NCP1308

NCP1308 PWM Current−Mode Controller for Free−Running Quasi−Resonant Operation The NCP1308 combines a true current mode modulator and a demagnetization detector to ensure full borderline/Critical Conduction Mode in any load/line conditions and minimum drain voltage switching (Quasi−Resonant operation). Due to its inherent skip cycle capability, the controller enters burst mode as soon as the power demand falls below a predetermined level. As this happens at low peak current, no audible noise can be heard. An internal 10 ms timer prevents the free−run frequency to exceed a high frequency (therefore below the 150 kHz CISPR−22 EMI starting limit), while the skip adjustment capability lets the user select the frequency at which the burst foldback takes place. The Dynamic Self−Supply (DSS) drastically simplifies the transformer design in avoiding the use of an auxiliary winding to supply the NCP1308. This feature is particularly useful in applications where the output voltage varies during operation (e.g. battery chargers). Thanks to its high−voltage technology, the IC is directly connected to the high−voltage DC rail. As a result, the short−circuit trip point is not dependent upon any VCC auxiliary level. The transformer core reset detection is done through an auxiliary winding which, brought via a dedicated pin. If an OVP is detected on the VCC pin, the IC permanently latches off. Finally, the continuous feedback signal monitoring implemented with an Overcurrent fault Protection circuitry (OCP) makes the final design rugged and reliable. Features http://onsemi.com MARKING DIAGRAM 8 1 SOIC−8 DR SUFFIX CASE 751 8 1308 ALYW G 1 A L Y W G = Assembly Location = Wafer Lot = Year = Work Week = Pb−Free Package PIN CONNECTIONS Dmg 1 FB 2 CS 3 GND 4 (Top View) 8 HV 7 6 VCC 5 Drv • • • • • • • • • • • • • • • • • • • Free−Running Borderline/Critical Mode Quasi−Resonant Operation Current−Mode with Adjustable Skip Cycle Capability Dynamic Self−Supply Type of VCC Auto−Recovery Overcurrent Protection Improved UVLO for VCC below 10 V Latching Overvoltage Protection on VCC 500 mA Peak Current Source/Sink Capability Internal 1.0 ms Soft−Start Internal 10 ms Minimum TOFF Adjustable Skip Level Internal Temperature Shutdown Internal Leading Edge Blanking Direct Optocoupler Connection SPICE Models Available for TRANsient Analysis This is a Pb−Free Device AC−DC Adapters for Notebooks, etc. Offline Battery Chargers Consumer Electronics (DVD Players, Set−Top Boxes, TVs, etc.) Auxiliary Power Supplies (USB, Appliances, TVs, etc.) 1 ORDERING INFORMATION Device NCP1308DR2G Package SOIC−8 (Pb−Free) Shipping† 2500/T ape & Reel †For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specifications Brochure, BRD8011/D. Typical Applications © Semiconductor Components Industries, LLC, 2005 December, 2005 − Rev. 2 Publication Order Number: NCP1308/D NCP1308 R* + 12 V @ 1 A GND NCP1308 + 1 Universal Network 2 3 4 8 7 6 5 + *Please refer to the application information section. Y1 Type Figure 1. Typical Application Schematic PIN FUNCTION DESCRIPTION Pin 1 2 Symbol Dmg FB Function Core reset detection Sets the peak current setpoint Description The auxiliary FLYBACK signal ensures discontinuous operation. By connecting an optocoupler to this pin, the peak current setpoint is adjusted accordingly to the output power demand. By bringing this pin below the internal skip level, the device shuts off. This pin senses the primary current and routes it to the internal comparator via an LEB By inserting a resistor in series with the pin, you control the level at which the skip operation takes place. − The driver’s output to an external MOSFET. This pin is connected to an external bulk capacitor of typically 10 mF. If an auxiliary winding brings this pin above 16 V typical, the circuit permanently latches off. This unconnected pin ensures adequate creepage distance. Connected to the high−voltage rail, this pin injects a constant current into the VCC bulk capacitor. 