oh v ol 5.0 6.2 0.8 1.1 5.0 6.2 0.8 0.8 1.1 1.2 v current sense section current sense input voltage gain (notes 4 & 5) (uc284xb, uc384xb) (uc384xbv) a v 2.85 3.0 3.15 2.85 2.85 3.0 3.0 3.15 3.25 v/v maximum current sense input threshold (note 4) (uc284xb, uc384xb) (uc384xbv) v th 0.9 1.0 1.1 0.9 0.85 1.0 1.0 1.1 1.1 v power supply rejection ratio v cc = 12 v to 25 v, note 4 psrr 70 70 db input bias current i ib 2.0 10 2.0 10 m a propagation delay (current sense input to output) t plh(in/out) 150 300 150 300 ns output section output voltage low state (i sink = 20 ma) (i sink = 200 ma) (uc284xb, uc384xb) (uc384xbv) high state (i source = 20 ma) (uc284xb, uc384xb) (uc384xbv) (i source = 200 ma) v ol v oh 13 12 0.1 1.6 13.5 13.4 0.4 2.2 13 12.9 12 0.1 1.6 1.6 13.5 13.5 13.4 0.4 2.2 2.3 v output voltage with uvlo activated v cc = 6.0 v, i sink = 1.0 ma v ol(uvlo) 0.1 1.1 0.1 1.1 v output voltage rise time (c l = 1.0 nf, t j = 25 c) t r 50 150 50 150 ns output voltage fall time (c l = 1.0 nf, t j = 25 c) t f 50 150 50 150 ns undervoltage lockout section startup threshold (v cc ) ucx842b, bv ucx843b, bv v th 15 7.8 16 8.4 17 9.0 14.5 7.8 16 8.4 17.5 9.0 v minimum operating voltage after turnon (v cc ) ucx842b, bv ucx843b, bv v cc(min) 9.0 7.0 10 7.6 11 8.2 8.5 7.0 10 7.6 11.5 8.2 v notes: 2. adjust v cc above the startup threshold before setting to 15 v. 3. low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible. t low =0 c for uc3842b, UC3843B t high = +70 c for uc3842b, UC3843B t low = 25 c for uc2842b, uc2843b t high = +85 c for uc2842b, uc2843b t low = 40 c for uc3842bv, UC3843Bv t high = +105 c for uc3842bv, UC3843Bv 4. this parameter is measured at the latch trip point with v fb = 0 v. 5. comparator gain is defined as: a v d v output compensation d v current sense input
uc3842b, 43b uc2842b, 43b 4 motorola analog ic device data electrical characteristics (v cc = 15 v [note 2], r t = 10 k, c t = 3.3 nf, for typical values t a = 25 c, for min/max values t a is the operating ambient temperature range that applies [note 3], unless otherwise noted.) uc284xb uc384xb, bv characteristics symbol min typ max min typ max unit pwm section duty cycle maximum (uc284xb, uc384xb) maximum (uc384xbv) minimum dc (max) dc (min) 94 96 0 94 93 96 96 0 % total device power supply current startup (v cc = 6.5 v for ucx843b, startup (v cc 14 v for ucx842b, bv) operating (note 2) i cc + i c 0.3 12 0.5 17 0.3 12 0.5 17 ma power supply zener voltage (i cc = 25 ma) v z 30 36 30 36 v notes: 2. adjust v cc above the startup threshold before setting to 15 v. 3. low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible. t low =0 c for uc3842b, UC3843B t high = +70 c for uc3842b, UC3843B t low = 25 c for uc2842b, uc2843b t high = +85 c for uc2842b, uc2843b t low = 40 c for uc3842bv, UC3843Bv t high = +105 c for uc3842bv, UC3843Bv 0.8 2.0 5.0 8.0 20 50 80 r t , timing resistor (k ) w 1.0 m 500 k 200 k 100 k 50 k 20 k 10 k f osc , oscillator frequency (khz) v cc = 15 v t a = 25 c figure 1. timing resistor versus oscillator frequency figure 2. output deadtime versus oscillator frequency 1.0 m 500 k 200 k 100 k 50 k 20 k 10 k f osc , oscillator frequency (khz) 1.0 2.0 5.0 10 20 50 100 % dt, percent output deadtime 1 2 figure 3. oscillator discharge current versus temperature figure 4. maximum output duty cycle versus timing resistor , discharge current (ma) 7.0 55 t a , ambient temperature ( c) 25 0 25 50 75 100 125 dischg i 7.5 8.0 8.5 9.0 v cc = 15 v v osc = 2.0 v , maximum output duty cycle (%) max d 40 0.8 r t , timing resistor (k w ) 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 50 60 70 80 90 100 i dischg = 7.5 ma v cc = 15 v c t = 3.3 nf t a = 25 c 1. c t = 10 nf 2. c t = 5.0 nf 3. c t = 2.0 nf 4. c t = 1.0 nf 5. c t = 500 pf 6. c t = 200 pf 7. c t = 100 pf 5 i dischg = 8.8 ma 7 3 6 4 v cc = 15 v t a = 25 c
