Quality is our message FUJI Power Supply Control IC FA7700V/01V Application Note Dec -2000 Fuji Electric Co., Ltd. Matsumoto Factory 1 Quality is our message WARNING 1.This Data Book contains the product specifications, characteristics, data, materials, and structures as of Dec 2000. The contents are subject to change without notice for specification changes or other reasons. When using a product listed in this Data Book, be sure to obtain the latest specifications. 2. All applications described in this Data Book exemplify the use of Fuji's products for your reference only. No right or license, either express or implied, under any patent, copyright, trade secret or other intellectual property right owned by Fuji Electric Co., Ltd. is (or shall be deemed) granted. Fuji makes no representation or warranty, whether express or implied, relating to the infringement or alleged infringement of other's intellectual property rights which may arise from the use of the applications described herein. 3. Although Fuji Electric is enhancing product quality and reliability, a small percentage of semiconductor products may become faulty. When using Fuji Electric semiconductor products in your equipment, you are requested to take adequate safety measures to prevent the equipment from causing a physical injury, fire, or other problem if any of the products become faulty. It is recommended to make your design fail-safe, flame retardant, and free of malfunction. 4.The products introduced in this Data Book are intended for use in the following electronic and electrical equipment which has normal reliability requirements. • Computers • OA equipment • Communications equipment (terminal devices) • Measurement equipment • Machine tools • Audiovisual equipment • Electrical home appliances • Personal equipment • Industrial robots etc. 5.If you need to use a product in this Data Book for equipment requiring higher reliability than normal, such as for the equipment listed below, it is imperative to contact Fuji Electric to obtain prior approval. When using these products for such equipment, take adequate measures such as a backup system to prevent the equipment from malfunctioning even if a Fuji's product incorporated in the equipment becomes faulty. • Transportation equipment (mounted on cars and ships) • Trunk communications equipment • Traffic-signal control equipment • Gas leakage detectors with an auto-shut-off feature • Emergency equipment for responding to disasters and anti-burglary devices • Safety devices 6. Do not use products in this Data Book for the equipment requiring strict reliability such as (without limitation) • Space equipment • Aeronautic equipment • Atomic control equipment • Submarine repeater equipment • Medical equipment 7. Copyright © 1995 by Fuji Electric Co., Ltd. All rights reserved. No part of this Data Book may be reproduced in any form or by any means without the express permission of Fuji Electric. 8. If you have any question about any portion in this Data Book, ask Fuji Electric or its sales agents before using the product. Neither Fuji nor its agents shall be liable for any injury caused by any use of the products not in accordance with instructions set forth herein. 2 Quality is our message CONTENTS page 1. Description ･･･････････････････ 4 2. Features ･･･････････････････ 4 3. Outline ･･･････････････････ 4 4. Block diagram ･･･････････････････ 5 5. Pin assignment ･･･････････････････ 5 6. Ratings and characteristics ･･･････････････････ 6 7. Characteristics curves ･･･････････････････ 9 8. Description of each circuit ･･･････････････････ 13 9. Design advice ･･･････････････････ 17 Application circuit ･･･････････････････ 18 10. Note • Parts tolerance and characteristics are not defined in all application described in this Data book. When design an actual circuit for a product, you must determine parts tolerances and characteristics for safe and stable operation. 