NCP4308DDR2G

NCP4308 Synchronous Rectifier Controller The NCP4308 is a synchronous rectifier controller for switch mode power supplies. The controller enables high efficiency designs for flyback, quasi resonant flyback and LLC topologies. Externally adjustable minimum off−time and on−time blanking periods provides flexibility to drive various MOSFET package types and PCB layout. A reliable and noise less operation of the SR system is insured due to the Self Synchronization feature. The NCP4308 also utilizes Kelvin connection of the driver to the MOSFET to achieve high efficiency operation at full load. The precise turn−off threshold, extremely low turn−off delay time and high sink current capability of the driver allow the maximum synchronous rectification MOSFET conduction time. The high accuracy driver and 5 V gate clamp make it ideally suited for directly driving GaN devices. Features • Self−Contained Control of Synchronous Rectifier in CCM, DCM and • • • • • • • • • • • • • QR for Flyback or LLC Applications Precise True Secondary Zero Current Detection Rugged Current Sense Pin (up to 150 V) Adjustable Minimum ON−Time Adjustable Minimum OFF-Time with Ringing Detection Adjustable Maximum ON−Time for CCM Controlling of Primary QR Controller Improved Robust Self Synchronization Capability 8 A / 4 A Peak Current Sink / Source Drive Capability Operating Voltage Range up to VCC = 35 V GaN Transistor Driving Capability (options A and C) Low Startup Current Consumption Maximum Operation Frequency up to 1 MHz SOIC-8, DFN−8 (4x4) and WDFN8 (2x2) Packages These are Pb−Free Devices Typical Applications • • • • February, 2017 − Rev. 1 MARKING DIAGRAMS 8 8 1 SOIC−8 D SUFFIX CASE 751 NCP4308x ALYW G G 1 1 DFN8 MN SUFFIX CASE 488AF 4308x ALYWG G 1 WDFN8 MT SUFFIX CASE 511AT 4308x Ex A L Y W M G ExMG G = Specific Device Code x = A, B, C, D or Q = Specific Device Code x = A or D = Assembly Location = Wafer Lot = Year = Work Week = Date Code = Pb−Free Package (Note: Microdot may be in either location) Notebook Adapters High Power Density AC/DC Power Supplies (Cell Phone Chargers) LCD TVs All SMPS with High Efficiency Requirements © Semiconductor Components Industries, LLC, 2016 www.onsemi.com 1 ORDERING INFORMATION See detailed ordering and shipping information on page 26 of this data sheet. Publication Order Number: NCP4308/D MIN_TON MIN_TOFF NCP4308 RTN D1 MIN_TON MIN_TOFF OK1 Figure 1. Typical Application Example − LLC Converter +Vout + Vbulk TR1 R1 C1 + C2 C5 D3 + VCC FLYBACK M2 D4 C3 CONTROL GND C4 CIRCUITRY DRV FB M1 CS R2 R3 R4 D5 R5 OK1 Figure 2. Typical Application Example − DCM, CCM or QR Flyback Converter www.onsemi.com 2 NCP4308 +Vout + Vbulk TR1 R1 C1 R3 + C2 C10 D3 VCC C4 R4 + PRIMARY ZCD SIDE M2 D4 C3 GND FLYBACK C7 CONTROLLER DRV M1 R7 COMP CS R2 R6 R5 C5 R8 C6 Figure 3. Typical Application Example − Primary Side Flyback Converter + Vbulk R4 TR1 R5 C1 +Vout C2 + R3 D2 D4 C7 + VCC QR CONTROL CIRCUITRY ZCD C4 DRV FB CS M3 D3 GND D1 R1 C5 M1 R11 R9 R10 R2 R8 R6 OK1 C3 M2 NCP4308 D6 R12 R7 D5 TR2 C6 Figure 4. Typical Application Example − QR Converter − Capability to Force Primary into CCM Under Heavy Loads utilizing MAX−TON www.onsemi.com 3 NCP4308 PIN FUNCTION DESCRIPTION ver. A, B, C, D ver. Q Pin Name 1 1 VCC 2 2 MIN_TOFF Adjust the minimum off time period by connecting resistor to ground. 