uc3842b, 43b uc2842b, 43b 5 motorola analog ic device data figure 5. error amp small signal transient response figure 6. error amp large signal transient response figure 7. error amp open loop gain and phase versus frequency figure 8. current sense input threshold versus error amp output voltage figure 9. reference voltage change versus source current figure 10. reference short circuit current versus temperature 20 a vol , open loop voltage gain (db) 10 m 10 f, frequency (hz) gain phase v cc = 15 v v o = 2.0 v to 4.0 v r l = 100 k t a = 25 c 0 30 60 90 120 150 180 100 1.0 k 10 k 100 k 1.0 m 0 20 40 60 80 100 , excess phase (degrees) f 0 v o , error amp output voltage (v) 0 , current sense input threshold (v) v th 0.2 0.4 0.6 0.8 1.0 1.2 2.0 4.0 6.0 8.0 v cc = 15 v t a = 25 c t a = 55 c t a = 125 c ???? ???? v cc = 15 v ??? ??? t a = 55 c ???? ???? t a = 25 c , reference voltage change (mv) 16 0 i ref , reference source current (ma) 20 40 60 80 100 120 ref v 12 8.0 4.0 0 d 20 24 ???? t a = 125 c ???? ???? v cc = 15 v r l 0.1 w , reference short circuit current (ma) sc i 50 55 t a , ambient temperature ( c) 25 0 25 50 75 100 125 70 90 110 1.0 m s/div 0.5 m s/div 20 mv/div / 2.55 v 2.50 v 2.45 v 3.0 v 2.5 v 2.0 v v cc = 15 v a v = 1.0 t a = 25 c v cc = 15 v a v = 1.0 t a = 25 c
uc3842b, 43b uc2842b, 43b 6 motorola analog ic device data ???? ???? sink saturation (load to v cc ) ???? t a = 55 c ??? ??? v cc ????? ????? source saturation (load to ground) 0 v sat , output saturation voltage (v) 800 0 i o , output load current (ma) 200 400 600 1.0 2.0 3.0 2.0 1.0 0 ??? ??? t a = 55 c figure 11. reference load regulation figure 12. reference line regulation figure 13. output saturation voltage versus load current figure 14. output waveform figure 15. output cross conduction figure 16. supply current versus supply voltage ??? t a = 25 c ???? ???? ???? ???? r t = 10 k c t = 3.3 nf v fb = 0 v i sense = 0 v t a = 25 c , supply current (ma) cc i 0 0 v cc , supply voltage (v) 10 20 30 40 5 10 15 20 25 ucx843b ucx842b ???? ???? t a = 25 c ?? ?? gnd ????? ????? ????? v cc = 15 v 80 m s pulsed load 120 hz rate 2.0 ms/div 2.0 ms/div v cc = 15 v i o = 1.0 ma to 20 ma t a = 25 c v cc = 12 v to 25 t a = 25 c , output voltage change (2.0 mv/div) v o d , output voltage change (2.0 mv/div) v o d v cc = 30 v c l = 15 pf t a = 25 c v cc = 15 v c l = 1.0 nf t a = 25 c 50 ns/div 100 ns/div 100 ma/div 20 v/div 90% 10% , output voltage o v , supply current cc i
uc3842b, 43b uc2842b, 43b 7 motorola analog ic device data pin function description pin fi dii 8pin 14pin function description 1 1 compensation this pin is the error amplifier output and is made available for loop compensation. 2 3 voltage feedback this is the inverting input of the error amplifier. it is normally connected to the switching power supply output through a resistor divider. 3 5 current sense a voltage proportional to inductor current is connected to this input. the pwm uses this information to terminate the output switch conduction. 4 7 r t /c t the oscillator frequency and maximum output duty cycle are programmed by connecting resistor r t to v ref and capacitor c t to ground. operation to 500 khz is possible. 5 gnd this pin is the combined control circuitry and power ground. 6 10 output this output directly drives the gate of a power mosfet. peak currents up to 1.0 a are sourced and sunk by this pin. 7 12 v cc this pin is the positive supply of the control ic. 8 14 v ref this is the reference output. it provides charging current for capacitor c t through resistor r t . 