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this is information on a product in full production. january 2013 doc id 15028 rev 5 1/32 32 viper28 off-line high voltage converters datasheet ? production data features 800 v avalanche rugged power section pwm operation with adjustable limiting current 30 mw stand-by power at 265 vac operating frequency: ? 60 khz for l type ? 115 khz for h type frequency jittering for low emc output overvoltage protection high primary current protection (2 nd ocp) extra power timer for peak current management on-board soft-start safe auto-restart after a fault condition hysteretic thermal shutdown application auxiliary power supply for consumer and home equipment atx auxiliary power supply low / medium power ac-dc adapters smps for set-top boxes, dvd players and recorders, white goods description the device is an off-line converter with an 800 v rugged power section, a pwm control, two levels of over-current protection, over-voltage and overload protections, hysteretic thermal protection, soft-start and safe auto-restart after any fault condition removal. burst mode operation and device very low consumption help to meet the standby energy saving regulations. advance frequency jittering reduces emi filter cost. the extra power timer allows the management of output peak power for a designed time window. the high voltage start-up circuit is embedded in the device. figure 1. typical application dip-7 so 16 so16 s1 dc input high voltage wide range - + dc output voltage - + viper28 drain drain ept vdd cont fb gnd table 1. device summary order codes package packaging viper28ln / viper28hn dip-7 tu b e viper28le / viper28he sdip10 viper28hd / VIPER28LD so16 narrow viper28hdtr / VIPER28LDtr tape and reel www.st.com
contents viper28 2/32 doc id 15028 rev 5 contents 1 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 typical power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4 electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1 maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2 thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.3 electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 typical electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6 typical circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 7 operation descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.1 power section and gate driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2 high voltage startup generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.3 power-up and soft-start up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.4 power down operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.5 auto restart operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.6 oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.7 current mode conversion with adjustable current limit set point . . . . . . . 18 7.8 overvoltage protection (ovp) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7.9 about cont pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 7.10 feed-back and overload protection (olp) . . . . . . . . . . . . . . . . . . . . . . . . 20 7.11 burst-mode operation at no load or very light load . . . . . . . . . . . . . . . . . . 23 7.12 extra power management function (ept) . . . . . . . . . . . . . . . . . . . . . . . . 24 7.13 2nd level overcurrent protection and hiccup mode . . . . . . . . . . . . . . . . . . 25 8 package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
viper28 block diagram doc id 15028 rev 5 3/32 1 block diagram 2 typical power figure 2. block diagram 2nd ocp th er mal shutdown burst otp leb ocp block ovp detection logic hv_on s r q rsense uvlo + - disable turn-on logic + - pwm leb + - ocp 2nd ocp logic oscillator olp tov l block supply & uvlo + - s r q hv_on pwm block burst-mode logic soft start ovp gnd fb drain cont ref erence voltages otp ovp olp & internal supply bus burst tovl 2nd ocp th er mal shutdown burst otp leb ocp block ovp detection logic hv_on s r q rsense uvlo + - disable turn-on logic + - pwm leb + - ocp 2nd ocp logic oscillator olp tov l block supply & uvlo + - s r q hv_on i ddch pwm block burst-mode logic soft start ovp gnd vdd fb drain cont ref erence voltages otp ovp olp & internal supply bus burst tovl ept ept table 2. typical power part number 230 v ac 85-265 v ac adapter (1) open frame (2) adapter (1) open frame (2) viper28 18 w 20 w 10 w 12 w 1. typical continuous power in non ventilated enclosed adapter measured at 50 c ambient. 2. maximum practical continuous power in an open fr ame design at 50 c ambient, with adequate heat sinking.
