NCP4330DR2G

NCP4330 Post Regulation Driver The NCP4330 houses a dual MOSFET driver intended to be used as a companion chip in AC−DC or DC−DC multi−output post regulated power supplies. Being directly fed by the secondary AC signal, the device keeps power dissipation to the lowest while reducing the surrounding part count. Furthermore, the implementation of a N−channel MOSFET gives NCP4330−based applications a significant advantage in terms of efficiency. http://onsemi.com MARKING DIAGRAM Features • • • • • • • • • Undervoltage Lockout Thermal Shutdown for Overtemperature Protection PWM Operation Synchronized to the Converter Frequency High Gate Drive Capability Bootstrap for N−MOSFET High−Side Drive Over−Lap Management for Soft Switching High Efficiency Post−Regulation Ideal for Frequencies up to 400 kHz This is a Pb−Free Device 8 SO−8 D SUFFIX CASE 751 8 1 1 4330D ALYW G 4330D = Device Number A = Assembly Location L = Wafer Lot Y = Year W = Work Week G = Pb−Free Package Typical Applications • ATX 3V3 Post−Regulation • Offline SMPS with MAGAMP Post−Regulation • Multi−Outputs DC−DC Converters VDD Band−Gap I_ramp UVD Iramp U4 VDD Current Mirror RST 3 6 VDD 5 I_ramp (Top View) ORDERING INFORMATION U3 OR Hysteresis Comparator 7 LS_DRV Buffer + - Iramp BST 2 HS_DRV Level Shifter 8 GND C_ramp 4 AR2 HS_DRV and LS_DRV low RESET Block GND BST U1 INVERTER 2.5 V/1.5 V C_ramp HS_DRV 1 Vref, UVDth VDD VDD Undervoltage Detection (UVD high if VDD < 4.9 V) RST PIN CONNECTIONS VDD AR3 Buffer Device Package Shipping† NCP4330DR2G SO−8 (Pb−Free) 2500 / Tape & Reel †For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specification Brochure, BRD8011/D. LS_DRV Figure 1. Block Diagram © Semiconductor Components Industries, LLC, 2011 April, 2011 − Rev. 1 1 Publication Order Number: NCP4330/D NCP4330 Converter Winding Voltage 0V Time Synchronization Signal 2.55 V Time Internal RESET Signal C_ramp Voltage VrefH VrefL Time 0V Time High−Side Driver (referenced to HS MOSFET source) Time Low−Side Driver 100 ns delay 100 ns delay Time Figure 2. Timing Diagram(s) DETAILED PIN DESCRIPTION(S) Pin Number Name 1 HS_DRV 2 BST “BST” is the bootstrap pin. A 0.1 mF to 1.0 mF ceramic capacitor should be connected between this pin and the node that is common to the coil and the two MOSFET. The “BST” voltage feeds the high−side driver (“HS_DRV”). 3 RST The “RST” pin resets the C_ramp voltage in order to synchronize the post−regulator free−wheeling sequence to the forward converter demagnetization phase. 4 C_ramp The capacitor connected to the C_ramp pin enables to adjust the delay in turning on the high−side MOSFET (in conjunction with “I_ramp” current). 5 I_ramp The “I_ramp” pin receives a current supplied by a regulation means. This current adjusts the delay after which the high−side MOSFET is turned on. By this way, it modules the high−side MOSFET on time in order to regulate the output voltage. 6 VDD “VDD” is the power supply input. A 0.1 mF to 1.0 mF ceramic capacitor should be connected from this pin to ground for decoupling. 