3 CS Current sense input and skip cycle level selection The IC ground Driving pulses Supplies the IC − High−voltage pin 4 5 6 7 8 GND Drv VCC NC HV http://onsemi.com 2 NCP1308 50 us Filter 7 mA + + 16 V + 12 V 10 V 5.3 V (Fault) + OVP S S R PON Dmg VUVLO + 10 us Blanking + 50 mV + HV + − Resd Dmg 10 V Q Q R Driver src = 20 sink = 10 To internal supply 20k Drv VCC To Internal Supply Fault Mngt. + − Soft−Start = 1 ms /3 1V 200 mA when DRV is OFF Time Reset Dmg 380 ns LEB FB GND Overload? 5 us Timeout CS Figure 2. Internal Circuit Architecture MAXIMUM RATINGS Rating Power Supply Voltage Maximum Voltage on all other pins except Pin 8 (HV), Pin 6 (VCC) and Pin 5 (Drv) and Pin 1 (Dmg) Maximum Current into all pins except VCC (6), HV (8) and Dmg (1) when 10 V ESD diodes are activated Maximum Current in Pin 1 Thermal Resistance, Junction−to−Case Thermal Resistance, Junction−to−Air Maximum Junction Temperature Temperature Shutdown Hysteresis in Shutdown Storage Temperature Range ESD Capability, Human Body Model (All pins except HV) ESD Capability, Machine Model Maximum Voltage on Pin 8 (HV), Pin 6 (VCC) Decoupled to Ground with 10 mF Minimum Voltage on Pin 8 (HV), Pin 6 (VCC) Decoupled to Ground with 10 mF Symbol VCC, Drv − − Idem RqJC RqJA TJMAX − − − − − VHVMAX VHVMIN Value 20 −0.3 to 10 5.0 +3.0/−2.0 57 178 150 155 30 −60 to +150 2.0 200 500 40 Unit V V mA mA °C/W °C/W °C °C °C °C kV V V V Maximum ratings are those values beyond which device damage can occur. Maximum ratings applied to the device are individual stress limit values (not normal operating conditions) and are not valid simultaneously. If these limits are exceeded, device functional operation is not implied, damage may occur and reliability may be affected. http://onsemi.com 3 NCP1308 ELECTRICAL CHARACTERISTICS (For typical values TJ = 25°C, for min/max values TJ = 0°C to +125°C, Max TJ = 150°C, VCC = 11 V unless otherwise noted.) Characteristic DYNAMIC SELF SUPPLY VCC Increasing Level at which the Current Source Turns−Off VCC Decreasing Level at which the Current Source Turns−On VCC Decreasing Level at which the Latchoff Phase Ends VCC Level at which pulses are disabled Internal IC Consumption, No Output Load on Pin 5, FSW = 60 kHz Internal IC Consumption, 1.0 nF Output Load on Pin 5, FSW = 60 kHz Internal IC Consumption, Latchoff Phase, VCC = 6.0 V INTERNAL STARTUP CURRENT SOURCE ( TJ = 0°C) High−Voltage Current Source, VCC = 10 V High−Voltage Current Source, VCC = 0 DRIVE OUTPUT Output Voltage Rise−Time @ CL = 1.0 nF, 10−90% of Output Signal Output Voltage Fall−Time @ CL = 1.0 nF, 10−90% of Output Signal Source Resistance Sink Resistance CURRENT COMPARATOR Input Bias Current @ 1.0 V Input Level on Pin 3 Maximum Internal Current Setpoint Propagation Delay from Current Detection to Gate OFF State Leading Edge Blanking Duration Internal Current Offset Injected on the CS Pin During OFF Time OVERVOLTAGE SECTION Voltage on the VCC above which the controller latches off Integration Time Constraint on the OVP comparator FEEDBACK SECTION (VCC = 11 V, Pin 5 loaded by 1.0 kW) Internal Pullup Resistor Pin 3 to Current Setpoint Division Ratio Internal Soft−Start DEMAGNETIZATION DETECTION BLOCK Input Threshold Voltage (Vpin 1 Decreasing) Hysteresis (Vpin 1 Decreasing) Input Clamp Voltage High State (Ipin 1 = 3.0 mA) Low State (Ipin 1 = −2.0 mA) Dmg Propagation Delay Internal Input Capacitance at Vpin 1 = 1.0 V Minimum TOFF (Internal Blanking Delay After TON) Timeout After Last Dmg Transition 1. Max value at TJ = 0°C, please see characterization curves. 1 1 1 1 1 1 1 1 Vth VH VCH VCL Tdem Cpar Tblank Tout 35 − 8.0 −0.9 − − − − 50 20 10 −0.7 210 10 10 5.0 90 − 12 −0.5 − − − − ns pF ms ms mV mV V 2 − − Rup Iratio Tss − − − 20 3.3 1.0 − − − kW − ms 6 6 VOVP Tint 14.3 − 16 50 17.8 − V ms 3 3 3 3 3 IIB ILimit TDEL TLEB Iskip − 0.92 − − − 0.02 1.0 100 380 200 − 1.12 160 − − mA V ns ns mA 5 5 5 5 Tr Tf ROH ROL − − 12 5.0 40 20 20 10 − − 36 20 ns ns W W 8 8 IC1 IC2 4.3 − 7.0 8.0 9.6 − mA mA 6 6 6 6 6 6 6 VCCOFF VCCON VCClatch VUVLO ICC1 ICC2 ICC3 10.8 9.1 − − − − − 12 10 5.3 VCCON − 200 mV 1.0 1.6 330 12.9 10.6 − − 1.3 (Note 1) 2.0 (Note 1) − V V V V mA mA mA Pin Symbol Min Typ Max Unit http://onsemi.com 4 NCP1308 TYPICAL CHARACTERISTICS 120 100 80 VTH, (mV) 60 40 20 0 −25 Ilimit, (V) 1.20 1.15 1.10 1.05 1.00 0.95 0.90 −25 0 25 50 75 100 125 