pin settings viper28 4/32 doc id 15028 rev 5 3 pin settings figure 3. connection diagram (top view) note: the copper area for heat dissipation has to be designed under the drain pins. table 3. pin description pin n. name function sdip10 dip-7 so16n 1 1 1...2 gnd this pin represents the device ground and the source of the power section. --4n.a. function: not available for user. it can be connected to gnd (pins 1-2) or left not connected. 225vdd supply voltage of the control section. this pin also provides the charging current of the external capacitor during start-up time. 3 3 6 cont control pin. the following functions can be selected: 1. current limit set point adjustment. the internal set default value of the cycle- by-cycle current limit can be reduced by connecting to ground an external resistor. 2. output voltage monitoring. a voltage exceeding v ovp threshold (see ta bl e 8 on page 7 ) shuts the ic down reducing the device consumption. this function is strobed and digitally filtered for high noise immunity. 447fb control input for duty cycle control. internal current generator provides bias current for loop regulation. a voltage below the threshold v fbbm activates the burst-mode operation. a level close to the threshold v fblin means that we are approaching the cycle-by-cycle over-current set point. 558ept this pin allows the connection of an external capacitor for the extra power management. if the function is not used, the pin has to be connected to gnd. 6...10 7,8 13...16 drain high voltage drain pin. the built-in high voltage switched start-up bias current is drawn from this pin too. pins connected to the metal frame to facilitate heat dissipation.
viper28 electrical data doc id 15028 rev 5 5/32 4 electrical data 4.1 maximum ratings 4.2 thermal data table 4. absolute maximum ratings symbol parameter value unit min max v drain drain-to-source (ground) voltage 800 v e av repetitive avalanche energy (limited by t j = 150 c) 3.5 mj i ar repetitive avalanche current (limited by t j = 150 c) 1 a i drain pulse drain current (limited by t j = 150c) 3 a v cont control input pin voltage -0.3 6 v v fb feed-back voltage -0.3 5.5 v v ept ept input pin voltage -0.3 5 v v dd supply voltage (i dd = 25 ma) -0.3 self limited v i dd input current 25 ma p tot power dissipation at t a < 40 c (dip-7) 1 w power dissipation at t a < 60 c (so16n, sdip-10) 1.5 t j operating junction temperature range -40 150 c t stg storage temperature -55 150 c table 5. thermal data symbol parameter max value unit so16n dip7 sdip10 r thjp thermal resistance junction pin (dissipated power = 1 w) 25 35 35 c/w r thja thermal resistance junction ambient (dissipated power = 1 w) 60 100 80 c/w r thja thermal resistance junction ambient (1) (dissipated power = 1 w) 1. when mounted on a standard single side fr4 board with 100 mm 2 (0.155 sq in) of cu (35 m thick) 50 80 65 c/w
electrical data viper28 6/32 doc id 15028 rev 5 4.3 electrical characteristics (t j = -25 to 125 c, v dd = 14 v (a) ; unless otherwise specified) a. adjust v dd above v ddon start-up threshold before setting to 14 v table 6. power section symbol parameter test condition min typ max unit v bvdss break-down voltage i drain = 1 ma, v fb = gnd, t j = 25 c 800 v i off off state drain current v drain = max rating,v fb = gnd, t j = 25c 60 a r ds(on) drain-source on state resistance i drain = 0.4 a, v fb = 3 v, v ept = gnd, t j = 25 c 7 i drain = 0.4 a, v fb = 3 v, v ept = gnd, t j = 125 c 14 c oss effective (energy related) output capacitance v drain = 0 to 640 v, t j = 25c 40 pf table 7. supply section symbol parameter test condition min typ max unit voltag e v drain _start drain-source start voltage 60 80 100 v i ddch start up charging current v drain = 120 v, v ept = gnd, v fb = gnd, v dd = 4 v -2 -3 -4 ma v drain = 120 v, v ept = gnd, v fb = gnd, v dd = 4 v after fault -0.4 -0.6 -0.8 ma v dd operating voltage range after turn-on 8.5 23.5 v v ddclamp v dd clamp voltage i dd = 20 ma 23.5 v v ddon v dd start up threshold v drain = 120 v, v ept = gnd, v fb = gnd 13 14 15 v v ddoff v dd under voltage shutdown threshold 7.588.5v v dd(restart) v dd restart voltage threshold v drain = 120 v, v ept = gnd, v fb = gnd 44.55 v current i dd0 operating supply current, not switching v fb = gnd, f sw = 0 khz v ept = gnd, v dd = 10 v 0.9 ma i dd1 operating supply current, switching v drain = 120 v, f sw = 60 khz 2.5 ma v drain = 120 v,f sw = 115 khz 3.5 ma i dd_fault operating supply current, with protection tripping v dd = 10 v 400 ua i dd_off operating supply current with v dd < v dd_off v dd = 7 v 270 ua