7 LS_DRV 8 GND Function “HS_DRV” is the gate driver of the high−side MOSFET. “LS_DRV” is the driver output of the low−side MOSFET gate. Ground. http://onsemi.com 2 NCP4330 MAXIMUM RATINGS Symbol Value Unit BST Bootstrap Input Rating −0.3, +40 V RST Reset Input −0.3, +5.0 V −0.3, VrampHL V C_ramp Timing Capacitor Node (Note 1) I_ramp Regulation Current Input (Note 1) −0.3, Vcl V VDD Supply Voltage −0.3, +20 V RqJA Thermal Resistance 180 °C/W −40, +125 °C 150 °C −65 to +150 °C 300 °C TJ Operating Junction Temperature Range (Note 2) TJmax Maximum Junction Temperature TSmax Storage Temperature Range TLmax Lead Temperature (Soldering, 10 s) 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. VrampHL and Vcl are the internal clamp levels of pins 4 and 5 respectively. 2. The maximum junction temperature should not be exceeded. ELECTRICAL CHARACTERISTICS (VDD = 10 V, VBST = 25 V, TJ from −25°C to +125°C, unless otherwise specified.) Symbol Characteristic Min Typ Max Unit 22.5 23.5 − V High−Side Output Stage VHS_H High−Side Output Voltage in High State @ Isource = −100 mA VHS_L High−Side Output Voltage in Low State @ Isink = 100 mA − 0.9 1.5 V Isource_HS Current Capability of the High−Side Drive Output in High State − 0.5 − A Isink_HS Current Capability of the High−Side Drive Output in Low State − 0.75 − A tr−HS High−Side Output Voltage Rise Time from 0.5 V to 12 V (CL = 1.0 nF) − 25 − ns tf−HS High−Side Output Voltage Fall Time from 20 V to 0.5 V (CL = 1.0 nF) − 25 − ns Delay from Low−Side Gate Drive Low (High) to High−Side Drive High (Low) − 100 − ns 7.4 8.2 − V TLS−HS Low−Side Output Stage VLS_H Low−Side Output Voltage in High State @ Isource = −500 mA VLS_L Low−Side Output Voltage in Low State @ Isink = 750 mA − 1.3 1.7 V Isource_LS Current Capability of the Low−Side Drive Output in High State − 0.5 − A Isink_LS Current Capability of the Low−Side Drive Output in Low State − 0.75 − A tr−LS Low−Side Output Voltage Rise Time from 0.5 V to 7.0 V (CL = 2.0 nF) − 25 − ns tf−LS Low−Side Output Voltage Fall Time from 9.5 V to 0.5 V (CL = 2.0 nF) − 25 − ns 90 1400 102 1590 110 1800 Pin5 Clamp Voltage @ Ipin5 = 1.5 mA 0.7 1.4 2.1 V VrefL Ramp Control Reference Voltage, Vpin4 Falling 1.3 1. 5 1.7 V VrefH Ramp Control Reference Voltage, Vpin4 Rising 2.25 2.5 2.75 V VrampHL Ramp Voltage Maximum Value @ Ipin5 = 1.5 mA 3.2 3.6 4.2 V VrampLL Ramp Voltage Low Voltage @ Ipin5 = 1.5 mA − − 100 mV Ramp Control Icharge Vcl C_ramp Current @ Ipin5 = 100 mA @ Ipin5 = 1.5 mA http://onsemi.com 3 mA NCP4330 ELECTRICAL CHARACTERISTICS (continued) (VDD = 10 V, VBST = 25 V, TJ from −25°C to +125°C, unless otherwise specified.) Symbol Characteristic Min Typ Max Unit UVDH Undervoltage Lockout Threshold (VDD Rising) 5.2 5.8 6.4 V UVDL Undervoltage Lockout Threshold (VDD Falling) 4.9 5.2 5.5 V HUVD Undervoltage Lockout Hysteresis 400 600 − mV IDD1 IDD2 IDD3 Consumption: @ Vpin4 = 3.0 V and Ipin5 = 500 mA @ Vpin4 = 0 V and Ipin5 = 500 mA @ Vpin4 Oscillating 0 to 3.0 V at 200 kHz, Ipin5 = 500 mA − − − 13 7.0 10 20 12 15 Reset Block Threshold 2.2 2.5 2.8 V Reset Comparator Hysteresis 0.8 1.0 − V − 250 500 ns VDD Management mA Reset Block Vrst_th Hrst Treset Reset Pulse Duration Ireset C_ramp Pin Average Current, a 200 kHz, 50% duty cycle Pulse Generator being applied to reset pin and 1.0 V to pin 4 (C_ramp) and @ Iramp = 0 0.3 0.7 − mA Negative Clamp Level @ Ipin3 = −2.0 mA −0.5 −0.3 0 V − 8.0 15 mA Vcl−neg BST Leakage BSTleakage BST Leakage Current (BST = 10 V, VDD = 1.5 V, other