NCP1238 Fixed Frequency Current Mode Controller for Flyback Converters The NCP1238 is a new generation of the NCP12xx fixed−frequency current−mode controllers featuring Dynamic Self−Supply (DSS), pin−to−pin compatible with the previous generation. The DSS function greatly simplifies the design of the auxiliary supply and the VCC capacitor by activating the internal startup current source to supply the controller during transients. Due to its proprietary Soft−Skip™ mode combined with frequency foldback, the controller exhibits excellent efficiency in light load condition while still achieving very low standby power consumption. This Soft−Skip feature also dramatically reduces the risk of acoustic noise, which enables the use of inexpensive transformers and capacitors in the clamping network. Internal frequency jittering, ramp compensation, and a versatile latch input make this controller an excellent candidate for converters where ruggedness and components cost are the key constraints. In addition, the controller includes a new high voltage circuitry that combines a startup current source and a brown−out / line OVP detector able to sense the input voltage either from the rectified ac line or the dc filtered bulk voltage. Finally, due to a careful design, the precision of critical parameters is well controlled over the entire temperature range (−40°C to +125°C), enabling easier design and increased safety (e.g. ±5% for the peak current limit, ±7% for the oscillator). Features • Timer−Based Overload Protections with Auto− • • • • • • • • • • • • • • Recovery (Option B) or Latched (Option A) Operation High−Voltage Current Source with DSS with Built−in Brown−out and Line Overvoltage Protections Fixed−Frequency Current−Mode Operation with Built−in Ramp Compensation Frequency Jittering for a Reduced EMI Signature Adjustable Overpower Compensation Latch−off Input for Severe Fault Conditions, with Direct Connection of an NTC for Overtemperature Protection (OTP) Protection Against Winding Short−Circuit Frequency Foldback transitioning into Soft−Skip for Improved Performance in Standby 65 kHz Oscillator (100 kHz and 133 kHz Versions Available Upon Request) http://onsemi.com MARKING DIAGRAM 8 SOIC−7 CASE 751U 1 38Xff ALYWX G 38Xff = Specific Device Code X = A or B ff = 65, 00, or 33 A = Assembly Location L = Wafer Lot Y = Year W = Work Week G = Pb−Free Package ORDERING INFORMATION See detailed ordering and shipping information in the package dimensions section on page 29 of this data sheet. VCC Operation up to 28 V Increased Precision on Critical Parameters ±1.0 A Peak Drive Capability 4.0 ms Soft−Start Internal Thermal Shutdown with Hysteresis These Devices are Pb−Free, Halogen Free/BFR Free and are RoHS Compliant* Typical Applications • • • • ac−dc Adapters for Notebooks, LCD, and Printers Offline Battery Chargers Consumer Electronic Power Supplies Auxiliary/Housekeeping Power Supplies *For additional information on our Pb−Free strategy and soldering details, please download the ON Semiconductor Soldering and Mounting Techniques Reference Manual, SOLDERRM/D. © Semiconductor Components Industries, LLC, 2010 February, 2010 − Rev. 1 1 Publication Order Number: NCP1238/D NCP1238 1 Latch 2 FB 3 4 HV 8 CS VCC 6 GND DRV 5 Figure 1. Pinout TYPICAL APPLICATION EXAMPLE VOUT VIN (dc) LATCH HV FB NCP1238 CS VCC GND DRV Figure 2. Typical Application PIN FUNCTION DESCRIPTION Pin No Pin Name Function Pin Description 1 LATCH Latch−off Input Pull the pin up or down to latch−off the controller. An internal current source allows the direct connection of an NTC for over temperature detection 2 FB Feedback 3 CS Current Sense 4 GND – 5 DRV Drive Output 6 VCC VCC Input 8 HV High−Voltage Pin A pull−down optocoupler controls the output regulation. Senses the primary current for current−mode operation, and provides a mean for overpower compensation adjustment. IC ground Drives an external MOSFET This supply pin accepts up to 28 Vdc Connects to the bulk capacitor or the rectified AC line to perform the functions of Dynamic Self−Supply and brown−out / line overvoltage detections http://onsemi.com 2 NCP1238 SIMPLIFIED INTERNAL BLOCK SCHEMATIC Brown−out + INTC INTC BO HV stop blanking + − OVM tLatch(OVP) VOVP − + VDD HV dc HV sample HV Latch + 1 kW − + VOTP S Q R Vclamp TSD TSD blanking tLatch(OTP) Dual HV start−up current source Latch TSD HV current Reset VCC UVLO management VDD Start Reset UVLO VDD IC Start Soft−start end Brown−out Reset FB Soft−skip ramp − + + VDD Reset VCC tSSKIP Vskip RFB(up) Skip KFB FB slope comp. HV sample PWM + − V to I IOPC = 0.5m x (VHV−125) Jitter Skip FB Sawtooth Soft−start + − Stop Foldback Oscillator tSSTART − + + Soft−start Start ramp Reset End VFB(OPC) DCMAX IC Start S Soft−start end blanking tLEB CS + blanking Q R + − VILIM tBCS Clamp DMAX IC Stop DRV ILIMIT IC stop GND UVLO ILIMIT DMAX S Q R + Latch Autorecovery protection mode only VCS(stop) Fault Flag timer timer tfault