3 3 MIN_TON Adjust the minimum on time period by connecting resistor to ground. 4 4 NC Leave this pin opened or tie it to ground. 5 − NC Leave this pin opened or tie it to ground. 6 6 CS Current sense pin detects if the current flows through the SR MOSFET and/or its body diode. Basic turn−off detection threshold is 0 mV. A resistor in series with this pin can decrease the turn off threshold if needed. 7 7 GND Ground connection for the SR MOSFET driver, VCC decoupling capacitor and for minimum on and off time adjust resistors. GND pin should be wired directly to the SR MOSFET source terminal/soldering point using Kelvin connection. DFN8 exposed flag should be connected to GND 8 8 DRV Driver output for the SR MOSFET − 5 MAX_TON MIN_TON Description Supply voltage pin Adjust the maximum on time period by connecting resistor to ground. ELAPSED ADJ NC Minimum ON time generator EN VDD 100mA CS CS_ON CS detection DRIVER CS_OFF DRV Out DRV Control logic CS_RESET V DD RESET MIN_TOFF ADJ Minimum OFF time generator ELAPSED EN VCC managment UVLO NC VCC GND Figure 5. Internal Circuit Architecture − NCP4308A, B, C, D www.onsemi.com 4 NCP4308 ELAPSED MIN_TON ADJ NC Minimum ON time generator EN VDD 100mA CS CS_ON CS detection DRIVER DRV Out DRV CS_OFF Control logic CS_RESET VDD RESET MIN_TOFF ADJ Minimum OFF time generator ELAPSED EN VCC managment UVLO VCC ELAPSED MAX_TON ADJ Maximum ON time generator GND EN Figure 6. Internal Circuit Architecture − NCP4308Q (CCM QR) with MAX_TON www.onsemi.com 5 NCP4308 ABSOLUTE MAXIMUM RATINGS Rating Symbol Value Unit VCC −0.3 to 37.0 V VMIN_TON, VMIN_TOFF, VMAX_TON −0.3 to VCC V Driver Output Voltage VDRV −0.3 to 17.0 V Current Sense Input Voltage VCS −4 to 150 V VCS_DYN −10 to 150 V IMIN_TON, IMIN_TOFF, IMAX_TON −10 to 10 mA Junction to Air Thermal Resistance, 1 oz 1 in2 Copper Area, SOIC8 RqJ−A_SOIC8 160 °C/W Junction to Air Thermal Resistance, 1 oz 1 in2 Copper Area, DFN8 RqJ−A_DFN8 80 °C/W RqJ−A_WDFN8 160 °C/W Maximum Junction Temperature TJMAX 150 °C Storage Temperature TSTG −60 to 150 °C ESD Capability, Human Body Model, Except Pin 6, per JESD22−A114E ESDHBM 2000 V ESD Capability, Human Body Model, Pin 6, per JESD22−A114E ESDHBM 1000 V ESD Capability, Machine Model, per JESD22−A115−A ESDMM 200 V ESD Capability, Charged Device Model, Except Pin 6, per JESD22−C101F ESDCDM 750 V ESD Capability, Charged Device Model, Pin 6, per JESD22−C101F ESDCDM 250 V Supply Voltage MIN_TON, MIN_TOFF, MAX_TON Input Voltage Current Sense Dynamic Input Voltage (tPW = 200 ns) MIN_TON, MIN_TOFF, MAX_TON, Input Current Junction to Air Thermal Resistance, 1 oz 1 in2 Copper Area, WDFN8 Stresses exceeding those listed in the Maximum Ratings table may damage the device. If any of these limits are exceeded, device functionality should not be assumed, damage may occur and reliability may be affected. 