pins grounded, TJ = 25°C) Temperature Protection Tlimit Thermal Shutdown Threshold − 150 − °C Htemp Thermal Shutdown Hysteresis − 50 − °C http://onsemi.com 4 18 9.0 16 8.2 14 7.4 IDD2 (mA) IDD1 (mA) NCP4330 12 5.8 10 8 −25 6.6 5.0 −25 125 25 Figure 3. IDD1 Consumption vs. Temperature 6.0 13 5.9 UVD_H (V) 12 IDD3 (mA) 125 Figure 4. IDD2 Consumption vs. Temperature 14 11 10 5.8 5.7 5.6 9 8 −25 25 TEMPERATURE (°C) TEMPERATURE (°C) 25 5.5 −25 125 TEMPERATURE (°C) 25 125 TEMPERATURE (°C) Figure 6. Undervoltage Lockout Upper Threshold vs. Temperature Figure 5. IDD3 Consumption vs. Temperature 5.22 700 660 H_UVD (V) UVD_L (V) 5.20 5.18 620 580 540 5.16 500 5.14 −25 460 −25 125 25 TEMPERATURE (°C) 25 TEMPERATURE (°C) Figure 7. Undervoltage Lockout Lower Threshold vs. Temperature Figure 8. Undervoltage Lockout Hysteresis vs. Temperature http://onsemi.com 5 125 NCP4330 108 1700 106 1660 Ipin4 (mA) Ipin4 (mA) 104 102 1620 1580 100 1540 98 96 −25 25 1500 −25 125 Figure 9. Pin4 Charge Current vs. Temperature (@ Ipin5 = 100 mA) Figure 10. Pin4 Charge Current vs. Temperature (@ Ipin5 = 1.5 mA) 3.75 3.70 VrampHL (V) 1.6 Vcl (V) 125 TEMPERATURE (°C) 1.8 1.4 1.2 1.0 3.65 3.60 3.55 3.50 0.8 −25 25 3.45 −25 125 25 125 TEMPERATURE (°C) TEMPERATURE (°C) Figure 11. Pin5 Clamp Voltage vs. Temperature Figure 12. Pin4 Clamp Voltage vs. Temperature 1.60 2.60 1.58 2.58 1.56 2.56 VrefH (V) VrefL (V) 25 TEMPERATURE (°C) 1.54 1.52 2.54 2.52 1.50 −25 25 2.50 −25 125 TEMPERATURE (°C) 25 TEMPERATURE (°C) Figure 13. Ramp Control Reference Voltage vs. Temperature (Vpin4 falling) Figure 14. Ramp Control Reference Voltage vs. Temperature (Vpin4 rising) http://onsemi.com 6 125 2.60 1.00 2.55 0.99 Hrst (V) Vrst_th (V) NCP4330 2.50 0.97 2.45 2.40 −25 25 0.96 −25 125 TEMPERATURE (°C) Figure 15. Reset Threshold vs. Temperature (Vpin3 rising) Figure 16. Reset Comparator Hysteresis vs. Temperature 125 1.0 0.9 300 250 Ireset (mA) T_reset (ns) 25 TEMPERATURE (°C) 350 200 150 0.8 0.7 0.6 0.5 100 −25 0.4 −25 125 25 25 TEMPERATURE (°C) TEMPERATURE (°C) Figure 17. Reset Time vs. Temperature Figure 18. Pin4 Average Sink Current vs. Temperature (@ f = 200 kHz, Vpin4 = 1 V and Iramp = 0 A) 125 2.45 −0.20 −0.24 24.0 VHS_H (V) Vcl_neg (V) 0.98 −0.28 −0.32 23.5 23.0 −0.36 −0.40 −25 25 125 22.5 −25 TEMPERATURE (°C) 25 125 TEMPERATURE (°C) Figure 19. Pin3 Negative Clamp Voltage vs. Temperature (@ Ipin3 = −2 mA) Figure 20. High−Side Drive Voltage vs. Temperature (high state, Isource = −100 mA, @ Vbst = 25 V) http://onsemi.com 7 1.1 160 1.0 150 0.9 140 TLS_HS (ns) VHS_L (V) NCP4330 0.8 0.7 0.6 0.5 −25 130 120 110 25 100 −25 125 TEMPERATURE (°C) 25 125 TEMPERATURE (°C) Figure 21. High−Side Drive Voltage vs. Temperature (low state, Isink = 100 mA, @ Vbst = 25 V) Figure 22. Low Side to High Side Delay vs. Temperature 9.0 1.35 8.8 1.30 VLS_L (V) VLS_H (V) 8.5 8.3 8.0 1.25 1.20 7.8 7.5 −25 25 125 1.15 −25 TEMPERATURE (°C) 25 125 TEMPERATURE (°C) Figure 23. Low−Side Drive Voltage vs. Temperature (high state, Isource = −500 mA, @ VDD = 10 V) Figure 24. Low−Side Drive Voltage vs. Temperature (low state, Isink = 750 mA, @ VDD = 10 V) http://onsemi.com 8 NCP4330 DETAILED OPERATING DESCRIPTION Introduction The NCP4330 is designed for forward, multiple output power supplies using synchronous