PWM HV stop Protection Mode release + − tautorec Fault Brown−out Reset Figure 3. Simplified Internal Block Schematic http://onsemi.com 3 TSD NCP1238 MAXIMUM RATINGS Symbol Value Unit Supply Pin (pin 6) (Note 2) Voltage range Current range Rating VCCMAX ICCMAX –0.3 to 28 $30 V mA High Voltage Pin (pin 8) (Note 2) Voltage range Current range VHVMAX IHVMAX –0.3 to 500 $20 V mA Driver Pin (pin 5) (Note 2) Voltage range Current range VDRVMAX IDRVMAX –0.3 to 20 $1000 V mA VMAX IMAX –0.3 to 10 $10 V mA All other pins (Note 2) Voltage range Current range Thermal Resistance Junction−to−Air, low conductivity PCB (Note 3) Junction−to−Air, medium conductivity PCB (Note 4) Junction−to−Air, high conductivity PCB (Note 5) RqJA Temperature Range Operating Junction Temperature Storage Temperature Range TJMAX TSTRGMAX ESD Capability (Note 1) Human Body Model (HBM) Model (All pins except HV) Machine Model (MM) 162 147 125 −40 to +150 −60 to +150 2000 190 °C/W °C V Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect device reliability. 1. This device series contains ESD protection and exceeds the following tests: Human Body Model 2000 V per JEDEC standard JESD22, Method A114E Machine Model Method 200 V per JEDEC standard JESD22, Method A115A 2. This device contains latch−up protection and exceeds 100 mA per JEDEC Standard JESD78 3. As mounted on a 80 x 100 x 1.5 mm FR4 substrate with a single layer of 100 mm2 of 1 oz copper traces and heat spreading area. As specified for a JEDEC 51−1 conductivity test PCB. Test conditions were under natural convection or zero air flow. 4. As mounted on a 80 x 100 x 1.5 mm FR4 substrate with a single layer of 100 mm2 of 2 oz copper traces and heat spreading area. As specified for a JEDEC 51−2 conductivity test PCB. Test conditions were under natural convection or zero air flow. 5. As mounted on a 80 x 100 x 1.5 mm FR4 substrate with a single layer of 650 mm2 of 1 oz copper traces and heat spreading area. As specified for a JEDEC 51−3 conductivity test PCB. Test conditions were under natural convection or zero air flow. http://onsemi.com 4 NCP1238 ELECTRICAL CHARACTERISTICS (For typical values TJ = 25°C, for min/max values TJ = −40°C to +125°C, VHV = 120 V, VCC = 11 V unless otherwise noted) Characteristics Test Condition Symbol Min Typ Max Unit VHV(min) − − 60 V Istart1 Istart2 0.2 4 0.5 8 0.8 12 mA Istart(off) − 25 50 mA Turn−on threshold level, VCC going up HV current source stop threshold VCC(on) 11.0 12.0 13.0 V HV current source restart threshold VCC(min) 9.5 10.5 11.5 V HIGH VOLTAGE CURRENT SOURCE Minimum voltage for current source operation Current flowing out of VCC pin @ VHV = 60 V VCC = 0 V VCC = VCC(on) − 0.5 V Off−state leakage current VHV = 500 V SUPPLY Turn−off threshold level VCC(off) 8.5 9.5 10.5 V tUVLO(blank) 7 10 13 ms VCC decreasing level at which the internal logic resets VCC(reset) 4.0 5.2 6.5 V VCC level for ISTART1 to ISTART2 transition VCC(inhibit) 0.4 0.65 0.9 V DRV open, VFB = 3 V ICC1 2.0 2.5 3.0 mA Cdrv = 1 nF, VFB = 3 V ICC2 2.3 3.3 4.3 Off mode (skip or before startup) Fault mode (fault or latch) ICC3 0.9 1.2 1.5 ICC4 0.4 0.7 1.0 VHV(start) VHV(stop) 104 97 112 105 120 113 V tHV 43 61 79 ms VHV(OV1) VHV(OV2) 400 395 430 425 460 455 V tOV(blank) − 250 − ms Oscillator frequency fOSC 60 65 70 kHz Maximum duty ratio DMAX 75 80 85 % Blanking duration on VCC(min) and VCC(off) detection Internal current consumption (Note 6) Guaranteed by design BROWN−OUT AND LINE OVERVOLTAGE Brown−out threshold voltage VHV going up VHV going down Timer duration for line cycle drop−out Overvoltage threshold VHV going up VHV going down Blanking duration on line overvoltage detection OSCILLATOR Frequency jittering amplitude, in percentage of FOSC Guaranteed by design Ajitter $4 $6 $8 % Frequency jittering modulation frequency Guaranteed by design Fjitter 85 125 165 Hz Rise time, 10% to 90% of VCC VCC = VCC(min) + 0.2 V, CDRV = 1 nF trise − 22 34 ns Fall time, 90% to 10% of VCC VCC = VCC(min) + 0.2 V, CDRV = 1 nF tfall − 22 34 ns Current Capability VCC = VCC(min) + 0.2 V, CDRV = 1 nF DRV high, VDRV = 0 V DRV low, VDRV = VCC OUTPUT DRIVER mA IDRV(source) IDRV(sink) − − 800 800 − − Clamping Voltage (Maximum Gate Voltage) VCC = VCCmax – 0.2 V, DRV high VDRV(clamp) 11 13.5 16 V High−State Voltage Drop VCC = VCC(min) + 0.2 V, RDRV = 33 kW, DRV high VDRV(drop) − − 1 V 6. Internal supply current only, current in FB pin not included (current flowing through GND pin only). http://onsemi.com 5 NCP1238 ELECTRICAL CHARACTERISTICS (For typical values TJ = 25°C, for min/max values TJ = −40°C to +125°C, VHV = 120 V, VCC = 11 V unless otherwise noted) Characteristics Test Condition Symbol Min Typ Max Unit Ibias − 0.02 − mA CURRENT SENSE Input Bias Current VCS = 0.7 V Maximum Internal Current Setpoint VFB > 3.5 V VILIM 0.665 0.7 0.735 V VCS(stop) 0.95 1.05 1.15 V tdelay − 80 110 ns Leading Edge Blanking Duration for VILIM tLEB 190 250 310 ns Leading Edge Blanking