0 25 50 75 100 125 TEMPERATURE (°C) TEMPERATURE (°C) Figure 3. Demagnetization Threshold vs. Temperature 18 18 17 ICC1, (mA) VCC, (V) 17 16 16 15 −25 1.60 1.40 1.20 1.00 0.80 0.60 0.40 −25 Figure 4. Maximum Peak Current Setpoint vs. Temperature 0 25 50 75 100 125 0 25 50 75 100 125 TEMPERATURE (°C) TEMPERATURE (°C) Figure 5. OVP Level Threshold vs. Temperature Figure 6. Internal IC Consumption (No Output Load) vs. Temperature 2.00 1.95 1.90 1.85 ICC2, (mA) 1.80 1.75 1.70 1.65 1.60 1.55 1.50 −25 0 25 50 75 100 125 TEMPERATURE (°C) Figure 7. Internal IC Consumption (1.0 nF Load) vs. Temperature http://onsemi.com 5 NCP1308 TYPICAL CHARACTERISTICS 12.90 11.0 10.8 12.40 VCCOFF, (V) VCCON, (V) 10.6 10.4 11.90 10.2 10.0 9.8 9.6 10.90 9.4 9.2 10.40 −25 0 25 50 75 100 125 9.0 −25 0 25 50 75 100 125 11.40 TEMPERATURE (°C) TEMPERATURE (°C) Figure 8. VCC Increasing Level at which the Current Source Turns−off vs. Temperature 12 11 10 ROH and ROL (W) 9 IC1, (mA) 8 7 6 5 4 3 2 −25 0 25 50 75 100 125 40 35 30 25 20 15 10 5 0 −25 Figure 9. VCC Decreasing Level at which the Current Source Turns−on vs. Temperature ROH ROL 0 25 50 75 100 125 TEMPERATURE (°C) TEMPERATURE (°C) Figure 10. Internal Startup Current Source, VCC = 10 V vs. Temperature Figure 11. Source and Sink Resistance vs. Temperature http://onsemi.com 6 NCP1308 APPLICATION INFORMATION INTRODUCTION The NCP1308 implements a standard current mode architecture where the switch−off time is dictated by the peak current setpoint, whereas the core reset detection triggers the turn−on event. This component represents the ideal candidate where low part−count is the key parameter, particularly in low−cost AC/DC adapters, consumer electronics, auxiliary supplies, etc. Due to its high−performance High−Voltage technology, the NCP1308 incorporates all the necessary components/features needed to build a rugged and reliable Switch−Mode Power Supply (SMPS): • Transformer Core Reset Detection: Borderline/critical operation is ensured whatever the operating conditions are. As a result, there are virtually no primary switch turn−on losses and no secondary diode recovery losses. The converter also stays a first−order system and accordingly eases the feedback loop design. • Quasi−Resonant Operation: By delaying the turn−on event, it is possible to restart the MOSFET in the minimum of the drain−source wave, ensuring reduced EMI/video noise perturbations. In nominal power conditions, the NCP1308 operates in Borderline Conduction Mode (BCM) also called Critical Conduction Mode (CCM). • Dynamic Self−Supply (DSS): Due to its Very High Voltage Integrated Circuit (VHVIC) technology, ON Semiconductor ’s NCP1308 allows for a direct pin connection to the high−voltage DC rail. A dynamic current source charges up a capacitor and thus provides a fully independent VCC level to the NCP1308. As a result, there is no need for an auxiliary winding to supply the IC, whose management is always a problem in variable output voltage designs (e.g. battery chargers). • Overvoltage Protection (OVP): By monitoring the VCC pin via a 50 ms time constant filter, the NCP1308 goes into latched fault condition whenever an overvoltage condition is detected. This occurs if VCC goes above 16 V typically. The controller stays fully latched in this position until the VCC is cycled down to 4 V, e.g. when the user unplugs the power supply from the mains outlet and re−plugs it. • Adjustable Skip Cycle Level: By offering the ability to tailor the level at which the skip cycle takes place, the designer can make sure that the skip operation only occurs at low peak current. This point guarantees • a noise−free operation with cheap transformer. This option also offers the ability to fix the maximum switching frequency when entering light load conditions. Overcurrent Protection (OCP): By continuously monitoring the FB line activity, NCP1308 enters burst mode as soon as the power supply undergoes an overload. The device enters a safe low power operation that prevents from any lethal thermal runaway. As soon as the default disappears, the power supply resumes operation. Unlike other controllers, overload detection is performed