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NCP1288 Fixed Frequency Current Mode Controller for Flyback Converters
The NCP1288 is a new generation of the NCP12xx fixed-frequency current-mode controllers featuring a high-voltage startup current, pin-to-pin compatible with the previous generation. Due to its proprietary Soft-SkipTM 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 (-40C to +125C), enabling easier design and increased safety (e.g. $5% for the peak current limit, $7% for the oscillator).
Features http://onsemi.com MARKING DIAGRAM
8 SOIC-7 CASE 751U 88Xff ALYWX G
1
88Xff = 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 30 of this data sheet.
* Timer-Based Overload Protections with Auto-
and Line Overvoltage Protections
* High-Voltage Current Source with Built-in Brown-out * Fixed-Frequency Current-Mode Operation with * Frequency Jittering for a Reduced EMI Signature * Adjustable Overpower Compensation * Latch-off Input for Severe Fault Conditions, with * * *
Built-in Ramp Compensation
Recovery (Option B) or Latched (Option A) Operation
* * * * * *
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* ac-dc Adapters for Notebooks, LCD, and Printers Offline Battery Chargers Consumer Electronic Power Supplies Auxiliary/Housekeeping Power Supplies
Typical Applications
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)
* * * *
*For additional information on our Pb-Free strategy and soldering details, please download the ON Semiconductor Soldering and Mounting Techniques Reference Manual, SOLDERRM/D.
(c) Semiconductor Components Industries, LLC, 2010
February, 2010 - Rev. 1
1
Publication Order Number: NCP1288/D
NCP1288
1
Latch
HV
8
2
FB
3
CS
VCC
6
4
GND
DRV
5
Figure 1. Pinout
TYPICAL APPLICATION EXAMPLE
VIN (dc)
VOUT
LATCH FB CS GND NCP1288
HV VCC DRV
Figure 2. Typical Application
PIN FUNCTION DESCRIPTION
Pin N5 1 2 3 4 5 6 8 Pin Name LATCH FB CS GND DRV VCC HV Function Latch-off Input Feedback Current Sense - Drive Output VCC Input High-Voltage Pin Pin Description 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 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 start-up current source and brown-out / line overvoltage detections
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NCP1288
SIMPLIFIED INTERNAL BLOCK SCHEMATIC
Brown-out blanking HV stop - + HV Dual HV start-up current source VCC FB Sawtooth Stop Foldback Oscillator IC Start IC Stop Clamp S + - Q R DRV GND UVLO ILIMIT DMAX + + - Protection Mode release Autorecovery protection mode only timer Fault Brown-out Reset HV stop Latch TSD
VDD INTC INTC
+
BO OVM HV sample HV dc
+ -
tLatch(OVP)
VOVP
Latch 1 kW Vclamp +
- +
VOTP
tLatch(OTP)
blanking
TSD S Q R Latch Reset UVLO VDD IC Start
TSD
Soft-start end Brown-out FB Reset
TSD HV current Reset VCC UVLO management VDD Start
Soft-skip ramp - + Vskip Reset
VDD
+
tSSKIP
RFB(up)
Skip
FB
KFB
slope comp. HV sample + - PWM Jitter Skip Soft-start + - Soft-start Start ramp Reset End Soft-start end blanking
V to I IOPC = 0.5m x (VHV-125)
VFB(OPC)
CS
tLEB
blanking
tBCS
S Q R
Fault Flag
PWM
+
- +
tSSTART
DCMAX DMAX
+
VILIM
ILIMIT
IC stop
VCS(stop) tfault
timer
tautorec
Figure 3. Simplified Internal Block Schematic
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NCP1288
MAXIMUM RATINGS
Rating Supply Pin (pin 6) (Note 2) Voltage range Current range High Voltage Pin (pin 8) (Note 2) Voltage range Current range Driver Pin (pin 5) (Note 2) Voltage range Current range 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) Temperature Range Operating Junction Temperature Storage Temperature Range ESD Capability (Note 1) Human Body Model (HBM) Model (All pins except HV) Machine Model (MM) Symbol VCCMAX ICCMAX VHVMAX IHVMAX VDRVMAX IDRVMAX VMAX IMAX RqJA Value -0.3 to 28 $30 -0.3 to 500 $20 -0.3 to 20 $1000 -0.3 to 10 $10 162 147 125 -40 to +150 -60 to +150 2000 190 Unit V mA V mA V mA V mA C/W
TSTRGMAX
TJMAX
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.