viper28 electrical data doc id 15028 rev 5 7/32 table 8. controller section symbol parameter test condition min typ max unit feed-back pin v fbolp over-load shut down threshold 4.5 4.8 5.2 v v fblin linear dynamics upper limit 3.2 3.5 3.7 v v fbbm burst mode threshold voltage falling 0.6 v v fbbmhys burst mode hysteresis voltage rising 100 mv i fb feed-back sourced current v fb = 0.3 v -150 -200 -280 ua 3.3 v < v fb < 4.8 v -3 ua r fb(dyn) dynamic resistance v fb < 3.3 v 14 20 k h fb v fb / i d 26v/a cont pin v cont_l low level clamp voltage i cont = -100 a 0.5 v v cont_h high level clamp voltage i cont = 1ma 5 5.5 6 v current limitation i dlim max drain current limitation v fb = 4 v, i cont = -10 a t j = 25 c 0.75 0.80 0.85 a t ss soft-start time 8.5 ms t on_min minimum turn on time 220 400 480 ns td propagation delay 100 ns t leb leading edge blanking 300 ns i d_bm peak drain current during burst mode v fb = 0.6 v 160 ma oscillator section f osc viper28 l v dd = operating voltage range, v fb = 1 v 54 60 66 khz viper28 h 103 115 127 khz fd modulation depth viper28 l4khz viper28 h 8 khz fm modulation frequency 250 hz d max maximum duty cycle 70 80 %
electrical data viper28 8/32 doc id 15028 rev 5 table 8. controller section (continued) symbol parameter test condition min typ max unit over-current protection (2 nd ocp) i dmax second over-current threshold 1.2 a over-voltage protection v ovp over-voltage protection threshold 2.7 3 3.3 v t strobe over-voltage protection strobe time 2.2 us extra power management i dlim_ept drain current limit with ept function i cont < -10 a t j = 25 c 85% idlim a v ept(stop) ept shut down threshold i cont < -10 a 4v v ept(restart) ept restart threshold 0.6 v i ept sourced ept current 5 a thermal shutdown t sd thermal shutdown temperature 150 160 c t hyst thermal shutdown hysteresis 30 c
viper28 electrical data doc id 15028 rev 5 9/32 figure 4. minimum turn-on time test circuit figure 5. ovp threshold test circuits note: adjust v dd above v ddon start-up threshold before setting to 14 v 14 v 3 .5 v 50 30 v gnd cont fb vdd drain ept drain v drain i drain i dlim time time t onmin 90 % 10 % gnd cont fb vdd drain ept drain v ovp v cont v drain 14 v 2 v 10 k 30 v time time
typical electrical characteristics viper28 10/32 doc id 15028 rev 5 5 typical electrical characteristics figure 6. current limit vs t j figure 7. switching frequency vs t j figure 8. drain start-up voltage vs t j figure 9. hfb vs t j figure 10. operating supply current (no switching) vs t j figure 11. operating supply current (switching) vs t j
viper28 typical electrical characteristics doc id 15028 rev 5 11/32 figure 12. current limit vs r lim figure 13. power mosfet on-resistance vs t j figure 14. power mosfet break down voltage vs t j
typical electrical characteristics viper28 12/32 doc id 15028 rev 5 figure 15. thermal shutdown t j v dd i drain v ddon time v ddoff v dd(restart) t sd time time t sd -t hyst shut down after over temperature normal operation normal operation
viper28 typical circuit doc id 15028 rev 5 13/32 6 typical circuit figure 16. min-features flyback application figure 17. full-feature flyback application ept ept
operation descriptions viper28 14/32 doc id 15028 rev 5 7 operation descriptions the device is a high-performance low-voltage pwm controller chip with an 800 v, avalanche rugged power section. the controller includes: the oscillator with jittering feature, the start up circuits with soft-start feature, the pwm logic, the current limit circuit with adjustable set point, the second overcurrent circuit, the burst mode management, the brown-out circuit, the uvlo circuit, the auto-restart circuit and the thermal protection circuit. the current limit set-point is set by the cont pin. the burst mode operation guaranties high performance in the stand-by mode and helps in the energy saving norm accomplishment. all the fault protections are built in auto restart mode with very low repetition rate to prevent ic's over heating. 7.1 power section and gate driver the power section is implemented with an avalanche ruggedness n-channel mosfet, which guarantees safe operation within the specified energy rating as well as high dv/dt capability. the power section has a b vdss of 800 v min. and a typical r ds(on) of 7 at 25 c. the integrated sensefet structure allows a virtually loss-less current sensing. the gate driver is designed to supply a controlled gate current during both turn-on and turn- off in order to minimize common mode emi. under uvlo conditions an internal pull-down circuit holds the gate low in order to ensure that the power section cannot be turned on accidentally. 7.2 high voltage startup generator the hv current generator is supplied through the drain pin and it is enabled only if the input bulk capacitor voltage is higher than v drain_start threshold, 80 v dc typically. when the hv current generator is on, the i ddch current (3 ma typical value) is delivered to the capacitor on the v dd pin. in case of auto restart mode after a fault event, the i ddch current is reduced to 0.6 ma, in order to have a slow duty cycle during the restart phase.