8 power ground this pin is a separate power ground return that is connected back to the power source. it is used to reduce the effects of switching transient noise on the control circuitry. 11 v c the output high state (v oh ) is set by the voltage applied to this pin. with a separate power source connection, it can reduce the effects of switching transient noise on the control circuitry. 9 gnd this pin is the control circuitry ground return and is connected back to the power source ground. 2,4,6,13 nc no connection. these pins are not internally connected.
uc3842b, 43b uc2842b, 43b 8 motorola analog ic device data operating description the uc3842b, UC3843B series are high performance, fixed frequency, current mode controllers. they are specifically designed for offline and dctodc converter applications offering the designer a costeffective solution with minimal external components. a representative block diagram is shown in figure 17. oscillator the oscillator frequency is programmed by the values selected for the timing components r t and c t . capacitor c t is charged from the 5.0 v reference through resistor r t to approximately 2.8 v and discharged to 1.2 v by an internal current sink. during the discharge of c t , the oscillator generates an internal blanking pulse that holds the center input of the nor gate high. this causes the output to be in a low state, thus producing a controlled amount of output deadtime. figure 1 shows r t versus oscillator frequency and figure 2, output deadtime versus frequency, both for given values of c t . note that many values of r t and c t will give the same oscillator frequency but only one combination will yield a specific output deadtime at a given frequency. the oscillator thresholds are temperature compensated to within 6% at 50 khz. also because of industry trends moving the uc384x into higher and higher frequency applications, the uc384xb is guaranteed to within 10% at 250 khz. these internal circuit refinements minimize variations of oscillator frequency and maximum output duty cycle. the results are shown in figures 3 and 4. in many noisesensitive applications it may be desirable to frequencylock the converter to an external system clock. this can be accomplished by applying a clock signal to the circuit shown in figure 20. for reliable locking, the freerunning oscillator frequency should be set about 10% less than the clock frequency. a method for multiunit synchronization is shown in figure 21. by tailoring the clock waveform, accurate output duty cycle clamping can be achieved. error amplifier a fully compensated error amplifier with access to the inverting input and output is provided. it features a typical dc voltage gain of 90 db, and a unity gain bandwidth of 1.0 mhz with 57 degrees of phase margin (figure 7). the noninverting input is internally biased at 2.5 v and is not pinned out. the converter output voltage is typically divided down and monitored by the inverting input. the maximum input bias current is 2.0 m a which can cause an output voltage error that is equal to the product of the input bias current and the equivalent input divider source resistance. the error amp output (pin 1) is provided for external loop compensation (figure 31). the output voltage is offset by two diode drops ( 1.4 v) and divided by three before it connects to the noninverting input of the current sense comparator. this guarantees that no drive pulses appear at the output (pin 6) when pin 1 is at its lowest state (v ol ). this occurs when the power supply is operating and the load is removed, or at the beginning of a softstart