independently of any auxiliary winding level. In presence of a bad coupling between both power and auxiliary windings, the short circuit detection can be severely affected. The DSS naturally shields you against these troubles. Dynamic Self−Supply The DSS principle is based on the charge/discharge of the VCC bulk capacitor from a low level up to a higher level. We can easily describe the current source operation with some simple logical equations: POWER−ON: IF VCC < VCCOFF THEN Current Source is ON, no output pulses IF VCC decreasing > VCCON THEN Current Source is OFF, output is pulsing IF VCC increasing < VCCOFF THEN Current Source is ON, output is pulsing Typical values are: VCCOFF = 12 V, VCCON = 10 V To better understand the operational principle, the diagram in Figure 12 offers the necessary light: Vripple = 2V VCC VCC OFF 12V = VCC ON= 10V CURRENT SOURCE ON OFF Output pulses Figure 12. The Charge/Discharge Cycle over a 10 mF VCC Capacitor http://onsemi.com 7 NCP1308 The DSS behavior actually depends on the internal IC consumption and the MOSFET’s gate charge Qg. If we select a MOSFET like the MTP2N60E, Qg equals 22 nC (max). With a maximum switching frequency selected at 75 kHz, the average power necessary to drive the MOSFET (excluding the driver efficiency and neglecting various voltage drops) is: FSW @ Qg @ VCC with: FSW = maximum switching frequency Qg = MOSFET’s gate charge VCC = VGS level applied to the gate To obtain the output current, simply divide this result by VCC: Idriver + FSW @ Qg + 1.6 mA. The total standby power consumption at no−load will therefore heavily rely on the internal IC consumption plus the above driving current (altered by the driver’s efficiency). Suppose that the IC is supplied from a 350 VDC line. The current flowing through Pin 8 is a direct image of the NCP1308 consumption (neglecting the switching losses of the HV current source). If ICC2 equals 2.3 mA @ TJ = 60°C, then the power dissipated (lost) by the IC is simply: 350 V x 2.3 mA = 805 mW. For design and reliability reasons, it would be interesting to reduce this source of wasted power that increases the die temperature. This can be achieved by using different methods: 1. Use a MOSFET with lower gate charge Qg 2. Connect a diode to the half−wave portion to directly supply the HV pin: HV 5 Cbulk 1 MAINS 2 1 2 3 4 8 7 6 5 6 1N4007 Skipping Cycle Mode The NCP1308 automatically skips switching cycles when the output power demand drops below a given level. This is accomplished by monitoring the FB pin. In normal operation, Pin 2 imposes a peak current accordingly to the load value. If the load demand decreases, the internal loop asks for less peak current. When this setpoint reaches a determined level, the IC prevents the current from decreasing further down and starts to blank the output pulses: the IC enters the so−called skip cycle mode, also named controlled burst operation. The power transfer now depends upon the width of the pulse bunches (Figure 14) and follows the following formula: 1 @ Lp @ Ip2 @ F SW @ Dburst with: 2 Lp = primary inductance FSW = switching frequency within the burst Ip = peak current at which skip cycle occurs Dburst = burst width/burst recurrence MAX PEAK CURRENT NORMAL CURRENT MODE OPERATION SKIP CYCLE CURRENT LIMIT CURRENT SENSE SIGNAL (mV) 300 200 100 0 WIDTH RECURRENCE Figure 14. The Skip Cycle Takes Place at Low Peak Currents which Guarantees Noise−Free Operation DRIVER DRIVER = HIGH ? I = 0 DRIVER = LOW ? I = 200 mA − + Rskip 3 Rsense 2 Figure 13. The Connection to the Half−Wave Signal Reduces the Dissipated Power on the Controller RESET 3. Permanently force the VCC level above VCCON with an auxiliary winding. It will automatically disconnect the internal startup source and the IC will be fully self−supplied from this winding. Again, the total power drawn from the mains will significantly decrease. Make sure the auxiliary voltage never exceeds the 16 V limit. When the power supply is switched off, an internal comparator makes sure that all output pulses are disable when VCC crosses VCCON. + Figure 15. A Patented Method Allows for Skip Level Selection via a Series Resistor