viper28 operation descriptions doc id 15028 rev 5 15/32 7.3 power-up and soft-start up if the input voltage rises up till the device start threshold, v drain_start , the v dd voltage begins to grow due to the i ddch current (see table 7 on page 6 ) coming from the internal high voltage start up circuit. if the v dd voltage reaches v ddon threshold (see ta b l e 7 o n page 6 ) the power mosfet starts switching and the hv current generator is turned off. see figure 18 on page 15 . the ic is powered by the energy stored in the capacitor on the vdd pin, c vdd , until when the self-supply circuit (typically an auxiliary winding of the transformer and a steering diode) develops a voltage high enough to sustain the operation. c vdd capacitor must be sized enough to avoid fast discharge and keep the needed voltage value higher than v ddoff threshold. in fact, a too low capacitance value could terminate the switching operation before the controller receives any energy from the auxiliary winding. the following formula can be used for the v dd capacitor calculation: equation 1 the t ssaux is the time needed for the steady state of the auxiliary voltage. this time is estimated by applicator according to the output stage configurations (transformer, output capacitances, etc.). during the converter start up time, the drain current limitation is progressively increased to the maximum value. in this way the stress on the secondary diode is considerably reduced. it also helps to prevent transformer saturation. the soft-start time lasts 8.5 ms and the feature is implemented for every attempt of start up converter or after a fault. figure 18. i dd current during start-up and burst mode c vdd i ddch t ssaux v ddon v ddoff ? ---------------------------------------- = burst mode normal mode start- up normal mode i ddch (-3 ma) i dd1 i dd0 i dd v fbbm v fb v drain v fbbmhys v fblin v fbolp v dd v ddoff v ddon t t t t
operation descriptions viper28 16/32 doc id 15028 rev 5 figure 19. timing diagram: normal power-up and power-down sequences figure 20. soft-start: timing diagram i dd v dd v drain v ddon time v in v drain_start power-on power-off normal operation regulation is lost here v in < v drain_start hv startup is no more activated v ddoff v dd(restart) i ddch (3ma) time time time v fb v fblin v fbolp i drain i dlim v out t t t olp-delay t t ss (soft start) regulated value
viper28 operation descriptions doc id 15028 rev 5 17/32 7.4 power down operation at converter power down, the system loses regulation as soon as the input voltage is so low that the peak current limitation is reached. the v dd voltage drops and when it falls below the v ddoff threshold (see table 7 on page 6 ) the power mosfet is switched off, the energy transfers to the ic interrupted and consequently the v dd voltages decreases, figure 19 on page 16 . later, if the v in is lower than v drain_start (see table 7 on page 6 ), the start up sequence is inhibited and the power down completed. this feature is useful to prevent converter?s restart attempts and ensures monotonic output voltage decay during the system power down. 7.5 auto restart operation if after a converter power down, the v in is higher than v drain_start, the start up sequence is not inhibited and will be activated only when the v dd voltage drops down the v dd(restart) threshold (see table 7 on page 6 ). this means that the hv start up current generator restarts the v dd capacitor charging only when the v dd voltage drops below v dd(restart) . the scenario above described is for instance a power down because of a fault condition. after a fault condition, the charging current, i ddch , is 0.6 ma (typ.) instead of the 3 ma (typ.) of a normal start up converter phase. this feature together with the low v dd(restart) threshold ensures that, after a fault, the restart attempts of the ic has a very long repetition rate and the converter works safely with extremely low power throughput. the figure 21 shows the ic behavioral after a short-circuit event. figure 21. timing diagram: behavior after short-circuit 7.6 oscillator the switching frequency is internally fixed to 60 khz or 115 khz. in both case the switching frequency is modulated by approximately 4 khz (60 khz version) or 8 khz (115 khz version) at 250 hz (typical) rate, so that the resulting spread-spectrum action distributes the energy of each harmonic of the switching frequency over a number of side- band harmonics having the same energy on the whole but smaller amplitudes. i dd v dd v ds v ddon time short circuit occurs here v ddof f v dd(restart) i ddc h (0.6ma) time time time v fb v fbolp v fblin t r epeti tion = 0.3 x t repetition
