unisonic technologies co., ltd p1785 preliminary linear integrated circuit www.unisonic.com.tw 1 of 7 copyright ? 2011 unisonic technologies co., ltd qw-r103-043.b pwm control 2a step-down converter ? description utc p1785 consists of 2a step-down switching regulator control which includes a reference voltage source, oscillation circuit, error amplifier, inte rnal pmos and etc with pwm. utc p1785 can provide low-ripple power, high efficiency, and excellent transient characteristics and an enable function, an over current protect function and a s hort circuit protect function are built inside, so the operation frequency will be reduced from 300khz to 30khz when ocp or scp happens. and the pwm control circuit can vary the duty ratio linearly from 100 down to 0%. this converter also includes an error amplifier circuit as well as a soft-start circuit that prevent s overshoot at startup. also, to minimum external component count an internal compensation block is built in. these ics can work as step-down switching regulators with the addition of an internal p-channel power mos, a coil, capacitors, and a diode connected externally. they provide such outstanding features: low current consumption. it is also suitable for the operation via an ac adapter because this converter can accommodate an input voltage up to 23v. ? features * soft-start, current limit, enable function * thermal shutdown function * built-in internal sw p-channel mos * input voltage: 3.6v to 23v. * output voltage: 0.8v to v cc . * duty ratio: 0% to 100% pwm control * oscillation frequency: 300khz typ. sop-8 ? ordering information ordering number lead free halogen free package packing p1785l-s08-r P1785G-S08-R sop-8 tape reel p1785l-s08-t p1785g-s08-t sop-8 tube
p1785 preliminary linear integrated circuit unisonic technologies co., ltd 2 of 7 www.unisonic.com.tw qw-r103-043.b ? pin configvrations ? pin descriptions pin no. pin name i/o description 1 fb i feedback pin. 2 en i chip enable, h: enable step-down function l: chip disable 3 ocset i add an external resist or to set max output current. 4 v cc ic power supply pin 5, 6 output o switch pin. connect external inductor/di ode here. minimize trace area at this pin to reduce emi. 7, 8 v ss gnd pin
p1785 preliminary linear integrated circuit unisonic technologies co., ltd 3 of 7 www.unisonic.com.tw qw-r103-043.b ? block diagram v cc fb vss en ocset output pwm-switched control circuit + - v en oscillation circuit reference voltage source with soft start thermal shutdown 100 a 1 7,8 2 3 4 5,6
p1785 preliminary linear integrated circuit unisonic technologies co., ltd 4 of 7 www.unisonic.com.tw qw-r103-043.b ? absolute maximum ratings parameter symbol rating unit v cc pin voltage v cc v ss -0.3 ~ v ss +25 v feedback pin voltage v fb v ss -0.3 ~ v cc v en pin voltage v en v ss -0.3 ~ v in +0.3 v switch pin voltage v out v ss -0.3 ~ v in +0.3 v power dissipation p d internally limited mw operating temperature t opr -20 ~ +125 storage temperature t stg -40 ~ +150 note: absolute maximum ratings are those values beyond which the device could be permanently damaged. absolute maximum ratings are stress ratings only and functional device oper ation is not implied. ? thermal data parameter symbol rating unit junction to ambient ja 65 c/w ? electrical characteristics (v in = 12v, t a =25c, unless otherwise specified) parameter symbol conditions min. typ. max. unit input voltage v in 3.6 23 v feedback voltage v fb i out =0.1a 0.784 0.8 0.816 v feedback bias current i fb(bias) i out =0.1a 0.1 0.5 a switch current i sw 2.15 a standby current i stn-by v en =0v 10 a line regulation v out /v out v in =6v~23v, i out =0.2a 1 2.1 % load regulation v out /v out i out =0.1~2a 0.2 0.5 % oscillation frequency f osc measure waveform at sw pin 240 300 360 khz frequency of current limit or short circuit protect f osc1 measure waveform at sw pin 10 khz v sh evaluate oscillation at sw pin 0.79 en pin input voltage v sl evaluate oscillation stop at sw pin 0.72 v i sh 20 a en pin input leakage current i sl -10 a ocset pin bias current i ocset 75 90 105 a soft-start time t ss 0.3 2 5 ms v in =5v, v fb =0v 180 230 internal mosfet r ds(on) r ds(on) v in =12v, v fb =0v 130 200 m ? efficiency v in =12v, v out =5v, i out =2a 91 %
p1785 preliminary linear integrated circuit unisonic technologies co., ltd 5 of 7 www.unisonic.com.tw qw-r103-043.b ? function description pwm control the utc p1785 consists of dc/dc converters which employ a pwm system. in this dc/dc converter, the pulse width varies in a range from 0 to 100% which is acco rding to the load current. cause the switching frequency remains constant the ripple voltage which produced by t he switching can be removed easily through a filter. therefore, these converters provi de a low-ripple power over broad ranges of input voltage and load current. under voltage lockout the under voltage lockout circuit makes sure that the outputs of high-side mosfet driver remain in the off state after the supply voltage drops below 3.3v. when v cc rises above 3.5v normal operation resumes. r ds(on) current limiting the current limit threshold is fixed by the external resistor which is connected from v cc supply to ocset. an over-current condition is triggered when the pwm voltage is less than the voltage at ocset. i load r ds(on) = i ocset r ocset above formula is setting the current limit value.
p1785 preliminary linear integrated circuit unisonic technologies co., ltd 6 of 7 www.unisonic.com.tw qw-r103-043.b ? test circuit + -
p1785 preliminary linear integrated circuit unisonic technologies co., ltd 7 of 7 www.unisonic.com.tw qw-r103-043.b ? typical application circuit v in =12v, i max =2a v out 2.5v 3.3v 5v l 1 value 15uh 18uh 22uh utc assumes no responsibility for equipment failures that result from using products at values that exceed, even momentarily, rated values (such as maximum ratings, operating condition ranges, or other parameters) listed in products specifications of any and all utc products described or contained herein. utc products are not designed for use in life support appliances, devices or systems where malfunction of these products can be reasonably expected to result in personal injury. reproduction in whole or in part is prohibited without the prior writ ten consent of the copyright owner. the information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice.
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