Inserted in Series with the Current http://onsemi.com 8 NCP1308 The skip level selection is done through a simple resistor inserted between the current sense input and the sense element. Every time the NCP1308 output driver goes low, a 200 mA source forces a current to flow through the sense pin (Figure 15): when the driver is high, the current source is off and the current sense information is normally processed. As soon as the driver goes low, the current source delivers 200 mA and develops a ground−referenced voltage across Rskip. If this voltage is below the feedback voltage, the current sense comparator stays in the high state and the internal latch can be triggered by the next clock cycle. Now, if because of a low load mode the feedback voltage is below Rskip level, then the current sense comparator permanently resets the latch and the next clock cycle (given by the demagnetization detection) is ignored: we are skipping cycles as shown in Figure 15. As soon as the feedback voltage goes up again, there can be two situations: the recurrent period is small and a new demagnetization detection (next wave) signal triggers the NCP1308. To the opposite, in low output power conditions, no more ringing waves are present on the drain and the toggling of the current sense comparator together with the internal 5 ms timeout initiates a new cycle start. In normal operating conditions, e.g. when the drain oscillations are generous, the demagnetization comparator can detect the 50 mV crossing and gives the “green light”, alone, to re−active the power switch. However, when skip cycle takes place (e.g. at low output power demands), the restart event slides along the drain ringing waveforms (actually the valley locations) which decays more or less quickly, depending on the Lprimary−Cparasitic network damping factor. The situation can thus quickly occur where the ringing becomes too weak to be detected by the demagnetization comparator: it then permanently stays locked in a given position and can no longer deliver the “green light” to the controller. To help in this situation, the NCP1308 implements a 5 ms timeout generator: each time the 50 mV crossing occurs, the timeout is reset. So, as long as the ringing becomes too low, the timeout generator starts to count and after 5 ms, it delivers its “green light”. If the skip signal is already present then the controller restarts; otherwise the logic waits for it to set the drive output high. Figure 16 depicts these two different situations: Drain Signal Timeout Signal Dmg Restart Current Sense and Timeout Restart Drain Signal Timeout Signal 5 ms 5 ms Figure 16. When the primary natural ringing becomes too low, the internal Timeout together with the sense comparator initiates a new cycle when FB passes the skip level. http://onsemi.com 9 NCP1308 Demagnetization Detection The core reset detection is done by monitoring the voltage activity on the auxiliary winding. This voltage features a FLYBACK polarity. The typical detection level is fixed at 50 mV as exemplified by Figure 17. Figure 19 portrays a typical Vds shot at nominal output power. 400 DRAIN VOLTAGE (V) 7.0 POSSIBLE RESTARTS 300 DMG SIGNAL (V) 5.0 200 3.0 100 1.0 0V 0 50 mV −1.0 Figure 19. The NCP1308 Operates in Borderline/Critical Operation Overvoltage Protection Figure 17. Core Reset Detection is Done through a Dedicated Auxiliary Winding Monitoring An internal timer prevents any restart within 10 μs further to the driver going−low transition. This prevents the switching frequency to exceed (1/(TON + 10 ms)) but also avoid false leakage inductance tripping at turn−off. In some cases, the leakage inductance kick is so energetic, that a slight filtering is necessary. The NCP1308 demagnetization detection pad features a specific component arrangement as detailed by Figure 18. In this picture, the Zener diodes network protect the IC against any potential ESD discharge that could appear on the pins. The first ESD diode connected to the pad, exhibits a parasitic capacitance. When this parasitic capacitance (10 pF typically) is combined with Rdem, a restart delay is created and the possibility to switch right in the