interval (figures 23, 24). the error amp minimum feedback resistance is limited by the amplifier's source current (0.5 ma) and the required output voltage (v oh ) to reach the comparator's 1.0 v clamp level: r f(min) 3.0 (1.0 v) + 1.4 v 0.5 ma = 8800 w current sense comparator and pwm latch the uc3842b, UC3843B operate as a current mode controller, whereby output switch conduction is initiated by the oscillator and terminated when the peak inductor current reaches the threshold level established by the error amplifier output/compensation (pin 1). thus the error signal controls the peak inductor current on a cyclebycycle basis. the current sense comparator pwm latch configuration used ensures that only a single pulse appears at the output during any given oscillator cycle. the inductor current is converted to a voltage by inserting the groundreferenced sense resistor r s in series with the source of output switch q1. this voltage is monitored by the current sense input (pin 3) and compared to a level derived from the error amp output. the peak inductor current under normal operating conditions is controlled by the voltage at pin 1 where: i pk = v (pin 1) 1.4 v 3 r s abnormal operating conditions occur when the power supply output is overloaded or if output voltage sensing is lost. under these conditions, the current sense comparator threshold will be internally clamped to 1.0 v. therefore the maximum peak switch current is: i pk(max) = 1.0 v r s when designing a high power switching regulator it becomes desirable to reduce the internal clamp voltage in order to keep the power dissipation of r s to a reasonable level. a simple method to adjust this voltage is shown in figure 22. the two external diodes are used to compensate the internal diodes, yielding a constant clamp voltage over temperature. erratic operation due to noise pickup can result if there is an excessive reduction of the i pk(max) clamp voltage. a narrow spike on the leading edge of the current waveform can usually be observed and may cause the power supply to exhibit an instability when the output is lightly loaded. this spike is due to the power transformer interwinding capacitance and output rectifier recovery time. the addition of an rc filter on the current sense input with a time constant that approximates the spike duration will usually eliminate the instability (refer to figure 26).
uc3842b, 43b uc2842b, 43b 9 motorola analog ic device data + reference regulator v cc uvlo + v ref uvlo 3.6v 36v s r q internal bias + 1.0ma oscillator 2.5v r r r 2r error amplifier voltage feedback input output/ compensation current sense comparator 1.0v v cc 7(12) gnd 5(9) v c 7(11) output 6(10) power ground 5(8) current sense input 3(5) r s q1 v cc v in 1(1) 2(3) 4(7) 8(14) r t c t v ref = sink only positive true logic pin numbers adjacent to terminals are for the 8pin dualinline package. pin numbers in parenthesis are for the d suffix so14 package. figure 17. representative block diagram figure 18. timing diagram large r t /small c t small r t /large c t pwm latch (see text) capacitor c t latch aseto input output/ compensation current sense input latch areseto input output