rectification. One output is traditionally regulated thanks to a regulation arrangement that modulates the forward converter duty cycle. The other outputs are regulated by a dual MOSFET arrangement driven by the NCP4330. The high−side MOSFET turns on during one part of the forward converter on−time, while the low−side power switch is ON for the rest of the period (free wheeling). The sequencing of the switching phases, includes over−laps that result in only one hard switching (high−side turn on). The three other transitions are soft for an optimum efficiency. The synchronous rectification enables to keep a Continuous Conduction Mode (CCM) operation whatever the load is, as this technique allows to send back some energy towards the input (light load conditions). In Continuous Conduction Mode (CCM), the forward duty cycle is simply given by the following equation: Pin 5 current is internally mirrored in order to charge the Cramp capacitor. An internal comparator (1.0 V hysteresis) detects when the capacitor voltage exceeds the 2.5 V internal reference. At that moment, the low−side MOSFET turns off. 100 ns later (typically), the high–side MOSFET switches on and keeps on until (following the turn off of the forward converter power switch) a RESET signal is applied to pin 3. At that time, an internal switch grounds the Cramp pin and abruptly discharges the Cramp capacitor. As a consequence, the internal comparator turns low and forces the low−side MOSFET on. The high−side MOSFET turns off 100 ns later. During the 100 ns during which both high and low side MOSFETs are on, the MOSFET Q1 of the application schematic is off and no energy can then be drawn from the converter transformer. Therefore, these 100 ns should not be considered as a part of the high−side MOSFET conduction time which can be computed as follows: ton_HS + Tsw * tRST * tLS, HS * tcharge where: − Tsw is the forward switching period, − tRST is the Cramp reset time during which the capacitor is kept grounded, − tLS,HS is the delay between the low−side turn off and the high−side switch on. During this time, 100 ns typically, the two drivers are in low state, − tcharge is the time necessary to charge the C_ramp capacitor up to the 2.5 V reference voltage. Given that: Vout1 df + (ns ń np) * Vin where: − df is the forward duty cycle, − ns/np is the transformer turn ratio (np: primary number of turns, ns: secondary number of turns), − Vin is the forward converter input voltage, − Vout1 is the main output voltage of the forward converter. The post−regulated output voltages are given by the following equation: tcharge + n Voutn + dn * ns * Vin, p Cramp * Vref Iramp where: − Cramp is the capacitor connected to the C_ramp pin, − Iramp is the current injected into the I_ramp pin, − Vref is the 2.5 V reference voltage, the following equation dictates the high−side MOSFET duty cycle: where dn is the duty cycle of the post−regulator n, with dn < df since (ns*Vin/np) is available only during the forward converter on−time. Post−regulated output voltages are then necessarily lower than the main regulated one. tRST ) tLS, HS ) don_HS + 1 * Tsw Sequencing and Regulation Block The timing diagram of page 2 portrays the phases sequencing. Typically, a regulation arrangement injects