1. This device meets latch−up tests defined by JEDEC Standard JESD78D Class I. RECOMMENDED OPERATING CONDITIONS Parameter Symbol Maximum Operating Input Voltage Min Max 35 V −40 125 °C VCC Operating Junction Temperature TJ Unit Functional operation above the stresses listed in the Recommended Operating Ranges is not implied. Extended exposure to stresses beyond the Recommended Operating Ranges limits may affect device reliability. www.onsemi.com 6 NCP4308 ELECTRICAL CHARACTERISTICS −40°C ≤ TJ ≤ 125°C; VCC = 12 V; CDRV = 0 nF; RMIN_TON = RMIN_TOFF = 10 kW; VCS = −1 to +4 V; fCS = 100 kHz, DCCS = 50%, unless otherwise noted. Typical values are at TJ = +25°C Parameter Test Conditions Symbol Min Typ Max Unit VCC rising, VCS = 0 V VCCON 8.3 8.8 9.4 V VCC falling, VCS = 0 V VCCOFF 7.3 7.8 8.3 SUPPLY SECTION VCC UVLO (ver. B & C) VCC UVLO Hysteresis (ver. B & C) VCC UVLO (ver. A, D & Q) VCCHYS VCC rising, VCS = 0 V VCC falling, VCS = 0 V VCC UVLO Hysteresis (ver. A, D & Q) 4.45 4.80 VCCOFF 3.70 3.95 4.20 V 75 125 ms 3.0 4.0 5.6 mA B, D, Q 3.5 4.5 6.0 A, C 4.5 6.0 7.5 B, D, Q 7.7 9.0 10.7 A, C 20 25 30 B, D, Q 40 50 60 CDRV = 0 nF, fCS = 500 kHz Current Consumption below UVLO 4.20 tSTART_DEL Current Consumption, RMIN_TON = RMIN_TOFF = 0 kW Current Consumption VCCON 0.5 VCC rising from 0 to VCCON + 1 V @ tr = 10 ms, VCS = 0 V A, C CDRV = 10 nF, fCS = 500 kHz V VCCHYS Start−up Delay CDRV = 1 nF, fCS = 500 kHz 1.0 ICC V No switching, VCS = 0 V, RMIN_TON = RMIN_TOFF = 0 kW ICC 1.5 2.0 2.5 mA No switching, VCS = 0 V, RMIN_TON = RMIN_TOFF = 0 kW, DFN8, WDFN8 ICC 1.0 2.0 2.5 mA No switching, VCC = VCCOFF – 0.1 V, VCS = 0 V ICC_UVLO 75 125 mA DRIVER OUTPUT Output Voltage Rise−Time CDRV = 10 nF, 10% to 90% VDRVMAX tr 40 55 ns Output Voltage Fall−Time CDRV = 10 nF, 90% to 10% VDRVMAX tf 20 35 ns RDRV_SOURCE 1.2 W Driver Source Resistance Driver Sink Resistance Output Peak Source Current Output Peak Sink Current Maximum Driver Output Voltage VCC = 35 V, CDRV > 1 nF (ver. B, D and Q) RDRV_SINK 0.5 W IDRV_SOURCE 4 A IDRV_SINK 8 A VDRVMAX 9.0 9.5 10.5 4.3 4.7 5.5 7.2 7.8 8.5 VCC = VCCOFF + 200 mV (ver. C) 4.2 4.7 5.3 VCC = VCCOFF + 200 mV (ver. A, D and Q) 3.6 4.0 4.4 VCC = 35 V, CDRV > 1 nF (ver. A, C) Minimum Driver Output Voltage VCC = VCCOFF + 200 mV (ver. B) VDRVMIN V V CS INPUT Total Propagation Delay From CS to DRV Output On VCS goes down from 4 to −1 V, tf_CS = 5 ns tPD_ON 35 60 ns Total Propagation Delay From CS to DRV Output Off VCS goes up from −1 to 4 V, tr_CS = 5 ns tPD_OFF 12 23 ns CS Bias Current VCS = −20 mV Turn On CS Threshold Voltage Turn Off CS Threshold