operation descriptions viper28 18/32 doc id 15028 rev 5 7.7 current mode conversion with adjustable current limit set point the device is a current mode converter: the drain current is sensed and converted in voltage that is applied to the non inverting pin of the pwm comparator. this voltage is compared with the one on the feed-back pin through a voltage divider on cycle by cycle basis. the device has a default current limit value, i dlim , that the designer can adjust according the electrical specification, by the r lim resistor connected to the cont, see figure 12 on page 11 . the cont pin has a minimum current sunk needed to activate the i dlim adjustment: without r lim or with high r lim (i.e. 100 k ) the current limit is fixed to the default value (see i dlim , table 8 on page 7 ). 7.8 overvoltage protection (ovp) the device has integrated the logic for the monitor of the output voltage using as input signal the voltage v cont during the off time of the power mosfet. this is the time when the voltage from the auxiliary winding tracks the output voltage, through the turn ratio the cont pin has to be connected to the auxiliary winding through the diode d ovp and the resistors r ovp and r lim as shows the figure 23 on page 20 when, during the off time, the voltage v cont exceeds, four consecutive times, the reference voltage v ovp (see ta bl e 8 on page 7 ) the overvoltage protection will stop the power mosfet and the converter enters the auto-restart mode. in order to bypass the noise immediately after the turn off of the power mosfet, the voltage v cont is sampled inside a short window after the time t strobe , see table 8 on page 7 and the figure 22 on page 19 . the sampled signal, if higher than v ovp , trigger the internal ovp digital signal and increments the internal counter. the same counter is reset every time the signal ovp is not triggered in one oscillator cycle. referring to the figure 23 , the resistors divider ratio k ovp will be given by: equation 2 equation 3 n aux n sec -------------- k ovp v ovp n aux n sec -------------- v outovp v dsec + () v daux ? ? ----------------------------------------------------------------------------------------------------- = k ovp r lim r lim r ovp + ---------------------------------- =
viper28 operation descriptions doc id 15028 rev 5 19/32 where: v ovp is the ovp threshold (see table 8 on page 8 ) v out ovp is the converter output voltage value to activate the ovp set by designer n aux is the auxiliary winding turns n sec is the secondary winding turns v dsec is the secondary diode forward voltage v daux is the auxiliary diode forward voltage r ovp together r lim make the output voltage divider than, fixed r lim, according to the desired i dlim , the r ovp can be calculating by: equation 4 the resistor values will be such that the current sourced and sunk by the cont pin be within the rated capability of the internal clamp. figure 22. ovp timing diagram r ovp r lim 1k ovp ? k ovp ---------------------- - = t v d s v aux t t t s trobe t counter re s et t counter s tatu s t 0 v cont t s trobe ovp fault 0 0 0 0 11 22 0 0 11 22 3 3 4 0 e r u l i a f p o o l k c a b d e e f e c n a b r u t s i d y r a r o p m e t n o i t a r e p o l a m r o n t v d s v ovp t t t s trobe t counter re s et t counter s tatu s t 0 ovp fault 0 0 0 0 11 22 0 0 11 22 3 3 4 0 e r u l i a f p o o l k c a b d e e f e c n a b r u t s i d y r a r o p m e t n o i t a r e p o l a m r o n t sampling time