drain−source wave exists. This guarantees QR operation with all the associated benefits (low EMI, no turn−on losses etc.). Rdem should be calculated to limit the maximum current flowing through Pin 1 to less than +3 mA / −2 mA: if during turn−on, the auxiliary winding delivers −30 V (at the highest line level), then the minimum Rdem value is defined by: (−30 + 0.7. This value will be further increased to introduce a restart delay and also a slight filtering in case of high leakage energy. TO INTERNAL COMPARATOR The overvoltage works by monitoring the VCC pin via a comparator and a reference voltage. Figure 20 portrays the internal arrangement: 50 us FILTER VCC + 16 V − + OVP COMPARATOR TO LATCH Figure 20. OVP Section Circuitry Resd 1 ESD ESD 4 Rdem A 50 ms time−constant filter prevents any parasitic spikes superimposed on the VCC to adversely trigger the OVP comparator. When the OVP comparator output goes high, the NCP1308 fully latches off and stays latched, being self−supplied by the DSS. The user must unplug the power supply and wait that the VCC comes down below a reset voltage of typically 4 V. Aux Figure 18. Internal Pad Implementation http://onsemi.com 10 NCP1308 Shutting off the NCP1308 Shutdown can easily be implemented through a simple NPN bipolar transistor as depicted by Figure 21. When OFF, Q1 is transparent to the operation. When forward biased, the transistor pulls the FB pin to ground (VCEsat ≈ 200 mV) and permanently disables the IC. A small time constant on the transistor base will avoid false triggering NCP1308 1 10 k ON/OFF Q1 10 nF 2 3 4 8 7 6 5 where IDSS is the peak DSS capability, DSSduty−cycle is the duty−cycle of the DSS, that is to say, the time it is on and the time it stays off (DSSduty−cycle = on/(on + off) ). Please refer to the application note AND8069/D available at www.onsemi.com/pub/ncp1200. If the power consumption budget is really too high for the DSS alone, connect a diode between the auxiliary winding and the VCC pin which will disable the DSS operation (VCC > 10 V). Overload Operation Figure 21. A Simple Bipolar Transistor Totally Disables the IC Power Dissipation The SOIC package offers a 178°C/W thermal resistor. Again, adding some copper area around the PCB footprint will help decreasing this number: 12 mm x 12 mm to drop RθJA down to 100°C/W with 35 mm copper thickness (1 oz) or 6.5 mm x 6.5 mm with 70 mm copper thickness (2 oz). As one can see, the designer must be cautious when using the SO−8 package to check if its thermal performance is compatible with the total power dissipation. The power dissipation is simply Vbulk (high line) x IDSS,AVG. The IDSS,AVG parameter can be measured by inserting an amp−meter in series with the HV pin and compute its average value. We therefore recommend the insertion of a resistor from the bulk connection to the HV pin. This will help to: 1. Avoid negative spikes at turn−off on the HV pin (see below) 2. Split the power budget between this resistor and the package. The resistor is calculated by leaving at least 50 V on pin 8 at minimum input voltage (suppose 100 Vdc in our case): V * 50 V Rdrop v bulkmin t 7.1 kW . 7.0 mA The power dissipated by the resistor is thus: Pdrop + + + VdropRMS2 Rdrop (IDSS @ Rdrop @ DSSduty−cycle)2 Rdrop (7.0 mA @ 7.1 kW @ 0.286) 2 7.1 kW In applications where the output current is purposely not controlled (e.g. wall adapters delivering raw DC level), it is interesting to implement a true short−circuit protection. A short−circuit actually forces the output voltage to be at a low level, preventing a bias current to circulate in the optocoupler LED. As a result, the FB pin level is pulled up to 4.2 V, as internally imposed by the IC. The peak current setpoint goes to the maximum and the supply delivers a rather high power with all the associated effects. Please note that this can also happen in case of feedback loss, e.g. a broken optocoupler. To account for this situation, NCP1308 hosts a dedicated overload detection circuitry. Once activated, this circuitry imposes to deliver pulses in a burst manner with a low duty−cycle. The system recovers when the fault condition disappears. During the startup