a current into pin 5, in order to adjust the high−side MOSFET duty cycle. Cramp*Vref Iramp The following curve gives don_HS versus the current Iramp in the following conditions: 400 kHz switching frequency, 250 ns reset pulse duration, 100 ns switching delay (between LS and HS), 100 pF Cramp capacitor. http://onsemi.com 9 NCP4330 free wheeling operation continues (refer to application schematic). However, such a situation should occur only during transient phases. Should this state occur too frequently, an excessive heating of the Q2 switch could be produced. 100 D (%) 75 70 50 60 25 0 0.5 1.0 1.5 2.0 D (%) 0 50 2.5 I_ramp (mA) 10 Conditions: Switching Frequency: 400 kHz, Reset Pulse Duration: 250 ns, Switching Delay (between LS and HS): 100 ns, Cramp = 100 pF. 0 0 One can note that the duty cycle increases when the I_ramp current increases. The duty cycle is zero if the I_ramp current is below about 110 μA. The duty cycle is limited to about 81% mainly by the reset time and the 100 ns delay that all together represent 14% of the period. In a 100 kHz application, the relative impact of these times would be reduced and the maximum duty cycle would be higher (in the range of 92%). In fact, the high−side on−times are useful only during the forward on−times. Finally, if the duty cycle of the forward converter is less than 80%, one can consider that the useful post regulator duty cycle can vary between 0 and 100%. It can also be noted that the HS MOSFET can be turned on while the forward power switch is off, the forward free−wheeling MOSFET (Q2) is on and then no voltage is applied to the post−regulator. This is not an issue since the MOSFETs Q2 and Q3 derive the L2 coil current so that the C_ramp 30 20 Figure 25. High−Side MOSFET Duty Cycle vs. I_ramp C 40 R3 RST pin 0.3 0.4 I_ramp (mA) 0.5 RESET Pin Voltage VDD Vdelay Ctrl - 1 + 2 HYST COMP 250 ns 0.6 RESET Block The “reset” pin should receive the free−wheeling drive signal of the forward (refer to application schematic). When this voltage exceeds the reset block threshold (2.55 V typically), the C_ramp capacitor is grounded by an internal switch for about 250 ns and the low−side MOSFET is turned on. The circuit is then initialized for a new cycle. The voltage that is applied to the “reset” pin, may be negative during one part of the period. The NCP4330 incorporates a negative clamp system to avoid that too negative voltages on the pin may cause carriers injection within the die. The negative clamp acts to force a minimum voltage of about –0.3 V in conjunction with the external resistor R3. It features a current capability of about 2.0 mA. VDD 2.55 V/ 1.55 V 0.2 Figure 26. HS MOSFET Duty Cycle vs. I_ramp (zoom) Negative Clamp Reset signal 0.1 3 Q? NPN AND Vdelay Ctrl 250 ns GND To HS and LS drivers Figure 27. Reset Block http://onsemi.com 10 NCP4330 Low−Side Driver Stage The timing diagram of page 2 portrays the sequencing driven by the NCP4330. The low−side drive is controlled by an internal comparator that compares the C_ramp voltage to the internal reference 2.5 V (1.0 V hysteresis). When the C_ramp exceeds the 2.5 V reference, the comparator turns high forcing the low−side MOSFET off. 100 ns later, the high−side MOSFET switches on. When a reset signal is applied to the reset pin, the C_ramp capacitor is grounded. As a consequence, the internal comparator turns low and forces the low−side MOSFET on. 100 ns later, the high−side MOSFET switches off. 80.0 0 6.50 16.0 The low−side drive is designed to drive on and off a 25 nC gate charge power MOSFET, in 25 ns typical. 