operation descriptions viper28 20/32 doc id 15028 rev 5 7.9 about cont pin referring to the figure 23 , through the cont pin, the below features can be implemented: 1. current limit set point 2. overvoltage protection on the converter output voltage the table 9 on page 20 referring to the figure 23 , lists the external components needed to activate one or plus of the cont pin functions. figure 23. cont pin configuration 7.10 feed-back and overload protection (olp) the device is a current mode converter: the feedback pin controls the pwm operation, controls the burst mode and actives the overload protection. figure 24 on page 22 and figure 25 show the internal current mode structure. with the feedback pin voltage between v fbbm and v fblin , (see table 8 on page 7 ) the drain current is sensed and converted in voltage that is applied to the non inverting pin of the pwm comparator. see figure 2 on page 3 . this voltage is compared with the one on the feedback pin through a voltage divider on cycle by cycle basis. when these two voltages are equal, the pwm logic orders the switch off of the power mosfet. the drain current is always limited to i dlim value. in case of overload the feedback pin increases in reaction to this event and when it goes higher than v fblin , the pwm comparator is disabled and the drain current is limited to i dlim by the ocp comparator, see figure 2 on page 3 . table 9. cont pin configurations function / component r lim (1) 1. r lim has to be fixed before of r ovp r ovp d aux i dlim reduction see figure 12 no no ovp 80 k see equation 4 ye s i dlim reduction + ovp see figure 12 see equation 4 ye s + - ovp r ov p soft start cont daux r lim from r sense auxiliary winding to gate driver ovp logic ocp block ocp
viper28 operation descriptions doc id 15028 rev 5 21/32 when the feedback pin voltage reaches the threshold v fblin an internal current generator starts to charge the feedback capacitor (c fb ) and when the feedback voltage reaches the v fbolp threshold, the converter is turned off and the start up phase is activated with reduced value of i ddch to 0.6 ma, see table 7 on page 6 . during the first start up phase of the converter, after the soft-start up time, t ss , the output voltage could force the feedback pin voltage to rise up to the v fbolp threshold that switches off the converter itself. to avoid this event, the appropriate feedback network has to be selected according to the output load. more the network feedback fixes the compensation loop stability. the figure 24 on page 22 and figure 25 show the two different feedback networks. the time from the overload detection (v fb = v fblin ) to the device shutdown (v fb = v fbolp ) can be set by c fb value (see figure 24 on page 22 and figure 25 ), using the formula: equation 5 in the figure 24 , the capacitor connected to fb pin (c fb ) is part of the compensation circuit as well as it needs to activate the overload protection (see equation 5). after the start up time, t ss , during which the feedback voltage is fixed at v fblin , the output capacitor could not be at its nominal value and the controller interprets this situation as an overload condition. in this case, the olp delay helps to avoid an incorrect device shut down during the start up phase. owing to the above considerations, the olp delay time must be long enough to by-pass the initial output voltage transient and check the overload condition only when the output voltage is in steady state. the output transient time depends from the value of the output capacitor and from the load. when the value of the c fb capacitor calculated for the loop stability is too low and cannot ensure enough olp delay, an alternative compensation network can be used and it is showed in figure 25 on page 22 . using this alternative compensation network, two poles (f pfb , f pfb1 ) and one zero (f zfb ) are introduced by the capacitors c fb and c fb1 and the resistor r fb1 . the capacitor c fb introduces a pole (f pfb ) at higher frequency than f zb and f pfb1 . this pole is usually used to compensate the high frequency zero due to the esr (equivalent series resistor) of the output capacitance of the fly-back converter. the mathematical expressions of these poles and zero frequency, considering the scheme in figure 25 are reported by the equations below: equation 6 t olp delay ? c fb v fbolp v fblin ? 3 a --------------------------------------- - = f zfb 1 2 c fb1 r fb1 ?? ? ------------------------------------------------------- - =