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NCP1288
VCC = 11 V unless otherwise noted)
ELECTRICAL CHARACTERISTICS (For typical values TJ = 25C, for min/max values TJ = -40C to +125C, VHV = 120 V,
Characteristics HIGH VOLTAGE CURRENT SOURCE Minimum voltage for current source operation Current flowing out of VCC pin @ VHV = 60 V Off-state leakage current SUPPLY Turn-on threshold level, VCC going up HV current source stop threshold UVLO and HV current source restart threshold Hysteresis between VCC(on) and VCC(min) Blanking duration on VCC(min) and VCC(off) detection VCC decreasing level at which the internal logic resets VCC level for ISTART1 to ISTART2 transition Internal current consumption (Note 6) DRV open, VFB = 3 V Cdrv = 1 nF, VFB = 3 V Off mode (skip or before startup) Fault mode (fault or latch) BROWN-OUT AND LINE OVERVOLTAGE Brown-out threshold voltage Timer duration for line cycle drop-out Overvoltage threshold Blanking duration on line overvoltage detection OSCILLATOR Oscillator frequency Maximum duty ratio Frequency jittering amplitude, in percentage of FOSC Frequency jittering modulation frequency OUTPUT DRIVER Rise time, 10% to 90% of VCC Fall time, 90% to 10% of VCC Current Capability VCC = VCC(min) + 0.2 V, CDRV = 1 nF VCC = VCC(min) + 0.2 V, CDRV = 1 nF VCC = VCC(min) + 0.2 V, CDRV = 1 nF DRV high, VDRV = 0 V DRV low, VDRV = VCC VCC = VCCmax - 0.2 V, DRV high VCC = VCC(min) + 0.2 V, RDRV = 33 kW, DRV high trise tfall - - 22 22 34 34 ns ns mA IDRV(source) IDRV(sink) VDRV(clamp) VDRV(drop) - - 11 - 800 800 13.5 - - - 16 1 V V Guaranteed by design Guaranteed by design fOSC DMAX Ajitter Fjitter 60 75 $4 85 65 80 $6 125 70 85 $8 165 kHz % % Hz VHV going up VHV going down VHV going up VHV going down VHV(start) VHV(stop) tHV VHV(OV1) VHV(OV2) tOV(blank) 104 97 43 400 395 - 112 105 61 430 425 250 120 113 79 460 455 - V ms V ms Guaranteed by design VCC(on) VCC(min) VCC(HYS) tUVLO(blank) VCC(reset) VCC(inhibit) ICC1 ICC2 ICC3 ICC4 15.0 9.5 5.0 7 4.0 0.4 2.0 2.3 0.9 0.4 16.0 10.5 - 10 5.2 0.65 2.5 3.3 1.2 0.7 17.0 11.5 - 13 6.5 0.9 3.0 4.3 1.5 1.0 V V V ms V V mA VCC = 0 V VCC = VCC(on) - 0.5 V VHV = 500 V VHV(min) Istart1 Istart2 Istart(off) - 0.2 4 - - 0.5 8 25 60 0.8 12 50 V mA mA Test Condition Symbol Min Typ Max Unit
Clamping Voltage (Maximum Gate Voltage) High-State Voltage Drop
6. Internal supply current only, current in FB pin not included (current flowing through GND pin only).
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NCP1288
VCC = 11 V unless otherwise noted)
ELECTRICAL CHARACTERISTICS (For typical values TJ = 25C, for min/max values TJ = -40C to +125C, VHV = 120 V,
Characteristics CURRENT SENSE Input Bias Current Maximum Internal Current Setpoint Threshold for Immediate Fault Protection Activation Propagation Delay from VILIM detection to DRV off Leading Edge Blanking Duration for VILIM Leading Edge Blanking Duration for VCS(stop) Slope of the Compensation Ramp Soft-Start Duration 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 KOPC IOPC(125) IOPC(162) IOPC(325) IOPC(max) VFB(OPC) VFB(OPCE) tWD TJ = 25C RFB(up) KFB VFB(ref) From CS reaching VILIMIT to DRV stop tfault tautorec - - - - 102 1.50 - 25 0.5 0 52 104 120 1.65 1.25 35 - - - - 138 1.80 - 45 mA / V mA From pulse to VCS = VILIM 1st VCS = VILIM VCS = 0.7 V VFB > 3.5 V Ibias VILIM VCS(stop) tdelay tLEB tBCS Scomp(65kHz) tSSTART - 0.665 0.95 - 190 90 - 2.8 0.02 0.7 1.05 80 250 120 -32.5 4.0 - 0.735 1.15 110 310 150 - 5.2 mA V V ns ns ns mV / ms ms Test Condition Symbol Min Typ Max Unit
FB voltage above which IOPC is applied FB voltage below which IOPC = 0 Refresh period for dc operation FEEDBACK Internal Pull-up Resistor VFB to Internal Current Setpoint Division ratio Internal Pull-up Voltage on the FB pin OVERCURRENT PROTECTION Fault Timer Duration Autorecovery Mode Latch-off Time Duration