uc3842b, 43b uc2842b, 43b 10 motorola analog ic device data undervoltage lockout two undervoltage lockout comparators have been incorporated to guarantee that the ic is fully functional before the output stage is enabled. the positive power supply terminal (v cc ) and the reference output (v ref ) are each monitored by separate comparators. each has builtin hysteresis to prevent erratic output behavior as their respective thresholds are crossed. the v cc comparator upper and lower thresholds are 16 v/10 v for the ucx842b, and 8.4 v/7.6 v for the ucx843b. the v ref comparator upper and lower thresholds are 3.6 v/3.4 v. the large hysteresis and low startup current of the ucx842b makes it ideally suited in offline converter applications where efficient bootstrap startup techniques are required (figure 33). the ucx843b is intended for lower voltage dctodc converter applications. a 36 v zener is connected as a shunt regulator from v cc to ground. its purpose is to protect the ic from excessive voltage that can occur during system startup. the minimum operating voltage (v cc ) for the ucx842b is 11 v and 8.2 v for the ucx843b. these devices contain a single totem pole output stage that was specifically designed for direct drive of power mosfets. it is capable of up to 1.0 a peak drive current and has a typical rise and fall time of 50 ns with a 1.0 nf load. additional internal circuitry has been added to keep the output in a sinking mode whenever an undervoltage lockout is active. this characteristic eliminates the need for an external pulldown resistor. the so14 surface mount package provides separate pins for v c (output supply) and power ground. proper implementation will significantly reduce the level of switching transient noise imposed on the control circuitry. this becomes particularly useful when reducing the i pk(max) clamp level. the separate v c supply input allows the designer added flexibility in tailoring the drive voltage independent of v cc . a zener clamp is typically connected to this input when driving power mosfets in systems where v cc is greater than 20 v. figure 25 shows proper power and control ground connections in a currentsensing power mosfet application. reference the 5.0 v bandgap reference is trimmed to 1.0% tolerance at t j = 25 c on the uc284xb, and 2.0% on the uc384xb. its primary purpose is to supply charging current to the oscillator timing capacitor. the reference has short circuit protection and is capable of providing in excess of 20 ma for powering additional control system circuitry. design considerations do not attempt to construct the converter on wirewrap or plugin prototype boards. high frequency circuit layout techniques are imperative to prevent pulsewidth jitter. this is usually caused by excessive noise pickup imposed on the current sense or voltage feedback inputs. noise immunity can be improved by lowering circuit impedances at these points. the printed circuit layout should contain a ground plane with lowcurrent signal and highcurrent switch and output grounds returning on separate paths back to the input filter capacitor. ceramic bypass capacitors (0.1 m f) connected directly to v cc , v c , and v ref may be required depending upon circuit layout. this provides a low impedance path for filtering the high frequency noise. all high current loops should be kept as short as possible using heavy copper runs to minimize radiated emi. the error amp compensation circuitry and the converter output voltage divider should be located close to the ic and as far as possible from the power switch and other noisegenerating components. current mode converters can exhibit subharmonic oscillations when operating at a duty cycle greater than 50% with continuous inductor current. this instability is independent of the regulator's closed loop characteristics and is caused by the simultaneous operating conditions of fixed frequency and peak current detecting. figure 19a shows the phenomenon graphically. at t 0 , switch conduction begins, causing the inductor current to rise at a slope of m 1 . this slope is a function of the input voltage divided by the inductance. at t 1 , the current sense input