operation descriptions viper28 22/32 doc id 15028 rev 5 equation 7 equation 8 the r fb(dyn) is the dynamic resistance seen by the fb pin. the c fb1 capacitor fixes the olp delay and usually c fb1 results much higher than c fb . the equation 5 can be still used to calculate the olp delay time but c fb1 has to be considered instead of c fb . using the alternative compensation network, the designer can satisfy, in all case, the loop stability and the enough olp delay time alike. figure 24. fb pin configuration (option 1) figure 25. fb pin configuration (option 2) f pfb r fb dyn () r fb1 + 2 c fb r fb dyn () r fb1 ? () ?? ? ------------------------------------------------------------------------------------------ = f pfb 1 2 c fb1 r fb1 r fb dyn () ? () ?? ? --------------------------------------------------------------------------------------------- = from sense fet 4.8v burst pwm control cfb to pwm logic burst-mode references burst-mode logic + - pwm + - olp comparator to disable logic 4.8v from sense fet pwm control + - pwm burst to disable logic + - olp comparator to pwm logic burst-mode logic cfb1 rfb1 cfb burst-mode references
viper28 operation descriptions doc id 15028 rev 5 23/32 7.11 burst-mode operation at no load or very light load when the load decrease the feedback loop reacts lowering the feedback pin voltage. if it falls down the burst mode threshold, v fbbm , the power mosfet is not more allowed to be switched on. after the mosfet stops, as a result of the feedback reaction to the energy delivery stop, the feedback pin voltage increases and exceeding the level, v fbbm + v fbbmhys , the power mosfet starts switching again. the burst mode thresholds are reported on ta b l e 8 and figure 26 shows this behavior. systems alternates period of time where power mosfet is switching to period of time where power mosfet is not switching; this device working mode is the burst mode. the power delivered to output during switching periods exceeds the load power demands; the excess of power is balanced from not switching period where no power is processed. the advantage of burst mode operation is an average switching frequency much lower then the normal operation working frequency, up to some hundred of hertz, minimizing all frequency related losses. during the burst-mode the drain current peak is clamped to the level, i d_bm , reported on ta bl e 8 . figure 26. burst mode timing diagram, light load management time time time v comp v fbbm +v fbbmhys v fbbm i dd1 i dd0 i dd i drain i d_bm burst mode
operation descriptions viper28 24/32 doc id 15028 rev 5 7.12 extra power management function (ept) some applications need an extra power for a limited time window during which the converter regulation has to be guaranteed. the extra power management function allows to design a converter that can satisfy this request and is provided by the ept pin, see ta bl e 8 o n page 8 . this function requires the use of a capacitor on ept pin (c ept ) that is charged or discharged by means of a 5 a current cycle by cycle. when the drain current raises over 85% of idlim value, see i dlim_ept ( table 8 on page 7 ), the current generator charges c ept while when the drain current is below i dlim_ept discharges the capacitor. if c ept ?s voltage reaches the v ept threshold (typical, 4 v), the converter is shut down. after the converter shut down, the v dd voltage will drop below the v dd(on) start up threshold (typ. 14.5 v) and according to the auto restart operation (see section 7.5 on page 17 ) the vdd pin voltage have to fall below the v dd(restart) threshold (typical, 4.5 v) in order to charge again the v dd capacitor. moreover the pwm operation is enabled again only when the voltage on ept pin, drop below the v ept(restart) (typical, 0.6 v). the low c ept discharge current in combination with its low restart threshold, ensures safe operations and avoids overheating in case of repeated overload events. the value of c ept has to be selected in order to prevent the device overheating. the ept pin can be connected to gnd if the function is not used.