V V ms
15 4.7 4.3
20 5.0 5.0
25 5.3 5.7
kW - V
64 1.0
78 1.4
98 1.8
ms s
6. Internal supply current only, current in FB pin not included (current flowing through GND pin only).
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NCP1288
VCC = 11 V unless otherwise noted)
ELECTRICAL CHARACTERISTICS (For typical values TJ = 25C, for min/max values TJ = -40C to +125C, VHV = 120 V,
Characteristics FREQUENCY FOLDBACK Feedback Voltage Threshold below which frequency foldback starts Minimum Switching Frequency Threshold below which the frequency foldback is finished and the controller switches at fOSC(min) SKIP CYCLE MODE Feedback Voltage Thresholds for Skip Mode Soft-Skip Duration LATCH-OFF INPUT High Threshold Low Threshold Current Source for Direct NTC Connection During normal operation During soft-start Blanking Duration on High Latch Detection Blanking Duration on Low Latch Detection Clamping Voltage TEMPERATURE SHUTDOWN Temperature Shutdown Temperature Shutdown Hysteresis TJ going up TJ going down TTSD TTSD(HYS) 135 20 150 30 165 40 C C ILatch = 0 mA ILatch = 1 mA VLatch going up VLatch going down VLatch = 0 V VOVP VOTP INTC 2.37 0.76 78 156 40 - 1.0 1.8 2.5 0.8 91 182 55 400 1.2 2.3 2.63 0.84 104 208 70 - 1.4 2.8 V V mA VFB going down VFB going up From 1st pulse to VCS = VFB(fold) / KFB Vskip(in) Vskip(out) tSSKIP 0.63 0.72 - 0.7 0.80 100 0.77 0.88 - V ms VFB = Vskip(in) + 0.2 V VFB(fold) fOSC(min) VFB(endfold) 1.3 21 - 1.4 27 1.0 1.5 31 - V kHz V Test Condition Symbol Min Typ Max Unit
INTC(SSTART) tLatch(OVP) tLatch(OTP) Vclamp0(Latch) Vclamp1(Latch)
ms ms V
6. Internal supply current only, current in FB pin not included (current flowing through GND pin only).
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NCP1288
APPLICATION INFORMATION
Introduction
The NCP1288 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 NCP1288 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 NCP1288 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. 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. * Adjustable Overpower Compensation: The high 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 NCP1288 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 NCP1288 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 NCP1288 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 NCP1288 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 NCP1288 runs at a fixed frequency (with jittering) in current-mode control, where the peak current (sensed on the CS pin) 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
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NCP1288
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. Overload: The NCP1288 features a timer-based 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 NCP1288 features on its HV pin a true AC line monitoring circuitry, which includes a 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).
*
*
*
*
*
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NCP1288
DETAILED DESCRIPTION
High-Voltage Current Source (Dynamic Self-Supply) with Built-in Brown-out Detection
The NCP1288 HV pin can be connected either to the rectified bulk voltage, or to the ac line through a rectifier.
Startup
HV
Istart
Control
TSD
IC Start
VCC
+
+ -
VCC(on)
R Q S
+
- +
tUVLO(blank)
blanking
UVLO
VCC(min)
+
- +
Reset
VCC(reset)
Figure 4. HV Startup Current Source Functional Schematic
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NCP1288
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. The current source is then turned off, saving additional power when the VCC is supplied externally. Once the controller has started, if VCC reaches VCC(min) the switching stops and the protection mode is activated: the controller must be supplied by an external voltage source.
VHV
VHV(start)
VHV(min) VCC VCC(on) VCC(min)
HV current source = Istart1
Waits next VCC(on) before starting
time
VCC(inhibit) DRV
HV current source = Istart2
time
Figure 5. Startup Timing Diagram
time
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NCP1288
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).
