general description the HT3706 is a high performance ac/dc power sup-ply controller for battery charger and adapter applications. the device uses pulse frequency modulation(pfm) method to build discontinuous conduction mode (dcm) flyback power supplies. the HT3706 provides cons tant voltage, constant cur-rent (cv/cc) regulation without requiring an opto-coupler and secondary control circuitry. it also eliminates the need of loop compensation circuitry while maintaining stability. the HT3706 achieves ex cellent regulation and high power efficiency, the no-load power consumption is less than 200mw at 265vac input. the HT3706 is available in soic-8 package. applications ? adapters/chargers for cell/cordless phones, pdas, mp3 and other portable apparatus ? standby and auxiliary power supplies features ? primary side control for rectangular constant current and constant voltage output ? eliminates opto-coupler and secondary cv/cc control circuitry ? eliminates control loop compensation circuitry ? flyback topology in dcm operation ? random frequency modulation to reduce system emi ? valley turn on of external power npn transistor ? built-in soft start ? open circuit protection ? over voltage protection ? short circuit protection figure 1. package type of HT3706 pin configuration figure 2 HT3706 1 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
pin description pin number pin name function 1 cs the primary current sense 2, vcc supply voltage 3 out this pin drives the base of external power npn switch 4 gnd ground 5 fb the voltage feedback from the auxiliary winding 6 vdd the 5v output of the internal voltage regulator 7 bias this pin sets the bias current insi de HT3706 with an external resistor to gnd 8 comp this pin connects a bypass capacitor for cc function functional block diagram figure3 HT3706 2 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
ordering information absolute maximum ratings (note 1) parameter value unit supply voltage vcc -0.3 to 30 v voltage at cs, bias, out, vdd, comp to gnd -0.3 to 7 v fb input (pin 5) -20 to 10 v output current at out internally limited a power dissipation at t a =25 o c 0.657 w operating junction temperature 150 o c storage temperature -65 to 150 o c lead temperature (soldering, 10s) 300 o c thermal resistance junction-to-ambient 190 o c/w esd (machine model) 200 v note 1: stresses greater than t hose listed under "absolute maximu m ratings" may cause permanent damage to the device.these are stress ratings only, and functional operation of t he device at these or any other conditions beyond those indicatedunder "recommended oper ating conditions" is not implied. exposure to "absolute maximum ratings" for ex tended periodsmay affect device reliability. HT3706 3 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
electrical characteristics (v cc =15v, t a =25 o c, unless otherwise specified.) parameter symbol conditions min typ max unit uvlo section start-up threshold v th (st) 17 2 18.5 19.8 v minimal operating voltage v opr(min) after turn on 6.5 7.3 8.1 v reference voltage section bias pin voltage v bias r bias =200k ? , before turn on 1.170 1.205 1.240 v vdd pin voltage v dd 4.75 5.0 5.25 v standby current section start-up current i st v cc =v th(st) -0.5v, r bias =200k ? , before turn on 70 80 a operating current i cc(opr) r bias =200k ? 680 900 a drive output section out maximum current sink i out r bias =200k ? 50 ma source 25 30 current sense section current sense threshold v cs 485 505 525 mv pre-current sense v cs(pre) 385 410 435 mv leading edge blanking 430 ns feedback input section feedback threshold v fb 3.90 4.00 4.10 v enable turn-on voltage v fb(en) -0.9 -0.7 -0.5 v comp threshold voltage section turn-on threshold voltage v comp 3.42 3.60 3.78 v protection section over voltage protection v fb(ovp) 7 8 9 v HT3706 4 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
typical performance characteristics HT3706 5 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
operation description figure 10 figure 10 illustrates a simplified flyback converter controlled by HT3706. constant primary peak current the primary current ip(t) is sensed by a current senseresistor rcs as shown in figure 10. the current rises up linearly at a rate of: as illustrated in figure 11, when the current ip(t) rises up to ipk, the switch q1 turns off. the constant peak current is given by: the energy stored in th e magnetizing inductance lm each cycle is therefore: so the power transferring from the input to the output is given by: where the fsw is the switching frequency. when the peak current ipk is constant, the output power depends on the switching frequency fsw. constant voltage operation HT3706 6 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
the HT3706 captures the auxiliary winding feedback voltage at fb pin and operates in constant-voltage (cv) mode to regulate the output voltage. assumi ng the secondary winding is master, the auxiliary winding is slave during the d1 on-time. the auxiliary voltage is given by: where the vd is the diode forward drop voltage. figure 12. auxiliary voltage waveform the output voltage is diff erent from the secondary voltage in a diode forward drop voltage. the diode drop voltage depends on the current. if the secondary voltage is always detected at a constant secondary cu rrent, the difference between the output voltage and the secondary voltage will be a fixed vd. the voltage detection point is at two-thirds of the d1 on-time. the cv loop control function of HT3706 then generates a d1 off-time to regulate the output voltage. constant current operation the HT3706 is designed to work in constant-current (cc) mo de. figure 13 shows the secondary current waveforms. figure 13. secondary current waveform in cc operation, the cc loop control function of HT3706 will keep a fixed proportion between d1 on-time tons and d1 off-time toffs by discharging or charging the capacitance connected in comp pin. the fixed proportion is the relationship between the output constant-current and se condary peak current ipks is given by: at the instant of d1 turn-on, the primary current transfers to the secondary at an amplitude of: thus the output constant -current is given by: leading edge blanking when the power switch is turned on, a turn-on spike will occur on the sense-resistor. to avoid false-termination of the switching pulse, a 430ns leading-edge blanking is built in. during this blanking period,the current sense comparator is disabled and the gatedriv er can not be switched off. ccm protection the HT3706 is designed to operate in discontinuous conduction mode (dcm) in both cv and cc modes.to avoid operating in continuous conduc tion mode (ccm), the HT3706 detects the falling edge of the fb input voltage on each cycle. if a 0.1v falling edge of fb is not detected, the HT3706 w ill stop switching. HT3706 7 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
ovp & ockp the HT3706 includes output over-voltage protection (ovp) and open circuit protection (ockp) circuitry as show n in figure 14. if the voltage at fb pin exceeds 8v, 100% above the normal detection voltage, or the -0.7v falling edge of the fb input can not be monitored, the HT3706 will immediately shut of f and enters hiccup mode. the HT3706 sends out a fault detection pulse every 8ms in hiccup mode until the fault has been removed. HT3706 8 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
typical application HT3706 9 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
package description sop8 package outl ine dimensions HT3706 10 of 10 4008-318-123 sales@twtysemi.com http://www.twtysemi.com product specification
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