Voltage Guaranteed by Design Turn Off Timer Reset Threshold Voltage CS Leakage Current VCS = 150 V ICS −105 −100 −95 mA −75 −40 mV 0 mV 0.6 V 0.4 mA VTH_CS_ON −120 VTH_CS_OFF −1 VTH_CS_RESET 0.4 ICS_LEAKAGE www.onsemi.com 7 0.5 NCP4308 ELECTRICAL CHARACTERISTICS −40°C ≤ TJ ≤ 125°C; VCC = 12 V; CDRV = 0 nF; RMIN_TON = RMIN_TOFF = 10 kW; VCS = −1 to +4 V; fCS = 100 kHz, DCCS = 50%, unless otherwise noted. Typical values are at TJ = +25°C Parameter Test Conditions Symbol Min Typ Max Unit RMIN_TON = 0 W tON_MIN 35 55 75 ns RMIN_TON = 0 W, DFN8, WDFN8 tON_MIN 25 50 75 ns RMIN_TOFF = 0 W tOFF_MIN 190 245 290 ns RMIN_TOFF = 0 W, DFN8, WDFN8 tOFF_MIN 160 245 290 ns Minimum tON time RMIN_TON = 10 kW tON_MIN 0.92 1.00 1.08 ms Minimum tOFF time RMIN_TOFF = 10 kW tOFF_MIN 0.92 1.00 1.08 ms Minimum tON time RMIN_TON = 50 kW tON_MIN 4.62 5.00 5.38 ms Minimum tOFF time RMIN_TOFF = 50 kW tOFF_MIN 4.62 5.00 5.38 ms Maximum tON Time VMAX_TON = 3 V tON_MAX 4.3 4.8 5.3 ms Maximum tON Time VMAX_TON = 0.3 V tON_MAX 41 48 55 ms Maximum tON Output Current VMAX_TON = 0.3 V IMAX_TON −105 −100 −95 mA MINIMUM tON and tOFF ADJUST Minimum tON time Minimum tOFF time MAXIMUM tON ADJUST Product parametric performance is indicated in the Electrical Characteristics for the listed test conditions, unless otherwise noted. Product performance may not be indicated by the Electrical Characteristics if operated under different conditions. www.onsemi.com 8 NCP4308 TYPICAL CHARACTERISTICS 4.7 9.3 4.6 9.1 VCCON 4.4 8.7 4.3 8.5 4.2 4.1 VCCOFF 4.0 VCCON 8.9 VCC (V) VCC (V) 4.5 8.3 8.1 VCCOFF 7.9 3.9 7.7 3.8 7.5 7.3 −40 −20 3.7 −40 −20 0 20 40 60 TJ (°C) 80 100 120 Figure 7. VCCON and VCCOFF Levels, VCS = 0 V, ver. A, D, Q TJ = 55°C 80 100 120 TJ = 125°C 100 ICC_UVLO (mA) 4 TJ = 0°C 3 TJ = −20°C TJ = −40°C 2 1 80 60 40 20 0 0 5 10 15 20 25 30 0 −40 35 −20 0 20 40 60 80 100 VCC (V) TJ (°C) Figure 9. Current Consumption, CDRV = 0 nF, fCS = 500 kHz, ver. D Figure 10. Current Consumption, VCC = VCCOFF − 0.1 V, VCS = 0 V, ver. D 30 120 60 CDRV = 10 nF CDRV = 10 nF 25 50 20 40 ICC (mA) ICC (mA) 40 60 TJ (°C) 120 TJ = 85°C 5 ICC (mA) 20 Figure 8. VCCON and VCCOFF Levels, VCS = 0 V, ver. B, C 6 TJ = 25°C 0 15 10 30 20 CDRV = 1 nF 5 CDRV = 1 nF 10 CDRV = 0 nF CDRV = 0 nF 0 −40 −20 0 20 40 60 80 100 0 −40 −20 120 0 20 40 60 80 100 120 TJ (°C) TJ (°C) Figure 11. Current Consumption, VCC = 12 V, VCS = −1 to 4 V, fCS = 500 kHz, ver. A Figure 12. Current Consumption, VCC = 12 V, VCS = −1 to 4 V, fCS = 500 kHz, ver. D www.onsemi.com 9 NCP4308 TYPICAL CHARACTERISTICS 0 −90 −92 −0.2 −94 −0.4 −98 ICS (mA) ICS (mA) −96 −100 −102 −104 TJ = 125°C TJ = 85°C TJ = 55°C TJ = 25°C TJ = 0°C TJ = −20°C TJ = −40°C −0.6 −0.8 −1.0 −106 −108 −110 −40 −1.2 −20 0 20 40 60 80 100 −1.4 −1.0 −0.8 −0.6 −0.4 −0.2 120 0.4 0.6 VCS (V) Figure 13. CS Current, VCS = −20 mV Figure 14. CS Current, VCC = 12 V 2.5 −50 VTH_CS_ON (mV) −30 2.0 ICC (mA) 0.2 TJ (°C) 3.0 TJ = 125°C