Duration for VCS(stop) tBCS 90 120 150 ns Scomp(65kHz) − −32.5 − mV / ms tSSTART 2.8 4.0 5.2 ms KOPC − 0.5 − mA / V IOPC(125) IOPC(162) IOPC(325) IOPC(max) − − − 102 0 52 104 120 − − − 138 mA FB voltage above which IOPC is applied VFB(OPC) 1.50 1.65 1.80 V FB voltage below which IOPC = 0 VFB(OPCE) − 1.25 − V tWD 25 35 45 ms RFB(up) 15 20 25 kW KFB 4.7 5.0 5.3 − VFB(ref) 4.3 5.0 5.7 V tfault 64 78 98 ms tautorec 1.0 1.4 1.8 s VFB(fold) 1.3 1.4 1.5 V fOSC(min) 21 27 31 kHz VFB(endfold) − 1.0 − V Vskip(in) Vskip(out) 0.63 0.72 0.7 0.80 0.77 0.88 V tSSKIP − 100 − ms Threshold for Immediate Fault Protection Activation Propagation Delay from VILIM detection to DRV off VCS = VILIM Slope of the Compensation Ramp Soft−Start Duration 1st From pulse to VCS = VILIM OVERPOWER COMPENSATION VHV to IOPC Conversion Ratio Current flowing out of CS pin VHV = 125 V VHV = 162 V VHV = 325 V VHV = VHV(OV2) − 5 V Refresh period for dc operation FEEDBACK Internal Pull−up Resistor TJ = 25°C VFB to Internal Current Setpoint Division ratio Internal Pull−up Voltage on the FB pin OVERCURRENT PROTECTION Fault Timer Duration From CS reaching VILIMIT to DRV stop Autorecovery Mode Latch−off Time Duration FREQUENCY FOLDBACK Feedback Voltage Threshold below which frequency foldback starts Minimum Switching Frequency VFB = Vskip(in) + 0.2 V Threshold below which the frequency foldback is finished and the controller switches at fOSC(min) SKIP CYCLE MODE Feedback Voltage Thresholds for Skip Mode VFB going down VFB going up Soft−Skip Duration From 1st pulse to VCS = VFB(fold) / KFB 6. Internal supply current only, current in FB pin not included (current flowing through GND pin only). http://onsemi.com 6 NCP1238 ELECTRICAL CHARACTERISTICS (For typical values TJ = 25°C, for min/max values TJ = −40°C to +125°C, VHV = 120 V, VCC = 11 V unless otherwise noted) Characteristics Test Condition Symbol Min Typ Max Unit LATCH−OFF INPUT High Threshold VLatch going up VOVP 2.37 2.5 2.63 V Low Threshold VLatch going down VOTP 0.76 0.8 0.84 V Current Source for Direct NTC Connection During normal operation During soft−start VLatch = 0 V INTC INTC(SSTART) 78 156 91 182 104 208 Blanking Duration on High Latch Detection tLatch(OVP) 40 55 70 ms Blanking Duration on Low Latch Detection tLatch(OTP) − 400 − ms Vclamp0(Latch) Vclamp1(Latch) 1.0 1.8 1.2 2.3 1.4 2.8 V TTSD 135 150 165 °C TTSD(HYS) 20 30 40 °C Clamping Voltage ILatch = 0 mA ILatch = 1 mA mA TEMPERATURE SHUTDOWN Temperature Shutdown TJ going up Temperature Shutdown Hysteresis TJ going down 6. Internal supply current only, current in FB pin not included (current flowing through GND pin only). http://onsemi.com 7 NCP1238 APPLICATION INFORMATION • Adjustable Overpower Compensation: The high Introduction The NCP1238 includes all of the necessary features to build a safe and efficient power supply based on a fixed−frequency flyback converter. It is particularly well suited for applications where low part count is a key parameter, without sacrificing safety. • Current−Mode Operation with slope compensation: The primary peak current is permanently controlled by the FB voltage, ensuring maximum safety: the DRV turn−off event is dictated by the peak current setpoint. It also ensures that the frequency response of the system remains first order if in DCM, which eases the design of the feedback loop. The controller can also be used in CCM with a wide input voltage range due to its fixed ramp compensation that prevents the appearance of sub−harmonic oscillations in most of the applications. • Fixed−Frequency Oscillator with Jittering: The NCP1238 is available in various frequency options to fit any application. The internal oscillator features a low−frequency jittering that helps to pass the EMI requirements by spreading out the energy content of frequency peaks in quasi−peak and average mode. • Latched / Autorecovery Timer−Based Overcurrent Protection: The overcurrent protection depends only on the FB signal, enabling it to work with any transformer, even with very poor coupling or high leakage inductance. The protection is fully latched on the A version (the power supply has to be unplugged then restarted in order to resume operation, even if the overload condition disappears), and autorecovery on the B version. The timer’s duration is fixed. The controller also enters the same protection mode if the voltage on the CS pin reaches 1.5 times the maximum internal setpoint, which enables to detect winding short circuits. • High Voltage Startup Current Source with Brown−Out and Line Overvoltage Detections: Due to On Semiconductor’s Very High Voltage technology, the NCP1238 can directly be connected to the high input voltage. The startup current source ensures a clean startup while ensuring low losses when it is off, and the Dynamic Self−Supply (DSS) restarts the startup current source to supply the controller if the VCC supply transiently drops. The high voltage pin also features a