phase, the peak current is pushed to the maximum until the output voltage reaches its target and the feedback loop takes over. This period of time depends on normal output load conditions and the maximum peak current allowed by the system. The timeout used by this IC works with the VCC decoupling capacitor: as soon as the VCC decreases from the VCCOFF level (typically 12 V) the device internally watches for an overload current situation. If this condition is still present when the VCCON level is reached, the controller stops the driving pulses, prevents the self−supply current source to restart and puts all the circuitry in standby, consuming as little as 330 mA typical (ICC3 parameter). As a result, the VCC level slowly discharges toward 0. When this level crosses 5.3 V typical, the controller enters a new startup phase by turning the current source on: VCC rises toward 12 V and again delivers output pulses at the VCCOFF crossing point. If the fault condition has been removed before VCCON approaches, then the IC continues its normal operation. Otherwise, a new fault cycle takes place. Figure 22 on the following page shows the evolution of the signals in presence of a fault. + 99.5 mW http://onsemi.com 11 NCP1308 VCC REGULATION OCCURS HERE LATCHOFF PHASE 12 V 10 V 5.3 V TIME DRV DRIVER PULSES TIME INTERNAL If the fault is relaxed during the VCC natural fall down sequence, the IC automatically resumes. If the fault still persists when VCC reached VCCON, then the controller cuts everything off until recovery. FAULT FLAG FAULT IS RELAXED STARTUP PHASE FAULT OCCURS HERE TIME Figure 22. Soft−Start The NCP1308 features an internal 1ms soft−start to soften the constraints occurring in the power supply during startup. It is activated during the power on sequence. As soon as VCC reaches VCCOFF, the peak current is gradually increased from nearly zero up to the maximum clamping level (e.g. 1.0 V). The soft−start is also activated during the over current burst (OCP) sequence. Every restart attempt is followed by a soft−start activation. Generally speaking, the soft−start will be activated when VCC ramps up either from zero (fresh power−on sequence) or 5.3 V, the latchoff voltage occurring during OCP. Calculating the VCC Capacitor must be less than the time needed to discharge from 12 V to 10 V, otherwise the supply will not properly start. The test consists in either simulating or measuring in the lab how much time the system takes to reach the regulation at full load. Let’s suppose that this time corresponds to 6ms. Therefore a VCC fall time of 10 ms could be well appropriated in order to not trigger the overload detection circuitry. If the corresponding IC consumption, including the MOSFET drive, establishes at 1.6 mA (e.g. with a 10 nC Qg), we can calculate the required capacitor using the following formula: Dt + DV @ C, with ΔV = 2 V. Then i for a wanted Δt of 10 ms, C equals 9 mF or 22 mF for a standard value. When an overload condition occurs, the IC blocks its internal circuitry and its consumption drops to 330 mA typical. This happens at VCC = 10 V and it remains stuck until VCC reaches 5.3 V: we are in latchoff phase. Again, using the calculated 22 mF and 330 mA current consumption, this latchoff phase lasts: 313 ms. As the above section describes, the fall down sequence depends upon the VCC level: how long does it take for the VCC line to go from 12 V to 10 V? The required time depends on the startup sequence of your system, i.e. when you first apply the power to the IC. The corresponding transient fault duration due to the output capacitor charging http://onsemi.com 12 NCP1308 Protecting Pin 8 Against Negative Spikes As any CMOS controller, NCP1308 is sensitive to negative voltages that could appear on its pins (Figure 23). To avoid any adverse latchup of the IC, we strongly recommend to insert a resistor in series with pin 8. This resistor prevents from adversely latching the controller in case of negative spikes appearing on the bulk capacitor during the power−off sequence. Please refer to the power dissipation section of this data sheet to see how to calculate this element. Vbulk Vcc Latch! Vbulk