Brown-out and Line Overvoltage
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
VCC VCC(on)
Waits next VCC(on) before starting
time
VCC(min)
time DRV
Figure 6. Brown-out or Line Overvoltage Timing Diagram
time
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NCP1288
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 VHV(start) VHV(stop)
Brown-out
DRV
Starts at next VCC(ON)
time t HV
Figure 7. AC Input Brown-out Timing Diagram
time
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
insensitive to spikes and voltage surges a blanking circuit is inserted after the output of the comparator, with a duration of tOV(blank).
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NCP1288
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 DRV time
Figure 8. AC Input Line Overvoltage Timing Diagram
time
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NCP1288
Oscillator with Maximum Duty Ratio and Frequency Jittering
The NCP1288 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%.
In order to improve the EMI signature, the switching frequency jitters around its nominal value, with a triangle-wave shape.
fOSC
fOSC + Ajitter Nominal fOSC fOSC - Ajitter
1 / Fjitter
Figure 9. Frequency Jittering Clamped Driver
Time
The supply voltage for the NCP1288 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.
This driver has a typical current capability of 1.0 A.
VCC
Clamp
DRV signal
DRV
Figure 10. Clamped Driver
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NCP1288
CURRENT-MODE CONTROL WITH OVERPOWER COMPENSATION AND SOFT-START
Current Sensing
NCP1288 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
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.
Jitter + - FB Soft-start Oscillator IC Start DCMAX
Soft-start ramp Start tSSTART Reset
IC Stop blanking tLEB CS + + - VILIM IC stop
S Q R
DRV Stage
blanking tBCS + - +
UVLO Protection Mode HV stop Latch TSD
VCS(stop) Fault
Figure 11. Current Sense Block Schematic
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NCP1288
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 CS Setpoint
Time
VILIMI
Figure 12. Soft-Start
Time
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. 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).
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NCP1288
Compensation for Overpower Detection
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)
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.
(in discontinuous conduction mode).
IP IP to be compensated High Line
ILIMIT Low Line
time tdelay tdelay
Figure 13. Line Compensation for True Overpower Protection
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.
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.
VDD
V to I HV HV Sensing IOPC = 0.5m x (VHV - 125)
FB +
+ - VFB(OPC) blanking t LEB ROPC Rsense CS
To CS block
Figure 14. Schematic Overpower Compensation Circuit
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NCP1288 IOPC
VHV
VFB(fold)
Feedback with Slope Compensation
VFB VFB(OPC)
to VILIM is 3.5 V. There is a pullup resistor of 20 kW (typical) from FB pin to the internal reference VFB(ref).
Figure 15. Overpower Compensation Current Relation to Feedback Voltage and Input Voltage
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
+ - blanking CS t LEB
PWM
Figure 16. FB Circuitry
In order to allow the NCP1288 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).
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NCP1288
OVERCURRENT PROTECTION WITH FAULT TIMER
Classical Overcurrent Protection
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
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. Fault Flag
FB
/5
+ -
PWM
R Q S
timer
tfault
Protection Mode
release Autorecovery protection mode only timer
Reset DRV
blanking
CS
tLEB
+
+ -
Brown-out Reset DC MAX DRV
t autorec
VILIM
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).
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NCP1288 Output Load Overcurrent
applied Fault disappears
Max Load
Fault Flag
time
Fault timer starts
VCC VCC(on)
time
VCC(min)
Restart At VCC(on) (new burst cycle if Fault
DRV
time
still present)
Controller stops
Fault timer
time
tfault
Figure 18. Autorecovery Timer-Based Protection Mode
tfault
tautorec
time
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NCP1288
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. No restart when fault disappears
Output Load
Overcurrent applied
Max Load
Fault Flag
Fault timer starts
time
VCC VCC(on)
time
VCC(min)
DRV
time
Controller latches off
Fault timer
time
tfault
tfault
Figure 19. Latched Timer-Based Overcurrent Protection
time
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NCP1288
If VCC reaches VCC(min) before the timer has elapsed, the controller enters protection mode anyway (in both protection mode versions). Here is the example with the latched version:
Output Load
Overcurrent applied Max Load No restart when fault disappears
time Fault Flag
Fault timer starts
time VCC VCC(on)
VCC(min) time DRV
Controller latches off
time Fault Timer tfault
time tfault
Figure 20. Overcurrent Protection Mode Triggered by VCC
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NCP1288
LOW LOAD OPERATION
Frequency Foldback
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 goes below VFB(fold), and is complete before VFB reaches Vskip(in),
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.