high−voltage sensing circuitry, which is able to turn the controller off if the input voltage is too low (brown−out condition) or too high (line overvoltage). This protection works either with a DC input voltage or a rectified AC input voltage, and is independent of the high voltage ripple. It uses a peak detector synchronized with line frequency, or with the internal watchdog timer if the HV pin is tied to a dc voltage. • • • • • • voltage sensed on the HV pin is converted into a current to add to the current sense voltage an offset proportional to the input voltage. By choosing the value of the resistor in series with the CS pin, the amount of compensation can be adjusted to the application. Frequency foldback then Soft−Skip mode for light load operation: In order to ensure a high efficiency in all load conditions, the NCP1238 implements a frequency foldback (the switching frequency is lowered to reduce switching losses) for light load condition; and a Soft−Skip (disabled in case of fast load transients) for extremely low load condition. Extended VCC range: The NCP1238 accepts a supply voltage as high as 28 V, making the design of the power supply easier. Clamped Driver Stage: Despite the high supply voltage, the voltage on DRV pin is safely clamped below 16 V; allowing the use of any standard MOSFET, and reducing the current consumption of the controller. Dual Latch−off Input: The NCP1238 can be latched off by an increasing voltage applied to its Latch pin (typically an overvoltage) or by a decreasing one, and an NTC can be directly connected to the latch pin thanks to the precise internal current source. Soft−Start: At every startup the peak current is gradually increased during 4 ms to minimize the stress on power components. Temperature Shutdown: The NCP1238 is internally protected against self−heating: if the die temperature is too high, the controller shuts all circuitries down (including the HV startup current source), allowing the silicon to cool down before attempting to restart. This ensures a safe behavior in case of failure. Typical Operation • Startup: The HV startup current source ensures the • charging of the VCC capacitor up to the startup threshold VCC(on), until the input voltage is high enough (above VHV(start)) to enable the switching. The controller then delivers pulses, starting with a soft−start period tSSTART during which the peak current linearly increases before the current−mode control takes over. During the soft−start period, the low level latch is ignored, and the latch current is double, to ensure a fast pre−charge of the decoupling capacitor on the Latch pin. Normal operation: As long as the feedback voltage is within the regulation range, the NCP1238 runs at a fixed frequency (with jittering) in current−mode control, where the peak current (sensed on the CS pin) http://onsemi.com 8 NCP1238 • • • Overload: The NCP1238 features a timer−based is set by the voltage on the FB pin. A fixed ramp compensation is applied internally to prevent sub−harmonic oscillations from occurring. The VCC must be supplied by an external source (such as an auxiliary winding), as the startup current source cannot permanently supply the controller without overheating. Light load operation: When the FB voltage decreases below VFB(fold), typically corresponding to a load of 16% of the maximum load (for a DCM design), the switching frequency starts to decrease down to fOSC(min). By lowering the switching losses, this feature helps to improve the efficiency in light load conditions. The frequency jittering is disabled in light load operation. No load operation: When the FB voltage decreases below Vskip(in), typically corresponding to a load of 1% of the maximum load, the controller enters Skip mode. By completely stopping the switching while the feedback voltage is below Vskip(out), the losses are further reduced, allowing to minimize the power dissipation under extremely low load conditions. In order to avoid audible noise, the peak current is gradually increased during the tSSKIP duration while exiting the skip mode (Soft−Skip function). In case of abrupt load increase during Soft−Skip mode, the soft−skip portion is bypassed and the peak current needed for regulation is directly applied. VCC can be maintained between VCC(on) and VCC(min) by the DSS. • • overload detection, solely dependent on the feedback information: as soon as the internal peak current setpoint hits the VILIM clamp, the internal overload timer starts to count. When the timer times out, the controller stops and enter the protection mode, autorecovery for the B version (the controller initiates a new start−up after tautorec elapses), or latched for the A version (the latch is released if a brown−out event occurs or VCC is reset). Brown−out: The NCP1238 features on its HV pin a true AC line monitoring circuitry which includes minimum startup threshold, brown−out protection, and overvoltage