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?2000 fairchild semiconductor international www.fairchildsemi.com rev. 5.0 features ? low start up current ? maximum duty clamp ? uvlo with hysteresis ? operating frequency up to 500khz description the uc3842/uc3843/UC3844/uc3845 are fixed fre- quency current-mode pwm controller. they are specially designed for off - line and dc-to-dc converter applica- tions with minimum external components. these inte- grated 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 totempole output ideally suited for driving a power mosfet. protection circuity includes built in under-volt- age lockout and current limiting. theuc3842 and UC3844 have uvlo thresholds of 16v (on) and 10v (off) the uc3843 and uc3845 are 8.5v (on) and 7.9v (off) the uc3842 and uc3843 can operate within 100% duty cycle. the UC3844and uc3845 can operate with 50% duty cycle. 8-dip 14-sop 1 1 internal block diagram uc3842/uc3843/UC3844/uc3845 smps controller
uc3842/uc3843/UC3844/uc3845 2 absolute maximum ratings parameter symbol value unit supply voltage v cc 30 v output current i o 1a analog inputs (pin 2.3) v (ana) -0.3 to 6.3 v error amp output sink current i sink (e.a) 10 ma power dissipation (t a = 25 c) p d 1w
uc3842/uc3843/UC3844/uc3845 3 electrical characteristics (v cc =15v, r t =10k ? , c t =3.3nf, t a = 0 c to +70 c, unless otherwise specified) parameter symbol conditions min. typ. max. unit reference section reference output voltage v ref t j = 25 c, i ref = 1ma 4.90 5.00 5.10 v line regulation ? v ref 12v v cc 25v - 6 20 mv load regulation ? v ref 1ma i ref 20ma - 6 25 mv short circuit output current i sc t a = 25 c - -100 -180 ma oscillator section oscillation frequency f t j = 25 c475257khz frequency change with voltage ? f/ ? v cc 12v v cc 25v - 0.05 1 % oscillator amplitude v osc - -1.6-v p-p error amplifier section input bias current i bias - --0.1-2 a input voltage v i(e>a) v 1 = 2.5v 2.42 2.50 2.58 v open loop voltage gain g vo 2v v o 4v 65 90 - db power supply rejection ratio psrr 12v v cc 25v 60 70 - db output sink current i sink v 2 = 2.7v, v 1 = 1.1v 2 7 - ma output source current i source v 2 = 2.3v, v 1 = 5v -0.6 -1.0 - ma high output voltage v oh v 2 = 2.3v, r l = 15k ? to gnd 5 6 - v low output voltage v ol v 2 = 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 3.15 v/v maximum input signal v i(max) v 1 = 5v(note 1) 0.9 1 1.1 v power supply rejection ratio psrr 12v v cc 25v (note 1) - 70 - db input bias current i bias - --3-10 a output section low output voltage v ol i sink = 20ma - 0.08 0.4 v i sink = 200ma - 1.4 2.2 v high output voltage v oh i source = 20ma 13 13.5 - v i source = 200ma 12 13.0 - v rise time t r t j = 25 c, c l = 1nf (note 3) - 45 150 ns fall time t f t j = 25 c, c l = 1nf (note 3) - 35 150 ns under-voltage lockout section start threshold v th(st) uc3842/UC3844 14.5 16.0 17.5 v uc3843/uc3845 7.8 8.4 9.0 v min. operating voltage (after turn on) v opr(min) uc3842/UC3844 8.5 10.0 11.5 v uc3843/UC3844 7.0 7.6 8.2 v
uc3842/uc3843/UC3844/uc3845 4 electrical characteristics (continued) (v cc =15v, r t =10k ? , c t =3.3nf, t a = 0 c to +70 c unless otherwise specified) adjust v cc above the start threshould before setting at 15v note: 1. parameter measured at trip point of latch 2. gain defined as: 3.these parameters, although guaranteed, are not 100 tested in production. figure 1. open loop test circuit high peak currents associated with capacitive loads necessitate careful grounding techniques timing and bypass capacitors should be connected close to pin 5 in a single point ground. the transistor and 5k ? potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to pin 3. parameter symbol conditions min. typ. max. unit pwm section max. duty cycle d (max) uc3842/uc3843 95 97 100 % d UC3844/uc3845 47 48 50 % min. duty cycle d (min) - --0% total standby current start-up current i st - -0.451ma operating supply current i cc(opr) v 3 =v 2 =on - 14 17 ma zener voltage v z i cc = 25ma 30 38 - v a ? v 1 ? v 3 ---------- = uc3842 ,0 v3 0.8v