viper28 operation descriptions doc id 15028 rev 5 25/32 7.13 2 nd level overcurrent protection and hiccup mode the device is protected against short-circuit of the secondary rectifier, short-circuit on the secondary winding or a hard-saturation of fly-back transformer. such as anomalous condition is invoked when the drain current exceed the threshold i dmax , see ta b l e 8 o n page 7 . to distinguish a real malfunction from a disturbance (e.g. induced during esd tests) a ?warning state? is entered after the first signal trip. if in the subsequent switching cycle the signal is not tripped, a temporary disturbance is assumed and the protection logic will be reset in its idle state; otherwise if the i dmax threshold is exceeded for two consecutive switching cycles a real malfunction is assumed and the power mosfet is turned off. the shutdown condition is latched as long as the device is supplied. while it is disabled, no energy is transferred from the auxiliary winding; hence the voltage on the v dd capacitor decays till the v dd under voltage threshold (v ddoff ), which clears the latch. the start up hv current generator is still off, until v dd voltage goes below its restart voltage, v dd(restart) . after this condition the v dd capacitor is charged again by 600 a current, and the converter switching restarts if the v ddon occurs. if the fault condition is not removed the device enters in auto-restart mode. this behavioral results in a low-frequency intermittent operation (hiccup-mode operation), with very low stress on the power circuit. see the timing diagram of figure 27 . figure 27. hiccup-mode ocp: timing diagram v drain v dd i drain v ddon time v ddof f v dd(re s tart) time time hiccup-mode secondary diode short circuit normal operation i dmax
package mechanical data viper28 26/32 doc id 15028 rev 5 8 package mechanical data in order to meet environmental requirements, st offers these devices in different grades of ecopack ? packages, depending on their level of environmental compliance. ecopack ? specifications, grade definitions and product status are available at: www.st.com . ecopack is an st trademark. 1- the leads size is comprehensive of the thickness of the leads finishing material. 2- dimensions do not include mold protrusion, not to exceed 0,25 mm in total (both side). 3- package outline exclusive of metal burrs dimensions. 4- datum plane ?h? coincident with the bottom of lead, where lead exits body. 5- ref. poa mother doc. 0037880 6- creepage distance > 800 v 7- creepage distance 250 v 8- creepage distance as shown in the 664-1 cei / iec standard. table 10. dip-7 mechanical data dim. mm typ min max a 5,33 a1 0,38 a2 3,30 2,92 4,95 b 0,46 0,36 0,56 b2 1,52 1,14 1,78 c 0,25 0,20 0,36 d 9,27 9,02 10,16 e 7,87 7,62 8,26 e1 6,35 6,10 7,11 e 2,54 ea 7,62 eb 10,92 l 3,30 2,92 3,81 m (6)(8) 2,508 n 0,50 0,40 0,60 n1 0,60 o (7)(8) 0,548
viper28 package mechanical data doc id 15028 rev 5 27/32 figure 28. package dimensions
package mechanical data viper28 28/32 doc id 15028 rev 5 table 11. so16 narrow mechanical data dim. mm min. typ. max. a 1.75 a1 0.1 0.25 a2 1.25 b 0.31 0.51 c 0.17 0.25 d 9.8 9.9 10 e 5.8 6 6.2 e1 3.8 3.9 4 e 1.27 h 0.25 0.5 l 0.4 1.27 k 0 8 ccc 0.1
viper28 package mechanical data doc id 15028 rev 5 29/32 figure 29. so16 narrow mechanical data
package mechanical data viper28 30/32 doc id 15028 rev 5 figure 30. sdip10 mechanical drawing table 12. sdip10 mechanical data dim. mm min. typ. max. a 5.33 a1 0.38 a2 2.92 4.95 b 0.36 0.56 b2 0.51 1.15 c 0.2 0.36 d 9.02 10.16 e 7.62 8.26 e1 6.1 7.11 e2 7.62 e3 10.92 e 1.77 l 2.92 3.81
viper28 revision history doc id 15028 rev 5 31/32 9 revision history table 13. document revision history date revision changes 30-sep-2008 1 initial release 22-jan-2009 2 updated figure 3 on page 4 21-oct-2009 3 added so16n and sdip10 packages 31-aug-2010 4 updated figure 3 , figure 4 , figure 5 on page 9 and ta b l e 3 o n page 4 08-jan-2013 5 minor text changes to improve readability in chapter 7.3 , chapter 7.4 , chapter 7.5 , chapter 7.7 , chapter 7.8 , chapter 7.9 , chapter 7.10 , chapter 7.11 , chapter 7.13 , in table 7 on page 6 , table 8 on page 7 and in figure 22 on page 19
viper28 32/32 doc id 15028 rev 5 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a particular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. unless expressly approved in writing by two authorized st representatives, st products are not recommended, authorized or warranted for use in military, air craft, space, life saving, or life sustaining applications, nor in products or systems where failure or malfunction may result in personal injury, death, or severe property or environmental damage. st products which are not specified as "automotive grade" may only be used in automotive applications at user?s own risk. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or register ed trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2013 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - philippines - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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