protection. All of these circuits are autorecovery and operate independently of any ripple on the input voltage. They can even work with an unfiltered, rectified AC input. All thresholds are fixed, but they are designed to fit most of the standard ac−dc conversion applications. Latch−off: When the Latch input is pulled up (typically by an overvoltage condition), or pulled low (typically by an overtemperature condition, using the provided current source with an NTC), the controller latches off. The latch is released when a brown−out condition occurs, or when VCC decreases below VCC(reset). http://onsemi.com 9 NCP1238 DETAILED DESCRIPTION High−Voltage Current Source (Dynamic Self−Supply) with Built−in Brown−out Detection The NCP1238 HV pin can be connected either to the rectified bulk voltage, or to the ac line through a rectifier. Startup HV Istart TSD Control IC Start VCC + + − VCC(on) + R Q S − + VCC(min) + blanking − + UVLO tUVLO(blank) VCC(off ) + − + Reset VCC(reset) Figure 4. HV Startup Current Source Functional Schematic http://onsemi.com 10 NCP1238 current source on and off, it can only be used in light load conditions, otherwise the power dissipation on the die would be too high. As a result, an auxiliary voltage source is needed to supply VCC during normal operation. The DSS is useful to keep the controller alive when no switching pulses are delivered, e.g. in a brown−out condition, or to prevent the controller from stopping during load transients when the VCC might drop below VCC(off). At startup, the current source turns on when the voltage on the HV pin is higher than VHV(min), and turns off when VCC reaches VCC(on). It turns on again when VCC reaches VCC(min). This sequence repeats until the input voltage is high enough to ensure a proper startup, i.e. when VHV reaches VHV(start). The switching actually starts the next time VCC reaches VCC(on), as shown in Figure 5. Even though the DSS is able to maintain the VCC voltage between VCC(on) and VCC(min) by turning the HV startup VHV VHV(start) Waits next VCC(on) before starting VHV(min) time VCC VCC(on) VCC(min) HV current source = Istart1 HV current source = Istart2 VCC(inhibit) time DRV Figure 5. Startup Timing Diagram http://onsemi.com 11 time NCP1238 Brown−out and Line Overvoltage To reduce the power dissipation in case the VCC pin is shorted to GND (in case of VCC capacitor failure, or external pulldown on VCC to disable the controller), the startup current is lowered when VCC is below VCC(inhibit). There are only two conditions for which the current source doesn’t turn on when VCC reaches VCC(min): the voltage on HV pin is too low (below VHV(min)), or a thermal shutdown condition (TSD) has been detected. In all other conditions, the HV current source always turns on and off to maintain VCC between VCC(min) and VCC(on). When the input voltage goes below VHV(stop), a brown−out condition is detected, and the controller stops. The HV current source alternatively turns on and off to maintain VCC between VCC(on) and VCC(min) until the input voltage is back above VHV(start). The same situation occurs when an overvoltage is detected on the ac line, i.e. when the input voltage goes above VHV(OV): the controller stops, and resumes normal operation when the overvoltage condition has gone. HV stop Brown-out or AC OVP detected Waits next VCC(on) before starting VCC time VCC(on) VCC(min) time DRV Figure 6. Brown−out or Line Overvoltage Timing Diagram http://onsemi.com 12 time NCP1238 When VHV crosses the VHV(start) threshold, the controller can start immediately. When it crosses VHV(stop), it triggers a timer of duration tHV: this ensures that the controller doesn’t stop in case of line cycle drop−out. VHV Brown-out VHV(start) VHV(stop) DRV time Starts at next VCC(ON) t HV Figure 7. AC Input Brown−out Timing Diagram The same scheme is used for the Line OVP, except that this time the controller must not stop instantaneously when the input voltage goes above VHV(OV1). In order to be time insensitive to spikes and voltage surges a blanking circuit is inserted after the output of the comparator, with a duration of tOV(blank). http://onsemi.com 13 NCP1238 Blanked voltage surge VHV VHV(OV1) VHV(OV2) time OVP detected HV timer starts HV timer restarts One Shot Restarts at VCC(on) t HV time DRV Figure 8. AC Input Line Overvoltage Timing Diagram http://onsemi.com 14 time NCP1238 Oscillator with Maximum Duty Ratio and Frequency Jittering In order to improve the EMI signature, the switching frequency jitters around its nominal value, with a triangle−wave shape. The NCP1238 includes an oscillator that sets the switching frequency with an accuracy of $7%. The maximum duty ratio of the DRV pin is 80% (typical), with an accuracy of $7%. fOSC fOSC + Ajitter Nominal fOSC fOSC - Ajitter 1 / Fjitter Time Figure 9. Frequency Jittering This driver has a typical current capability of ±1.0 A. Clamped Driver The supply voltage for the NCP1238 can