reaches the threshold established by the control voltage. this causes the switch to turn off and the current to decay at a slope of m 2 , until the next oscillator cycle. the unstable condition can be shown if a perturbation is added to the control voltage, resulting in a small d i (dashed line). with a fixed oscillator period, the current decay time is reduced, and the minimum current at switch turnon (t 2 ) is increased by d i + d i m 2 /m 1 . the minimum current at the next cycle (t 3 ) decreases to ( d i + d i m 2 /m 1 ) (m 2 /m 1 ). this perturbation is multiplied by m 2 /m 1 on each succeeding cycle, alternately increasing and decreasing the inductor current at switch turnon. several oscillator cycles may be required before the inductor current reaches zero causing the process to commence again. if m 2 /m 1 is greater than 1, the converter will be unstable. figure 19b shows that by adding an artificial ramp that is synchronized with the pwm clock to the control voltage, the d i perturbation will decrease to zero on succeeding cycles. this compensating ramp (m 3 ) must have a slope equal to or slightly greater than m 2 /2 for stability. with m 2 /2 slope compensation, the average inductor current follows the control voltage, yielding true current mode operation. the compensating ramp can be derived from the oscillator and added to either the voltage feedback or current sense inputs (figure 32). control voltage inductor current oscillator period control voltage inductor current oscillator period (a) (b) figure 19. continuous current waveforms m 1 m 2 t 0 t 1 t 2 t 3 m 3 m 2 t 4 t 5 t 6 d i m 1 d i l l m 2 m 1 l l m 2 m 1 m 2 m 1
uc3842b, 43b uc2842b, 43b 11 motorola analog ic device data figure 20. external clock synchronization figure 21. external duty cycle clamp and multiunit synchronization figure 22. adjustable reduction of clamp level figure 23. softstart circuit 2(3) ea bias + osc r r r 2r 5(9) 1(1) 4(7) 8(14) r t c t v ref 0.01 the diode clamp is required if the sync amplitude is large enough to cause the bottom side of c t to go more than 300 mv below ground. external sync input 47 + r r r 2r bias osc ea 5(9) 1(1) 2(3) 4(7) 8(14) to additional ucx84xbs r s q 8 4 6 5 2 1 c 3 7 r a r b 5.0k 5.0k 5.0k mc1455 f 1.44 (r a 2r b )c d (max) r b r a 2r b + 5.0v ref + s r q bias + osc r r r 2r ea 1.0v 5(9) 7(11) 6(10) 5(8) 3(5) r s q1 v cc v in 1(1) 2(3) 4(7) 8(14) r 1 v clamp r 2 5.0v ref + s r q bias + 1.0ma osc r r r 2r ea 1.0v 5(9) 1(1) 2(3) 4(7) 8(14) c 1.0m t softstart 3600c in m f 7(12) comp/latch 1.0 ma i pk(max) v clamp r s where: 0 v clamp 1.0 v v clamp 1.67 r 2 r 1 1 + 0.33x10 3 r 1 r 2 r 1 r 2
uc3842b, 43b uc2842b, 43b 12 motorola analog ic device data figure 24. adjustable buffered reduction of clamp level with softstart figure 25. current sensing power mosfet figure 26. current waveform spike suppression figure 27. mosfet parasitic oscillations + + s r + r r r 2r v clamp 1.67 r 2 r 1 1 i pk(max) v clamp r s 5.0v ref q bias osc ea 1.0v 5(9) 7(11) 6(10) 5(8) 3(5) r s q1 v cc v in 1(1) 2(3) 4(7) 8(14) r 1 v clamp r 2 where: 0 v clamp 1.0 v c mpsa63 t soft-start in 1 v c 3v clamp c r 1 r 2 r 1 r 2 + 5.0v ref + s r q (11) (10) (8) comp/latch (5) r s 1/4 w v cc v in k m d sensefet g s power ground: to input source return control circuitry ground: to pin (9) virtually lossless current sensing can be achieved with the implementation of a sensefet power switch. for proper operation during overcurrent conditions, a reduction of the i pk(max) clamp level must be implemented. refer to figures 22 and 24. v pin 5 r s i pk r ds(on) r dm(on) r s if: sensefet = mtp10n10m r s = 200 then : v pin 5 0.075 i pk 7(12) 1.0 ma comp/latch (12) + 5.0v ref + s r q 7(11) 6(10) 5(8) 3(5) r s q1 v cc v in s r 5.0v ref q 7(11) 6(10) 5(8) 3(5) r s q1 v cc v in c r the addition of the rc filter will eliminate instability caused by the leading edge spike on the current waveform. series gate resistor r g will damp any high frequency parasitic oscillations caused by the mosfet input capacitance and any series wiring inductance in the gatesource circuit. 7(12) 7(12) r g comp/latch comp/latch + +
