1 pin connection ( top view ) description the uc3842a-bw/43a-bw/44a-bw/45a-bw are fixed frequency current mode pwm controller. they are specially designed for off ? line and dc to dc converter applications with a minimal external components. interna lly implemented circuits include a trimmed oscillator for precise duty cycle control, a temperature compensated reference, high gain error amplifier, current sensing comparator, an d a high current totempole output ideally suited for driving a power mosfet. protection circuitry includes buil t undervoltage lockout and current limiting. the uc3842a-bw and uc3844a-bw have uvlo thresholds of 16 v (on) and 10 v (off). the corresponding thresholds for the uc38 43a-bw/45a-bw are 8.4v (on) and 7.6v (off). the uc3842a-bw and UC3843A-BW can operate within 100% duty cycle. the uc3844a-bw and uc3845a-bw can operate within 50% duty cycle. the uc384xa-bw has start-up current 0.17ma (typ). the uc384xa-bw are revised uc384xam and differ by higher unity gain bandwidth of error amplifier. features ? low start-up and operating current ? high current totem pole output ? undervoltage lockout with hysteresis ? operating frequency up to 500khz block diagram (toggle flip flop used only in uc3844, uc3845) absolute maximum ratings characteristic symbol value unit supply voltage (low impedance source) v cc 30 v output current i o 1 a input voltage (analog inputs pins 2,3) v i ? 0.3 to 5.5 v error amp output sink current i sink (e.a) 10 ma power dissipation (t a =25 0 c) p o 1 w storage temperature range tstg -65 to150 o c lead temperature (soldering 5 sec.) t l 260 o c uc384xa-bw current mode pwm controller (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw)
2 electrical characteristics (*v cc =15v, r t =10k ? , c t =3.3nf, t a =0 0 c to +70 0 c, unless otherwise specified) characteristics symbol test condition min typ max unit reference section reference output voltage v ref t j = 25 c, i ref = 1 ma 4.9 5.0 5.1 v line regulation ? v ref 12v v cc 25 v 6.0 20 mv load regulation ? v ref 1 ma i ref 20ma 6.0 25 short circuit output current i sc t a = 25 c -100 -180 ma oscillator section oscillation frequency f t j = 25 c 47 52 57 khz frequency change with voltage ? f/ ? v cc 12v v cc 25 v 0.05 1.0 % oscillator amplitude v (osc) (peak to peak) 1.6 v error amplifier section input bias current i bias v fb =3v -0.1 -2 a input voltage v i(e.a) v pin1 = 2.5v 2.42 2.5 2.58 v open loop voltage gain a vol 2v v 0 4v 65 90 db unity gain bandwidth ugbw t j =25 0 c, note 3 0.5 0.6 mhz power supply rejection ratio psrr 12v v cc 25 v 60 70 db output sink current i sink v pin2 = 2.7v, v pin1 = 1.1v 2 7 ma output source current i source v pin2 = 2.3v, v pin1 = 5v -0.5 -1.0 ma high output voltage v oh v pin2 = 2.3v, r l = 15k ? to gnd 5.0 6.0 low output voltage v ol v pin2 = 2.7v, r l = 15k ? to pin 8 0.8 1.1 v current sense section gain g v (note 1 & 2) 2.85 3.0 3.15 v/v maximum input signal v i(max) v pin1 = 5v (note1) 0.9 1.0 1.1 v supply voltage rejection svr 12v v cc 25 v (note 1) 70 db input bias current i bias v pin3 = 3v -3.0 -10 a output section low output voltage v ol i sink = 20 ma 0.08 0.4 i sink = 200 ma 1.4 2.2 high output voltage v oh i sink = 20 ma 13 13.5 i sink = 200 ma 12 13.0 v rise time t r t j = 25 c, c l = 1nf (note 3) 45 150 fall time t f t j = 25 c, c l = 1nf (note 3) 35 150 ns undervoltage lockout section start theshold v th(st) uc3842a-bw/44a-bw 14.5 16.0 17.5 UC3843A-BW/45a-bw 7.8 8.4 9.0 v min. operating voltage v opr(min) uc3842a-bw/44a-bw 8.5 10 11.5 (after turn on) UC3843A-BW/45a-bw 7.0 7.6 8.2 v pwm section max. duty cycle d (max) uc3842a-bw/43a-bw 95 97 100 uc3844a-bw/45a-bw 47 48 50 % min. duty cycle d (max) 0 total standby current start ? up current i st uc384xa-bw 0.17 0.3 operating supply current i cc (opr) v pin3 = v pin2 = 0v 13 17 ma zener voltage v z i cc =25 ma 30 38 v * adjust v cc above the start threshold before setting it to 15v. note 1: parameter measured at trip point of latch with v pin2 =0. note 2: gain defined as a= ? v pin1 / ? v pin3 ; 0 v pin3 0.8v. note 3: these parameters, although guaran teed, are not 100% tested in production. uc384xa-bw current mode pwm controller (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw)
3 pin function n function description 1 comp this pin is the error amplifier ou tput and is made for loop compensation. 2 v fb this is the inverting input of the error amplifier. it is normally connected to the switching power supply output through a resistor divider. 3 i sense a voltage proportional to inductor curre nt is connected to this input. the pwm uses this information to terminate the output switch conduction. 4 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. 5 ground this pin is the combined control circuitry and power ground. 6 output this output directly drives the gate of a power mosfet. peak currents up to 1a are sourced and sink by this pin. 7 v cc this pin is the positive suppl y of the integrated circuit. 8 v ref this is the reference output. it prov ides charging current for capacitor c t through resistor r t . application information figure 1. error amp configuration figure 2. undervoltage lockout (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw) uc384xa-bw current mode pwm controller
4 figure 3. current sense circuit figure 4. slope compensation techniques (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw) uc384xa-bw current mode pwm controller
5 scr must be selected for a holding current of less than 0.5ma. the simple two transistor circuit can be used in place of the scr as shown. figure 5. latched shutdown figure 6. error amplifier compensation (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw) uc384xa-bw current mode pwm controller
6 figure 7. external clock synchronization figure 8. soft-start circuit (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw) uc384xa-bw current mode pwm controller
7 typical performance characteristics figure 1. timing resistor vs. os cillator frequency figure 2. ou tput dead-time vs. oscillator frequency figure 3. maximum output duty cycle vs. figure 4. error amp open-loop gain vs. timing resistor (uc3 842/43) frequency figure 5. current sense input threshold vs. figure 6. refe rence short circuit current vs. error amp output voltage temperature (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw) uc384xa-bw current mode pwm controller
8 figure 7. output saturation voltage vs. load current figure 8. supply current vs. supply voltage t a = 25 c figure 9. oscillator and output waveforms (uc3842a-bw/ 4 3a-bw/ 44a-bw/ 45a-bw) uc384xa-bw current mode pwm controller
9 pad location chip size: 2.38 x 1.63 mm pad location coordinates coordinates p m pad n pad name x y 1 comp 90 110 2 v fb 1050 110 3 i sense 1310 110 4 r t /c t 2000 150 5 power gnd 1700 1280 6 gnd 1680 1450 7 out 1310 1410 8 power v cc 990 1410 9 v cc 815 1410 10 v ref 460 1390 (uc384 3a-bw/ 45a-bw) uc384xa-bw current mode pwm controller uc384xa-bw current mode pwm controller
uc384xa-bw current mode pwm controller (uc3842a-bw/ 44a-bw)
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