be as high as 28 V, but most of the MOSFETs that will be connected to the DRV pin cannot tolerate a gate−to−source voltage greater than 20 V on their gate. The driver pin is therefore clamped safely below 16 V. VCC Clamp DRV signal DRV Figure 10. Clamped Driver http://onsemi.com 15 NCP1238 CURRENT−MODE CONTROL WITH OVERPOWER COMPENSATION AND SOFT−START Current Sensing one input of the PWM comparator through the LEB block. On the other input the FB voltage divided by KFB sets the threshold: when VCS reaches this threshold, the output driver is turned off. The maximum value for the peak current, VILIM, is set by a dedicated comparator. NCP1238 is a current−mode controller, which means that the FB voltage sets the peak current flowing in the inductance and the MOSFET. This is done through a PWM comparator: the current is sensed across a resistor and the resulting voltage is applied to the CS pin. VCS is applied to VDD KFB + − RFB(up) PWM Jitter Soft−start + − Oscillator Soft−start ramp Start tSSTART Reset FB DCMAX IC Start IC Stop S Q R blanking tLEB CS + − + VILIM IC stop UVLO blanking tBCS + − + Protection Mode VCS(stop) Fault Figure 11. Current Sense Block Schematic http://onsemi.com 16 HV stop Latch TSD DRV Stage NCP1238 Each time the controller is starting (i.e. the controller was off and starts, or restarts, when VCC reaches VCC(on)), a soft−start is applied: the current sense setpoint is linearly increased from 0 (the minimum level can be higher than 0 because of the LEB and propagation delay) until it reaches VILIM (after a duration of tSSTART), or until the FB loop imposes a setpoint lower than the one imposed by the soft−start (the 2 comparators outputs are OR’ed). VFB VFB(fault) Time Soft-start ramp VFB takes over soft-start VILIM tSSTART Time CS Setpoint VILIMI Figure 12. Soft−Start Under some conditions, like a winding short−circuit for instance, not all the energy stored during the on time is transferred to the output during the off time, even if the on time duration is at its minimum (imposed by the propagation delay of the detector added to the LEB duration). As a result, the current sense voltage keeps on increasing above VILIM, because the controller is blind during the LEB blanking Time time. Dangerously high current can grow in the system if nothing is done to stop the controller. In order to protect against this, an additional comparator is included, that senses when VCS reaches VCS(stop) ( = 1.5 x VILIM ). As soon as this comparator toggles, the controller immediately enters the protection mode (latched or autorecovery according to the chosen option). http://onsemi.com 17 NCP1238 Compensation for Overpower Detection Unfortunately, due to the inherent propagation delay of the logic, the actual peak current is higher at high input voltage than at low input voltage, as shown in Figure 13. This leads to a significant difference in the maximum output power delivered by the power supply. The power delivered by a flyback power supply is proportional to the square of the peak current: P OUT + 1 @ h @ L P @ F SW @ IP 2 2 (eq. 1) (in discontinuous conduction mode). IP IP to be compensated ILIMIT High Line Low Line time tdelay tdelay Figure 13. Line Compensation for True Overpower Protection Since in light load conditions this offset is in the same order of magnitude as the current sense signal, it must be removed. Therefore the compensation current is only added when the FB voltage is higher than VFB(OPC), as shown in Figure 15. To compensate this and have an accurate overpower protection, an offset proportional to the input voltage is added to the CS signal by turning on an internal current source (IOPC): by adding an external resistor (ROPC) in series between the sense resistor and the CS pin, a voltage offset is created across it by the current. The compensation can be adjusted by changing the value of the ROPC resistor. VDD V to I HV Sensing HV IOPC = 0.5m x (VHV − 125) + FB − + VFB(OPC) CS To CS block blanking t LEB ROPC Rsense Figure 14. Schematic Overpower Compensation Circuit http://onsemi.com 18 NCP1238 IOPC VHV VFB VFB(fold) VFB(OPC) Figure 15. Overpower Compensation Current Relation to Feedback Voltage and Input Voltage Feedback with Slope Compensation to VILIM is 3.5 V. There is a pullup resistor of 20 kW (typical) from FB pin to the internal reference VFB(ref). The ratio from the FB voltage to the current sense setpoint is typically 5. This means that the FB voltage corresponding VDD 20 kW FB K FB slope comp. + Oscillator PWM − blanking CS t LEB Figure 16. FB Circuitry In order to allow the NCP1238 to operate in CCM with a duty cycle above 50%, a fixed slope compensation is internally applied to the current−mode control. The slope appearing on the internal voltage setpoint for the PWM comparator is −32.5 mV/ms typical for the 65 kHz version (and respectively −50 mV/ms and −67 mV/ms for the 100 kHz and 133 kHz versions). http://onsemi.com 19 NCP1238 