uc3842b, 43b uc2842b, 43b 13 motorola analog ic device data figure 28. bipolar transistor drive figure 29. isolated mosfet drive figure 30. latched shutdown figure 31. error amplifier compensation 6(10) 5(8) 3(5) r s q1 v in c1 base charge removal the totem pole output can furnish negative base current for enhanced transistor turnoff, with the addition of capacitor c 1 . s r 5.0v ref q 7(11) 6(10) 5(8) 3(5) r s q1 v cc i b + 0 v in isolation boundary v gs waveforms + 0 + 0 50% dc 25% dc i p k v (pin1) 1.4 3r s n s n p comp/latch 7(12) r c n s n p + + bias + osc r r r 2r ea 5(9) 1(1) 2(3) 4(7) 8(14) the mcr101 scr must be selected for a holding of < 0.5 ma @ t a(min) . the simple two transistor circuit can be used in place of the scr as shown. all resistors are 10 k. mcr 101 2n 3905 2n 3903 + r 2r 1.0ma ea 2(3) 5(9) 2.5v 1(1) r f c f r d r i from v o error amp compensation circuit for stabilizing any current mode topology except for boost and flyback converters operating with continuous inductor current. r f 8.8 k + r 2r 1.0ma ea 2(3) 5(9) 2.5v 1(1) r f c f r d r p from v o error amp compensation circuit for stabilizing current mode boost and flyback topologies operating with continuous inductor current. c p r i 1.0 ma
+ + 5.0v ref 36v s r q bias + 1.0ma osc r r r 2r ea 1.0v 7(12) 7(11) 6(10) 5(8) 3(5) r s v cc v in 1(1) 2(3) 4(7) 8(14) r t c t the buffered oscillator ramp can be resistively summed with either the voltage feedback or current sense inputs to provide slope compensation. figure 32. slope compensation m 3.0m m r f c f r i r d from v o r slope mps3904 figure 33. 27 w offline flyback regulator 5(9) comp/latch mur110 + + s r + r r 5.0v ref q bias ea 5(9) 7(11) 6(10) 5(8) 3(5) 0.5 mtp 4n50 1(1) 2(3) 4(7) 8(14) 10k 4700pf 470pf 150k 100 pf 18k 4.7k 0.01 100 + 1.0k 115 vac 4.7 w mda 202 250 56k 4.7k 3300 pf 1n4935 1n4935 ++ 68 47 1n4937 1n4937 680pf 2.7k l3 l2 l1 ++ ++ ++ 1000 1000 2200 10 10 1000 5.0v/4.0a 5.0v rtn 12v/0.3a 12v rtn 12v/0.3a primary: 45 turns #26 awg secondary 12 v: 9 turns #30 awg (2 strands) bifiliar wound secondary 5.0 v: 4 turns (six strands) #26 hexfiliar wound secondary feedback: 10 turns #30 awg (2 strands) bifiliar wound core: ferroxcube ec353c8 bobbin: ferroxcube ec35pcb1 gap: 0.10o for a primary inductance of 1.0 mh mur110 mbr1635 t1 22 osc t1 7(12) comp/latch l1 l2, l3 15 m h at 5.0 a, coilcraft z7156 25 m h at 5.0 a, coilcraft z7157 1n5819 uc3842b, 43b uc2842b, 43b 14 motorola analog ic device data test conditions results line regulation: 5.0 v 12 v v in = 95 to 130 vac d = 50 mv or 0.5% d = 24 mv or 0.1% load regulation: 5.0 v 12 v v in = 115 vac, i out = 1.0 a to 4.0 a v in = 115 vac, i out = 100 ma to 300 ma d = 300 mv or 3.0% d = 60 mv or 0.25% output ripple: 5.0 v 12 v v in = 115 vac 40 mv pp 80 mv pp efficiency v in = 115 vac 70% all outputs are at nominal load currents, unless otherwise noted