1. Low−Side MOSFET Turn On: In nominal operation, the body diode is already ON when the low−side MOSFET turns on. The energy Qg to be supplied is then approximately half the energy necessary if the drain source voltage was high. The necessary current capability is then: 25 nC Ils * on + 1 * , that is 500 mA. 2 25 ns Vin 40.0 vin1 i(l1) lind 12.0 8.00 20.0 ls_drv in volts 4.50 irl in amps i(l1) in amps 40.0 −80.0 vin1 in volts v(vsn) in volts 60.0 ls_drv −160 2.50 20.0 −240 500 M 4.00 −20.0 0 −320 −1.50 0 −40.0 LS_DRV 0 irl IQ VQ v(vsn) 4.5010 M 4.5015 M 4.5020 M Time in Secs Figure 28. Low−Side MOSFET Turn ON http://onsemi.com 11 4.5025 M 4.5030 M NCP4330 2. Low−Side MOSFET Turn Off: 25 nC Ils * off + 1 * , that is 500 mA. 2 25 ns The high−side MOSFET turns on about 100 ns after the low−side one is switched off. During this time when both switches are off, the body diode of the low−side MOSFET derives the inductor current (in nominal load condition, when the coil current is positive, i.e., it flows toward the output). As a result, the LS MOSFET turns off while its drain−source voltage keeps around zero due to its body diode activation. Again, the energy Qg to be supplied is then approximately half its value if the drain source voltage was high. The necessary current capability is then: High dV/dt occur in the application. When the high−side MOSFET turns on, the drain−source voltage of the low−side MOSFET sharply increases, producing a huge current through the Crss capacitor. This current may produce some parasitic turn on of the LS MOSFET if the driver impedance is not low enough to absorb this current without significant increase of the driver voltage. The driver current capability has then been increased to 750 mA so that it can effectively face a 30 V variation in 10 ns with a MOSFET exhibiting a 250 pF Crss. 40.0 60.0 −80.0 4.50 12.0 20.0 40.0 20.0 −160 2.50 −240 500 M 8.00 4.00 ls_drv in volts 16.0 irl in amps 6.50 i(l1) in amps 0 vin1 in volts v(vsn) in volts vin1 80.0 Vin lind LS_DRV 0 ls_drv −20.0 IQ 0 −320 −1.50 0 i(l1) v(vsn) VQ irl −40.0 4.4990 M 4.4995 M 4.5000 M Time in Secs Figure 29. Low−Side MOSFET Turn Off http://onsemi.com 12 4.5005 M 4.5010 M NCP4330 High−Side Driver Stage The high−side turn on (turn off) is 100 ns delayed behind the low−side turn off (turn on). The high−side drive is designed to drive on and off 12.5 nC gate charge power MOSFET, in 25 ns typical. 1. High−Side Turn On: As portrayed by in figure 30, the high−side turn on is a “hard” switching (large dV/dt and dI/dt). The necessary current capability is then: 12.5 nC Ihs * on + , that is 500 mA. 25 ns 40.0 4.00 60.0 −80.0 12.0 20.0 0 −4.00 40.0 20.0 −160 8.00 −240 4.00 ls_drv in volts 16.0 irl in amps 0 i(l1) in amps 80.0 vin1 in volts v(vsn) in volts vin1 8.00 Vin hs_drv HS_DRV 0 ihs −20.0 VQ −8.00 0 −320 0 i(l1) lind IQ vdshs −40.0 4.4990 M 4.4995 M 4.5000 M Time in Secs Figure 30. High−Side MOSFET Turn On http://onsemi.com 13 4.5005 M 