OVERCURRENT PROTECTION WITH FAULT TIMER Classical Overcurrent Protection controller is either latched off (latched protection, Version A), or it enters an autorecovery mode (Version B). The timer is reset when the CS setpoint goes back below VILIM before the timer elapses. The fault timer is also started if the driver signal is reset by the max duty ratio. When an overcurrent occurs on the output of the power supply, the feedback loop asks for more power than the controller can deliver, and the CS setpoint reaches VILIM. When this event occurs, an internal tfault timer is started: once the timer times out, DRV pulses are stopped and the FB + − /5 Fault Flag PWM R Q S timer tfault Protection Mode Autorecovery protection mode only blanking CS Reset DRV release timer tLEB + + − Brown−out Reset VILIM DC MAX DRV Figure 17. Timer−Based Overcurrent Protection In autorecovery mode, the controller tries to restart after tautorec. If the fault has gone, the supply resumes operation; if not, the system starts a new burst cycle (see Figure 18). http://onsemi.com 20 t autorec NCP1238 Fault disappears Output Load Overcurrent applied Max Load Fault Flag time Fault timer starts VCC time VCC(on) VCC(min) Restart At VCC(on) (new burst cycle if Fault DRV time still present) Controller stops Fault timer time tfault tfault tautorec Figure 18. Autorecovery Timer−Based Protection Mode http://onsemi.com 21 time NCP1238 In the latched version (Figure 19), the controller can restart only if a brown−out or a VCC reset occurs. In a real application this can only happen if the power supply is unplugged from the mains line. Output Load No restart when fault disappears Overcurrent applied Max Load Fault Flag time Fault timer starts VCC time VCC(on) VCC(min) DRV time Controller latches off Fault timer time tfault tfault Figure 19. Latched Timer−Based Overcurrent Protection http://onsemi.com 22 time NCP1238 LOW LOAD OPERATION Frequency Foldback whatever the nominal switching frequency option is. The current−mode control is still active while the oscillator frequency decreases, but the frequency jittering is off. Note that the frequency foldback is disabled if the controller runs at its maximum duty cycle. In order to improve the efficiency in light load conditions, the frequency of the internal oscillator is linearly reduced from its nominal value down to fOSC(min). This frequency foldback starts when the voltage on FB pin VFB goes below VFB(fold), and is complete before VFB reaches Vskip(in), fOSC Nominal fOSC Skip fOSC(min) FB Vskip(in) VFB(fold) Figure 20. Frequency Foldback when the FB Voltage Decreases Skip Cycle Mode with Soft−Skip + − + Soft−skip ramp Reset tSSKIP Sawtooth Oscillator DCMAX Vskip FB CS S Q R K FB blanking tLEB DRV stage − + + − Figure 21. Skip Cycle with Soft−Skip Schematic When VFB reaches Vskip(in) while decreasing, the skip mode is activated: the driver stops, and the internal consumption of the controller is decreased. While VFB is below Vskip(out), the controller remains in this state. When VFB crosses Vskip(out), the DRV pin starts to pulse again, and the controller restarts with a short Soft−Skip duration (tSSKIP). The soft−skip imposes the peak current from nearly 0, in a voltage−mode manner: it doesn’t have the same behavior as the startup soft−start which is current−mode driven. http://onsemi.com 23 NCP1238 VFB VFB(fold) Vskip(out) Vskip(in) Exits skip Time Enters skip Enters skip DRV Exits soft-skip Exits skip Time CS setpoint Soft-skip Softskip Figure 22. Skip Cycle with Soft−Skip Timing Diagram If during the Soft−Skip duration the FB voltage goes above VFB(fold), the Soft−Skip ends instantaneously, and the peak current follows the setpoint imposed by the Time current−mode control. This transient load detection feature avoids large output voltage drops if a load transient occurs while the controller is in soft−skip mode. http://onsemi.com 24 NCP1238 Latch−off Input VDD + − + INTC blanking tLatch(OVP) VOVP INTC Latch + 1 kW − + blanking tLatch(OTP) S Q R Latch VOTP Vclamp Brown−out Reset Soft−start end Figure 23. Latch Detection Schematic The Latch pin is dedicated to the latch−off function. It includes two thresholds that define a working window, between a high latch and a low latch. Within these 2 thresholds; the controller is allowed to run; but as soon as either the low or the high threshold is crossed, the controller is latched off. The lower threshold is intended to be used with an NTC thermistor, with the internal current source INTC providing the necessary bias current. An active clamp prevents the voltage from reaching the high threshold if it is only pulled up by the ILatch current. To reach the high threshold, the pullup current has to be higher than the pulldown capability of the clamp (typically 1.5 mA at VOVP). To avoid any false triggering, noise spikes shorter than tLatch(OVP) or tLatch(OTP) respectively are blanked, and only longer events can actually