TJ = 85°C TJ = 55°C TJ = 25°C TJ = 0°C TJ = −20°C TJ = −40°C 1.5 1.0 0.5 0 −4 0 −3 −2 0.8 1.0 −70 −90 −110 −130 −1 0 1 2 3 −150 −40 −20 4 0 20 40 60 80 100 VCS (V) TJ (°C) Figure 15. Supply Current vs. CS Voltage, VCC = 12 V Figure 16. CS Turn−on Threshold 1.0 120 0.60 VTH_CS_RESET (V) VTH_CS_OFF (mV) 0.5 0 −0.5 −1.0 0.55 0.50 0.45 −1.5 −2.0 −40 −20 0 20 40 60 80 100 0.40 −40 120 −20 0 20 40 60 80 TJ (°C) TJ (°C) Figure 17. CS Turn−off Threshold Figure 18. CS Reset Threshold www.onsemi.com 10 100 120 NCP4308 0.80 200 0.75 180 0.70 160 0.65 140 ICS_LEAKAGE (nA) VTH_CS_RESET (V) TYPICAL CHARACTERISTICS 0.60 0.55 0.50 0.45 100 80 60 0.40 40 0.35 0.30 20 0 −40 0 5 10 15 20 25 30 35 −20 0 20 40 60 80 100 VCC (V) TJ (°C) Figure 19. CS Reset Threshold Figure 20. CS Leakage, VCS = 150 V 60 24 55 22 120 20 50 18 tPD_OFF (ns) tPD_ON (ns) 120 45 40 35 16 14 12 10 30 8 25 6 −20 0 20 40 60 80 100 4 −40 120 −20 0 20 40 60 80 100 120 TJ (°C) TJ (°C) Figure 21. Propagation Delay from CS to DRV Output On Figure 22. Propagation Delay from CS to DRV Output Off 75 1.08 70 1.06 65 1.04 tMIN_TON (ms) tMIN_TON (ns) 20 −40 60 55 50 1.02 1.00 0.98 45 0.96 40 0.94 35 −40 −20 0 20 40 60 80 100 0.92 −40 120 −20 0 20 40 60 80 100 120 TJ (°C) TJ (°C) Figure 23. Minimum On−time RMIN_TON = 0 W Figure 24. Minimum On−time RMIN_TON = 10 kW www.onsemi.com 11 NCP4308 TYPICAL CHARACTERISTICS 5.4 290 5.3 280 270 tMIN_TOFF (ns) tMIN_TON (ms) 5.2 5.1 5.0 4.9 240 230 210 4.7 4.6 −40 −20 0 20 40 60 80 100 200 190 −40 120 −20 0 20 40 60 80 100 120 TJ (°C) TJ (°C) Figure 25. Minimum On−time RMIN_TON = 50 kW Figure 26. Minimum Off−time RMIN_TOFF = 0 W 1.08 5.4 1.06 5.3 1.04 5.2 tMIN_TOFF (ms) tMIN_TOFF (ms) 250 220 4.8 1.02 1.00 0.98 5.1 5.0 4.9 0.96 4.8 0.94 4.7 0.92 −40 −20 0 20 40 60 80 100 4.6 −40 120 −20 0 20 40 60 80 100 TJ (°C) TJ (°C) Figure 27. Minimum Off−time RMIN_TOFF = 10 kW Figure 28. Minimum Off−time RMIN_TOFF = 50 kW 1.04 1.08 1.03 1.06 1.02 1.04 tMIN_TOFF (ms) tMIN_TON (ms) 260 1.01 1.00 0.98 120 1.02 1.00 0.98 0.96 0.96 0.94 0.94 092 0.92 0 5 10 15 20 25 30 0 35 5 10 15 20 25 30 VCC (V) VCC (V) Figure 29. Minimum On−time RMIN_TON = 10 kW Figure 30. Minimum Off−time RMIN_TOFF = 10 kW www.onsemi.com 12 35 NCP4308 TYPICAL CHARACTERISTICS 5.5 10.4 VCC = 12 V, CDRV = 0 nF VCC = 12 V, CDRV = 1 nF VCC = 12 V, CDRV = 10 nF VCC = 35 V, CDRV = 0 nF VCC = 35 V, CDRV = 1 nF VCC = 35 V, CDRV = 10 nF VDRV (V) 10.0 9.8 5.1 VDRV (V) 10.2 VCC = 12 V, CDRV = 0 nF VCC = 12 V, CDRV = 1 nF VCC = 12 V, CDRV = 10 nF VCC = 35 V, CDRV = 0 nF VCC = 35 V, CDRV = 1 nF VCC = 35 V, CDRV = 10 nF 5.3 9.6 4.9 4.7 9.4 4.5 9.2 9.0 −40 −20 0 20 40 60 80 4.3 −40 −20 120 100 20 40 60 80 100 120 TJ (°C) TJ (°C) Figure 31. Driver and Output Voltage, ver. B, D and Q Figure 32. Driver Output Voltage, ver. A and C 50 5.3 TJ = 125°C TJ = 85°C TJ = 55°C TJ = 25°C 45 40 TJ = 0°C TJ = −20°C TJ = −40°C 5.2 5.1 tMAX_TON (ms) 35 30 25 20 5.0 4.9 4.8 4.7 