uc3842b, 43b uc2842b, 43b 15 motorola analog ic device data outline dimensions n suffix plastic package case 62605 issue k d1 suffix plastic package case 75106 (so8) issue t notes: 1. dimension l to center of lead when formed parallel. 2. package contour optional (round or square corners). 3. dimensioning and tolerancing per ansi y14.5m, 1982. 14 5 8 f note 2 a b t seating plane h j g d k n c l m m a m 0.13 (0.005) b m t dim min max min max inches millimeters a 9.40 10.16 0.370 0.400 b 6.10 6.60 0.240 0.260 c 3.94 4.45 0.155 0.175 d 0.38 0.51 0.015 0.020 f 1.02 1.78 0.040 0.070 g 2.54 bsc 0.100 bsc h 0.76 1.27 0.030 0.050 j 0.20 0.30 0.008 0.012 k 2.92 3.43 0.115 0.135 l 7.62 bsc 0.300 bsc m 10 10 n 0.76 1.01 0.030 0.040 seating plane 1 4 5 8 a 0.25 m cb ss 0.25 m b m h c x 45 l dim min max millimeters a 1.35 1.75 a1 0.10 0.25 b 0.35 0.49 c 0.19 0.25 d 4.80 5.00 e 1.27 bsc e 3.80 4.00 h 5.80 6.20 h 0 7 l 0.40 1.25 0.25 0.50 notes: 1. dimensioning and tolerancing per asme y14.5m, 1994. 2. dimensions are in millimeter. 3. dimension d and e do not include mold protrusion. 4. maximum mold protrusion 0.15 per side. 5. dimension b does not include dambar protrusion. allowable dambar protrusion shall be 0.127 total in excess of the b dimension at maximum material condition. d e h a b e b a1 c a 0.10
uc3842b, 43b uc2842b, 43b 16 motorola analog ic device data outline dimensions d suffix plastic package case 751a03 (so14) issue f notes: 1. dimensioning and tolerancing per ansi y14.5m, 1982. 2. controlling dimension: millimeter. 3. dimensions a and b do not include mold protrusion. 4. maximum mold protrusion 0.15 (0.006) per side. 5. dimension d does not include dambar protrusion. allowable dambar protrusion shall be 0.127 (0.005) total in excess of the d dimension at maximum material condition. a b g p 7 pl 14 8 7 1 m 0.25 (0.010) b m s b m 0.25 (0.010) a s t t f r x 45 seating plane d 14 pl k c j m dim min max min max inches millimeters a 8.55 8.75 0.337 0.344 b 3.80 4.00 0.150 0.157 c 1.35 1.75 0.054 0.068 d 0.35 0.49 0.014 0.019 f 0.40 1.25 0.016 0.049 g 1.27 bsc 0.050 bsc j 0.19 0.25 0.008 0.009 k 0.10 0.25 0.004 0.009 m 0 7 0 7 p 5.80 6.20 0.228 0.244 r 0.25 0.50 0.010 0.019 motorola reserves the right to make changes without further notice to any products herein. motorola makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does motorola assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. atypicalo parameters which may be provided in motorola data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. all operating parameters, including atypicalso must be validated for each customer application by customer's technical experts. motorola does not convey any license under its patent rights nor the rights of others. motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the motorola product could create a situation where personal injury or death may occur. should buyer purchase or use motorola products for any such unintended or unauthorized application, buyer shall indemnify and hold motorola and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that motorola was negligent regarding the design or manufacture of the part. motorola and are registered trademarks of motorola, inc. motorola, inc. is an equal opportunity/affirmative action employer. mfax is a trademark of motorola, inc. how to reach us: usa / europe / locations not listed : motorola literature distribution; japan : nippon motorola ltd.: spd, strategic planning office, 141, p.o. box 5405, denver, colorado 80217. 13036752140 or 18004412447 4321 nishigotanda, shagawaku, tokyo, japan. 0354878488 customer focus center: 18005216274 mfax ? : rmfax0@email.sps.mot.com touchtone 1 6022446609 asia / pacific : motorola semiconductors h.k. ltd.; 8b tai ping industrial park, motorola fax back system us & canada only 18007741848 51 ting kok road, tai po, n.t., hong kong. 85226629298 http://sps.motorola.com/mfax/ home page : http://motorola.com/sps/ uc3842b/d ?

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