latch the controller. Reset occurs when a brown−out condition is detected or the VCC is cycled down to a VCC(reset), which in a real application can only happen if the power supply is unplugged from the AC line. Upon startup, the internal references take some time before reaching their nominal values; and one of the comparators could toggle inadvertantly. Therefore the internal logic ignores the latch signal before the controller is ready to start. Once VCC reaches VCC(on), the latch pin High latch state is enabled and the DRV switching starts only if it is allowed; whereas the Low latch (typically sensing an overtemperature) is taken into account only after the soft−start is finished. In addition, the NTC current is doubled during the soft−start period, to speed up the charging of the Latch pin capacitor. http://onsemi.com 25 NCP1238 VCC VCC(on) VCC(min) Start-up initiated by VCC(on) Internal Latch Signal Noise spike ignored (tLatch blanking) Latch signal high during pre-start phase time time DRV Latch-off Switching allowed (no latch event) Figure 24. Latch−off Function Timing Diagram Temperature shutdown time instantaneously, the HV current source is turned off, and the internal logic state is reset. When the temperature falls below the low threshold, the HV startup current source is enabled, and a regular startup sequence takes place. The die includes a temperature shutdown protection with a turn−off threshold guaranteed between 140°C and 160°C, and a typical hysteresis of 30°C. When the temperature rises above the high threshold, the controller stops switching http://onsemi.com 26 NCP1238 STATE DIAGRAMS HV Startup Current Source VCC > VCC(inhibit) Istart1 No TSD Istart2 VCC < VCC(inhibit) TSD TSD Stop TSD VCC < VCC(min) VCC > VCC(on) TSD Off Figure 25. HV Startup Current Source State Diagram http://onsemi.com 27 NCP1238 Controller Operation (Latched Version: A Option) High Latch VCC > VCC(on) Soft−start Soft-start ends S Brown-out S HV OVP S TSD S Brown-out S HV OVP S TSD Stopped S Brown-out S HV OVP S TSD S Brown-out S VCC reset S Brown-out S HV OVP S TSD Skip S High Latch S Low Latch S Brown-out S HV OVP S TSD Skip in Latch Skip out Running Soft−skip S High Latch S Low Latch S Soft-skip ends S VFB > V FB(fold) S Fault S High Latch S Low Latch With Fault = S tfault expires S VCS > VCS(stop) S VCC < VCC(off) Figure 26. Controller Operation State Diagram (Latched Protection) http://onsemi.com 28 NCP1238 Controller Operation (Autorecovery Version: B Option) High Latch VCC > VCC(on) Soft−start Soft-start ends S Brown-out S HV OVP S TSD S t autorec counting S Brown-out S HV OVP S TSD Stopped S Brown-out S HV OVP S TSD S Brown-out S VCC reset S Fault S Brown-out S HV OVP S TSD Skip S High Latch S Low Latch S Brown-out S HV OVP S TSD Latch Skip in Skip out Running Soft−skip S High Latch S Low Latch S Soft-skip ends S V FB > VFB(fold) S High Latch S Low Latch With Fault = S tfault expires S VCS > VCS(stop) S VCC < VCC(off) Figure 27. Controller Operation State Diagram (Autorecovery Protection) Table 1. ORDERING INFORMATION Overload Protection Switching Frequency Package Shipping† NCP1238AD65R2G Latched 65 kHz SOIC−7 (Pb−Free) 2500 / Tape & Reel NCP1238BD65R2G Autorecovery 65 kHz SOIC−7 (Pb−Free) 2500 / Tape & Reel Part No. †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. http://onsemi.com 29 NCP1238 PACKAGE DIMENSIONS SOIC−7 CASE 751U−01 ISSUE D −A− 8 NOTES: 1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982. 2. CONTROLLING DIMENSION: MILLIMETER. 3. DIMENSION A AND B ARE DATUMS AND T IS A DATUM SURFACE. 4. DIMENSION A AND B DO NOT INCLUDE MOLD PROTRUSION. 5. MAXIMUM MOLD PROTRUSION 0.15 (0.006) PER SIDE. 5 −B− S 0.25 (0.010) B M M 1 4 G C R X 45 _ J −T− SEATING PLANE H 0.25 (0.010) K M D 7 PL M T B S A DIM A B C D G H J K M N S MILLIMETERS MIN MAX 4.80 5.00 3.80 4.00 1.35 1.75 0.33 0.51 1.27 BSC 0.10 0.25 0.19 0.25 0.40 1.27 0_ 8_ 0.25 0.50 5.80 6.20 INCHES MIN MAX 0.189 0.197 0.150 0.157 0.053 0.069 0.013 0.020 0.050 BSC 0.004 0.010 0.007 0.010 0.016 0.050 0_ 8_ 0.010 0.020 0.228 0.244 S The product described herein (NCP1238) may be covered by one or more of the following U.S. patents: 5,073,850, 6,271,735, 6,362,067, 6,385,060, 6,597,221, 6,633,193, 6,587,351, 6,940,320. There may be other patents pending. Soft−Skip is a trademark of Semiconductor Components Industries, LLC (SCILLC). ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner. PUBLICATION ORDERING INFORMATION LITERATURE FULFILLMENT: Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada Email: [email protected] N. American Technical Support: 800−282−9855 Toll Free USA/Canada Europe, Middle East and Africa Technical Support: Phone: 421 33 790 2910 Japan Customer Focus Center Phone: 81−3−5773−3850 http://onsemi.com 30 ON Semiconductor Website: www.onsemi.com Order Literature: http://www.onsemi.com/orderlit For additional information, please contact your local Sales Representative NCP1238/D