fOSC Nominal fOSC
Skip
fOSC(min) FB Vskip(in) VFB(fold)
Figure 21. Frequency Foldback when the FB Voltage Decreases Skip Cycle Mode with Soft-Skip
- +
Soft-skip ramp
Reset
+
tSSKIP
Sawtooth
Oscillator
DCMAX
Vskip
FB CS
KFB
blanking
S Q R - +
DRV stage
tLEB
+ -
Figure 22. 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.
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NCP1288 VFB VFB(fold) Vskip(out) Vskip(in)
Exits skip Enters skip
Time
Enters skip Exits soft-skip Exits skip
DRV
Time CS setpoint
Soft-skip Softskip
Figure 23. Skip Cycle with Soft-Skip Timing Diagram
Time
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
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.
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NCP1288
Latch-off Input
VDD INTC INTC
+
+ -
blanking
tLatch(OVP)
VOVP
Latch
1 kW Vclamp
+
- +
blanking
tLatch(OTP)
S Q R
Latch
VOTP Brown-out Reset Soft-start end
Figure 24. 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.
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NCP1288 VCC VCC(on)
VCC(min)
Start-up initiated by
VCC(on)
Internal Latch Signal
Latch signal high during pre-start phase
Noise spike ignored (tLatch blanking)
time
time DRV
Switching allowed (no latch event) Latch-off
Figure 25. Latch-off Function Timing Diagram Temperature shutdown
time
The die includes a temperature shutdown protection with a turn-off threshold guaranteed between 140C and 160C, and a typical hysteresis of 30C. When the temperature rises above the high threshold, the controller stops switching
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.
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NCP1288
STATE DIAGRAMS
HV Startup Current Source
VCC > VCC(inhibit)
No TSD
Istart1
VCC < VCC(inhibit) TSD TSD Stop
Istart2
TSD VCC < VCC(min) VCC > VCC(on)
TSD Off
Figure 26. HV Startup Current Source State Diagram
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NCP1288
Controller Operation (Latched Version: A Option)
High Latch VCC > VCC(on)
Soft-start
Soft-start ends S Brown-out S HV OVP S TSD Stopped 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 out
Soft-skip
S Brown-out S HV OVP S TSD
S Brown-out S VCC reset
S Brown-out S HV OVP S TSD
Skip in
Running S Soft-skip ends S VFB > V FB(fold) S Fault S High Latch S Low Latch
S High Latch S Low Latch
With Fault =
S tfault expires S VCS > VCS(stop) S VCC < VCC(min)
Figure 27. Controller Operation State Diagram (Latched Protection)
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NCP1288
Controller Operation (Autorecovery Version: B Option)
High Latch VCC > VCC(on)
Soft-start
Soft-start ends
S t autorec counting S Brown-out S HV OVP S TSD Stopped S Brown-out S VCC reset S High Latch S Low Latch
S Brown-out S HV OVP S TSD S Brown-out S HV OVP S TSD Skip S Fault S Brown-out S HV OVP S TSD Skip in Skip out
Soft-skip
Latch
S Brown-out S HV OVP S TSD
Running S Soft-skip ends S V FB > VFB(fold) S High Latch S Low Latch S tfault expires S VCS > VCS(stop) S VCC < VCC(min)
S High Latch S Low Latch
With Fault =
Figure 28. Controller Operation State Diagram (Autorecovery Protection)
Table 1. ORDERING INFORMATION
Part No. NCP1288BD65R2G Overload Protection Autorecovery Switching Frequency 65 kHz Package SOIC-7 (Pb-Free) Shipping 2500 / Tape & 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.
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NCP1288
PACKAGE DIMENSIONS
SOIC-7 CASE 751U-01 ISSUE D
-A-
8 5 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. 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
-B- S
1 4
0.25 (0.010)
M
B
M
G C -T- H D 7 PL 0.25 (0.010)
M SEATING PLANE
R
X 45 _
J M TB
S
K
A
S
The product described herein (NCP1288) 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: orderlit@onsemi.com 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 ON Semiconductor Website: www.onsemi.com Order Literature: http://www.onsemi.com/orderlit For additional information, please contact your local Sales Representative
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NCP1288/D

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