15 4.6 10 4.5 5 0 4.4 0 0.5 1.0 1.5 2.0 2.5 4.3 −40 3.0 −20 0 20 40 60 80 100 120 VMAX_TON (V) TJ (°C) Figure 33. Maximum On−time, ver. Q Figure 34. Maximum On−time, VMAX_TON = 3 V, ver. Q 55 53 51 tMAX_TON (ms) tMAX_TON (ms) 0 49 47 45 43 41 −40 −20 0 20 40 60 80 100 TJ (°C) Figure 35. Maximum On−time, VMAX_TON = 0.3 V, ver. Q www.onsemi.com 13 120 NCP4308 APPLICATION INFORMATION General description resistors connected to GND. If needed, blanking periods can be modulated using additional components. An extremely fast turn−off comparator, implemented on the current sense pin, allows for NCP4308 implementation in CCM applications without any additional components or external triggering. An output driver features capability to keep SR transistor closed even when there is no supply voltage for NCP4308. SR transistor drain voltage goes up and down during SMPS operation and this is transferred through drain gate capacitance to gate and may turn on transistor. NCP4308 uses this pulsing voltage at SR transistor gate (DRV pin) and uses it internally to provide enough supply to activate internal driver sink transistor. DRV voltage is pulled low (not to zero) thanks to this feature and eliminate the risk of turned on SR transistor before enough VCC is applied to NCP4308. Some IC versions include a MAX_TON circuit that helps a quasi resonant (QR) controller to work in CCM mode when a heavy load is present like in the example of a printer’s motor starting up. The NCP4308 is designed to operate either as a standalone IC or as a companion IC to a primary side controller to help achieve efficient synchronous rectification in switch mode power supplies. This controller features a high current gate driver along with high−speed logic circuitry to provide appropriately timed drive signals to a synchronous rectification MOSFET. With its novel architecture, the NCP4308 has enough versatility to keep the synchronous rectification system efficient under any operating mode. The NCP4308 works from an available voltage with range from 4 V (A, D & Q options) or 8 V (B & C options) to 35 V (typical). The wide VCC range allows direct connection to the SMPS output voltage of most adapters such as notebooks, cell phone chargers and LCD TV adapters. Precise turn-off threshold of the current sense comparator together with an accurate offset current source allows the user to adjust for any required turn-off current threshold of the SR MOSFET switch using a single resistor. Compared to other SR controllers that provide turn-off thresholds in the range of −10 mV to −5 mV, the NCP4308 offers a turn-off threshold of 0 mV. When using a low RDS(on) SR (1 mW) MOSFET our competition, with a −10 mV turn off, will turn off with 10 A still flowing through the SR FET, while our 0 mV turn off turns off the FET at 0 A; significantly reducing the turn-off current threshold and improving efficiency. Many of the competitor parts