4.5010 M NCP4330 2. High−Side Turn Off: The high−side MOSFET turns in a very soft way (no current, no voltage). The turn off drive is in fact designed to face the high dV/dt that occurs when the MOSFET Q1 8.00 80.0 0 16.0 abruptly turns on. The current capability has been set to 750 mA so that a 30 V variation in 10 ns cannot parasitically switch on a high−side MOSFET exhibiting a 250 pF Crss. vin1 Vin 40.0 lind 12.0 −160 8.00 −240 4.00 i(l1) 20.0 ls_drv in volts irl in amps 40.0 −80.0 i(l1) in amps 0 60.0 vin1 in volts v(vsn) in volts 4.00 hs_drv HS_DRV 0 IQ −4.00 20.0 ihs −20.0 VQ −8.00 0 −320 0 vdshs −40.0 4.5010 M 4.5015 M 4.5020 M 4.5025 M 4.5030 M Time in Secs Figure 31. High−Side MOSFET Turn Off Therefore, the maximum value of the pulsed input voltage should be chosen lower than the maximum source−gate voltage the HS MOSFET can sustain. Traditional MOSFET Vgs maximum ratings are generally +/−20 V. Therefore, with this kind of MOSFETs, the input voltage must keep below 20 V (possible spikes being included). 3. Input Voltage Limitation: Traditional high−side drivers turn off the MOSFET they control, by forcing nearly 0 V between gate and source. The NCP4330 high−side stage is not referenced to the MOSFET source but to ground, and the HS MOSFET is forced off by grounding its gate. This technique that saves the pin that is traditionally connected to the MOSFET source, allows a robust turn off of the power switch. In effect, the low−side MOSFET that is ON when the high−side is off, forces the High−side MOSFET source to approximately 0 V. However the high−side MOSFET turn on is preceded by a 100 ns phase during which both the low−side and high−side power switches are off. During this 100 ns phase, the drain source voltage of the low−side MOSFET may get high, given that in light load operation, the L2 coil current may get negative and flow from the load toward the input through the HS MOSFET body diode. In this case, the gate source voltage of the high−side MOSFET becomes negative and substantially equal to the input voltage amplitude of the post−regulator in absolute value. Undervoltage Lockout An undervoltage lockout comparator is incorporated to guarantee that the device is fully functional before enabling the output stages. The NCP4330 starts to operate when the power supply VDD exceeds 5.8 V. A 600 mV hysteresis avoids that some noise on the VDD might produce some erratic turns on and off of the device. When the NCP4330 detects an undervoltage lockout condition, it keeps both the high−side and low−side drivers in low state. The undervoltage lockout has a 4.9 V minimum threshold (falling). As a consequence, around 3.4 V are available on the driver outputs to force on the MOSFET. Such a level allows to properly drive most MOSFETs. http://onsemi.com 14 MC33152 OPTO1 NPN−PHOTO R7 RES1 Forward Control Circuitry Buffer Buffer 15 http://onsemi.com Regulation Block Vout D1 DIODE C7 CAPACITOR T2 TRANS1 C3 CAPACITOR S1 SW C2 CAP S2 SW C5 CAP T1 TRANSFO Active Clamp VCC Input Voltage GND (8) I_ramp (5) C_ramp (4) R3 RST (3) C5 CAP VDD (6) VDD Q1 VDD Q2 Iramp Iramp U? OR HS_DRV and LS_DRV low Hysteresis Comparator VDD − + U4 + Vref, UVDth DIODE D2 C1 INDUCTOR Band−Gap UVD Current Mirror 2.5V / 1.5V RESET Block (UVD high if VDD