uc3842/uc3843/UC3844/uc3845 5 figure 2. under voltage lockout during under-voltage lock-out, the output driver is biased to a high impedance state. pin 6 should be shunted to ground with a bleeder resistor to prevent activating the power switch with output leakage current. figure 3. error amp configuration figure 4. current sense circuit peak current (i s ) is determined by the formula: a small rc filter may be required to suppress switch transients. uc3842/44 uc3843/45 i s max () 1.0v r s ------------ =
uc3842/uc3843/UC3844/uc3845 6 figure 5. oscillator waveforms and maximum duty cycle oscillator timing capacitor, c t , is charged by v ref through r t , and discharged by an internal current source. during the dis- charge time, the internal clock signal blanks the output to the low state. selection of r t and c t therefore determines both oscillator frequency and maximum duty cycle. charge and discharge times are determined by the formulas: t c = 0.55 r t c t frequency, then, is: f=(t c + t d ) -1 figure 8. shutdown techniques figure 6. oscillator dead time & frequency figure 7. timing resistance vs frequency t d r t c t i n 0.0063r t 2.7 ? 0.0063r t 4 ? ---------------------------------------- ?? ?? = forrt 5k ? f 1.8 r t c t -------------- - = , > (deadtime vs c t rt > 5k ?)
uc3842/uc3843/UC3844/uc3845 7 shutdown of the uc3842 can be accomplished by two methods; either raise pin 3 above 1v or pull pin 1 below a voltage two diode drops above ground. either method causes the output of the pwm comparator to be high (refer to block diagram). the pwm latch is reset dominant so that the output will remain low until the next clock cycle after the shutdown condition at pins 1 and/or 3 is removed. in one example, an externally latched shutdown may be accomplished by adding an sor which will be reset by cycling voc below the lower uvlo threshold. at this point the reference turns off, allowing the scr to reset. figure 9. slope compensation a fraction of the oscillator ramp can be resistively summed with the current sense signal to provide slope compensation for converters requiring duty cycles over 50%. note that capacitor, c, forms a filter with r2 to suppress the leading edge switch spikes. temperature ( c) figure 10. temperature drift (vref) temperature ( c) figure 11. temperature drift (ist) temperature ( c) figure 12. temperature drift (icc) uc3842/uc3843
uc3842/uc3843/UC3844/uc3845 8 mechanical dimensions package 6.40 0.20 3.30 0.30 0.130 0.012 3.40 0.20 0.134 0.008 #1 #4 #5 #8 0.252 0.008 9.20 0.20 0.79 2.54 0.100 0.031 () 0.46 0.10 0.018 0.004 0.060 0.004 1.524 0.10 0.362 0.008 9.60 0.378 max 5.08 0.200 0.33 0.013 7.62 0~15 0.300 max min 0.25 +0.10 ?.05 0.010 +0.004 ?.002 8-dip
uc3842/uc3843/UC3844/uc3845 9 mechanical dimensions (continued) package 8.56 0.20 0.337 0.008 1.27 0.050 5.72 0.225 1.55 0.10 0.061 0.004 0.05 0.002 6.00 0.30 0.236 0.012 3.95 0.20 0.156 0.008 0.60 0.20 0.024 0.008 8.70 0.343 max #1 #7 #8 0~8 #14 0.47 0.019 () 1.80 0.071 max0.10 max0.004 max min + 0.10 -0.05 0.20 + 0.004 -0.002 0.008 + 0.10 -0.05 0.406 + 0.004 -0.002 0.016 14-sop
uc3842/uc3843/UC3844/uc3845 10 ordering information product number package operating temperature uc3842n 8 dip 0 ~ + 70 c uc3843n UC3844n uc3845n uc3842d 14 sop uc3843d UC3844d uc3845d
uc3842/uc3843/UC3844/uc3845 11
uc3842/uc3843/UC3844/uc3845 7/12/00 0.0m 001 stock#dsxxxxxxxx ? 2000 fairchild semiconductor international life support policy fairchild?s products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of fairchild semiconductor international. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user. 2. a critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com

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