maintain a drain source voltage across the MOSFET causing the SR MOSFET to operate in the linear region to reduce turn−off time. Thanks to the 8 A sink current of the NCP4308 significantly reduces turn off time allowing for a minimal drain source voltage to be utilized and efficiency maximized. To overcome false triggering issues after turn-on and turn−off events, the NCP4308 provides adjustable minimum on-time and off-time blanking periods. Blanking times can be adjusted independently of IC VCC using external Current Sense Input Figure 36 shows the internal connection of the CS circuitry on the current sense input. When the voltage on the secondary winding of the SMPS reverses, the body diode of M1 starts to conduct current and the voltage of M1’s drain drops approximately to −1 V. The CS pin sources current of 100 mA that creates a voltage drop on the RSHIFT_CS resistor (resistor is optional, we recommend shorting this resistor). Once the voltage on the CS pin is lower than VTH_CS_ON threshold, M1 is turned−on. Because of parasitic impedances, significant ringing can occur in the application. To overcome false sudden turn−off due to mentioned ringing, the minimum conduction time of the SR MOSFET is activated. Minimum conduction time can be adjusted using the RMIN_TON resistor. www.onsemi.com 14 NCP4308 Figure 36. Current Sensing Circuitry Functionality Figure 37). Therefore the turn−off current depends on MOSFET RDSON. The −0.5 mV threshold provides an optimum switching period usage while keeping enough time margin for the gate turn-off. The RSHIFT_CS resistor provides the designer with the possibility to modify (increase) the actual turn−on and turn−off secondary current thresholds. To ensure proper switching, the min_tOFF timer is reset, when the VDS of the MOSFET rings and falls down past the VTH_CS_RESET. The minimum off−time needs to expire before another drive pulse can be initiated. Minimum off−time timer is started again when VDS rises above VTH_CS_RESET. The SR MOSFET is turned-off as soon as the voltage on the CS pin is higher than VTH_CS_OFF (typically −0.5 mV minus any voltage dropped on the optional RSHIFT_CS). For the same ringing reason, a minimum off-time timer is asserted once the VCS goes above VTH_CS_RESET. The minimum off-time can be externally adjusted using RMIN_TOFF resistor. The minimum off−time generator can be re−triggered by MIN_TOFF reset comparator if some spurious ringing occurs on the CS input after SR MOSFET turn−off event. This feature significantly simplifies SR system implementation in flyback converters. In an LLC converter the SR MOSFET M1 channel conducts while secondary side current is decreasing (refer to www.onsemi.com 15 NCP4308 VDS = VCS ISEC V TH_CS _RESET – (RSHIFT _CS*ICS ) VTH_CS_OFF – (RSHIFT _CS*ICS ) VTH_CS _ON – (RSHIFT _CS*ICS ) VDRV Turn−on delay Turn −off delay Min ON−time tMIN_TON Min t OFF timer was stopped here because of VCS