3 Quality is our message 1． ． Description FA7700V/FA7701V are the PWM type DC to DC converter control ICs with 1ch output that can directly drive power MOSFETs. CMOS devices with high breakdown voltage are used in these ICs and low power consumption is achieved. These ICs have the many functions equivalent to those which our conventional version bipolar ICs – FA76XX series- have,and have merits of output ON/OFF control function,directly driving Nch/Pch MOSFETs,low power consumption , higher frequency operation, and less external discrete components. 2． ． Features ・Wide range of supply voltage.: VCC=2.5 to 20V ・FA7700V—for boost, flyback converter (Maximum output duty cycle is 80%) FA7701V—for buck converter (Maximum output duty cycle is 100%) ・output stage consist of CMOS push-pull circuit, and achieves a high speed switching of external MOSFETs. (FA7700V: for Nch-MOSFET driving, FA7701V: for Pch-MOSFET driving) ・High accuracy reference voltage (Error amplifier): 0.88V±2% ・Soft start function. ・Adjustable built-in timer latch for short-circuit protection. ・Output ON/OFF control function ・Less external discrete components needed (2 components less than conventional version of the equivalent products) ・Low power consumption Stand-by current: 40μA(typ.) Operating current: 1.2mA(typ.) (including error amplifier output current and oscillator current) ・High frequency operation: 50kHz to 1MHz ・Package: TSSOP-8(thin and small) 3． ． Outline units:mm 4 Quality is our message 4． ． Block diagram ＦＡ７７００Ｖ ＲＴ 1 ＵＶＬＯ ＶＲＥＦ 0.3V VREF ＲＥＦ 2 2.2V ＯＳＣ 5.5V 1.5V S.C.DET BIAS 1.5V + - Power Good Signal ＩＮ− 3 0.88V ＣＳ 7 ＶＣＣ 6 ＯＵＴ 5 ＧＮＤ ＰＷＭ ＥＲ．ＡＭＰ ＦＢ OFF ON/OFF + + + + - ON/OFF + S.C.P + 2.2V 8 4 ＦＡ７７０１Ｖ ＲＴ 1 ＵＶＬＯ ＶＲＥＦ 0.3V VREF ＲＥＦ 2 2.2V ＯＳＣ 5.5V 1.5V S.C.DET BIAS 1.5V + - Power Good Signal ＩＮ− 3 0.88V + - + + ON/OFF + S.C.P + 2.2V OFF ON/OFF 8 ＣＳ 7 ＶＣＣ 6 ＯＵＴ 5 ＧＮＤ ＰＷＭ ＥＲ．ＡＭＰ ＦＢ 4 5． ． Pin assignment Pin No. Pin Name 1 RT 2 REF Internal bias voltage 3 IN(-) Error amplifier inverting input 4 FB Error amplifier output 5 GND Ground 6 OUT Output for driving switching device 7 VCC Power supply 8 CS Function Oscillator timing resistor ON/OFF, Soft start, Timer latched short circuit protection 5 Quality is our message 6． ． Ratings and Characteristics (1) Absolute maximum ratings Item Power supply voltage REF terminal output current Symbol Vcc IREF ISOpeak ISOcont ISIpeak ISIcont VRT，VREF VIN-，VFB OUT terminal source current OUT terminal sink current RT，REF，IN−，FB terminal voltage CS terminal voltage VCS CS terminal sink current Power dissipation Operating ambient temperature Operating junction temperature Storage temperature ICS Pd Ta Tj Tstg Ratings 20 2 -400(peak) -50(continuos) +150(peak) +50(continuos) +2.5(max.) -0.3(min.) Self Limiting≒5.5(max.) -0.3(min.) 200 250(Ta≦25℃) -30∼+85 +125 -40∼+150 Units V mA mA mA V V μA mW ℃ ℃ ℃ Maximum power dissipation curve Maximum power dissipation [mW］ 300 250 200 150 100 50 0 -30 0 30 60 90 Ambient temperature [℃] 120 150 (2) Recommended operating conditions Item Symbol MIN. TYP. MAX. Supply voltage VCC 2.5 6 18 DC feedback resistor 100 RNF of error amplifier VCC terminal capacitance CVCC 0.1 REF terminal capacitance CREF 0.047 0.1 1 CS terminal capacitance CS 0.01 10 CS terminal sink current Icsin 1 (*1) 50 Oscillation frequency fosc 50 1000 (*1)Lower Limit of ICSIN does not include leak current “IＬ” for capacitor Cs. Set a resistor “Rcs［ＭΩ］” connected between VCC terminal and CS terminal to satisfy the following equation. VCC − 1.5 VCC − 1.5 < RCS[ MΩ] < 50uA + IL 1uA + IL 6 Units V kΩ μF μF μF μA kHz Quality is our message (3) Electrical characteristics (Unless otherwise standard, Ta=25℃,Vcc=6V,RT=22kΩ) (1)Internal Bias Section (REF terminal voltage) Item Symbol Conditions REF terminal source current Output Voltage VREF IREF = 0mA Line Regulation VLINE Vcc = 2.5 to 20V，IREF = 0mA Load Regulation VLOAD IREF = 0 to 2mA VTC1 Ta = -30 to 25℃ Variation with temperature VTC2 Ta = 25 to 85℃ (2)Oscillator Section (Frequency set by RT terminal ) Item Symbol Conditions Oscillation fosc RT = 22kΩ frequency Line Regulation fLINE Vcc = 2.5 to 20V f TC1 Ta = -30 to 25℃, 50k to 1MHz Variation with temperature fTC2 Ta = 25 to 85℃, 50k to 1MHz (3)Error Amplifier Section (IN- terminal , FB terminal ） Item Symbol Conditions IN- terminal, FB terminal Reference Voltage VB :shorted (Voltage Follower) Input current IIN“VB” Vcc = 2.5 to 20V VBLINE Line Regulation VBTC1 Ta = -30 to 25℃ “VB” variation with temperature VBTC2 Ta = 25 to 85℃ Open Loop Gain AVO Unity Gain fT Bandwidth Source IOHE FB terminal = VREF- 0.5V Output Current FB terminal = 0.5V Sink IOLE MIN. TYP. MAX. Units 2.16 2.23 2.30 V ±2 ±2 ±0.3 ±0.3 ±14 ±12 mV mV % % MIN. TYP. MAX. Units 155 185 215 kHz ±0.1 ±2 ±3 MIN. TYP. MAX. Units 0.863 0.880 0.897 Ｖ +500 nA ±5 mV -500 ±1 ±0.3 ±0.3 % % dB 1.5 MHz 70 -220 3 (4)Pulse Width Modulation (PWM) Section (FB terminal voltage and Duty Cycle) Item Symbol Conditions MIN. FB 0% threshold VFB0 Duty Cycle = 0% 0.560 FB 50% threshold VFB50 Duty Cycle = 50% DMAX1 85 RT = 100kΩ,ｆ≒ 50kHz Maximum FA7700 DMAX2 83 RT = 22kΩ,ｆ≒185kHz Duty DMAX3 80 RT = 3kΩ,ｆ≒1MHz Cycle 100 DMAX FA7701 (5)Under Voltage Lock-Out Section (VCC terminal voltage) Item Symbol Conditions ON threshold VCCON OFF threshold VCCOF Hysteresis Voltage VCCHY Ta = -30 to 25℃ Variation with VCCHY temperature Ta = 25 to 85℃ % % % MIN. 1.60 0.04 -160 6 -100 12 μA mA TYP. 0.660 0.880 90 88 86 MAX. 0.760 Units Ｖ Ｖ % % % % TYP. 2.07 1.93 0.14 ＋0.2 −0.2 MAX. 2.30 7 95 93 92 0.24 Units Ｖ Ｖ Ｖ mV/℃ mV/℃ Quality is our message (6)ON/OFF Section (CS terminal voltage) Item Symbol ON/OFF threshold VONOF Threshold Variation with temperature VONTC Conditions MIN. 0.150 MIN. 0.560 (8)Timer Latched Short circuit Protection Section (FB terminal, CS terminal) Item Symbol Conditions MIN. Short Detection Threshold Voltage Latched Mode Threshold Voltage Latched Mode Reset Voltage Latched Mode Hysteresis ＣＳ terminal Clamped Voltage MAX. 0.450 +0.5 Ta = -30 to 85℃ (7)Soft Start Section (CS terminal voltage) Item Symbol Conditions Threshold Voltage 1 VCS0 Duty Cycle = 0% Threshold Voltage 2 VCS50 Duty Cycle = 50% TYP. 0.300 Units Ｖ mV/℃ TYP. 0.660 0.880 MAX. 0.760 Units Ｖ Ｖ TYP. MAX. Units VFBTH FB terminal voltage 1.350 1.500 1.650 Ｖ VCSTH CS terminal voltage 2.050 2.200 2.350 Ｖ VCSRE CS terminal voltage 1.700 2.030 2.300 Ｖ VCSHY CS terminal voltage 50 170 350 mV 1.400 1.500 1.600 Ｖ 4.500 5.500 6.500 Ｖ MIN. TYP. 10 18 5 5 20 25 45 40 MAX. 20 36 10 10 Units Ω Ω Ω Ω ns ns ns ns MIN. TYP. MAX. Units 40 100 μA 0.9 1.5 mA 1.2 2.0 mA 0.9 1.5 mA VCSCL1 VCSCL2 FB terminal<1.35V CS sink current = +1μA FB terminal>1.65V CS sink current = +150μA (9)Output Stage Section (OUT terminal) Item Symbol Conditions VCC = 6V, Source Current = -50ｍA RONH High Side On Resistance VCC = 2.5V,Source Current = -50ｍA RONH VCC = 6V, Sink Current = +50ｍA RONL Low Side On Resistance VCC = 2.5V, Sink Current = +50ｍA RONL 330pF Load to GND terminal FA7700 Rise Time tr 330pF Load to VCC terminal FA7701 330pF Load to GND terminal FA7700 Fall Time tf 330pF Load to VCC terminal FA7701 (10)Overall Section (Supply Current to VCC terminal) Item Symbol Conditions OFF mode CS terminal=0V ICCST1 Supply Current Duty Cycle = 0%, OUT:open ICC0 IN- =0V, FB:open Operating mode Supply Current Duty Cycle = 50%, OUT:open ICC1 IN-, FB:shorted Latched mode CS terminal >2.35V ICCLAT Supply Current IN- = 0V, FB:open 8 Quality is our message 7． ． characteristics curve Oscillation frequency vs. ambient temperature Timing resistor vs. Oscillation frequency Oscillation frequency variation[％] Oscillation frequency ［kHz］ 10000 1000 100 10 1 10 5 4 3 2 1 0 -1 -2 -3 -4 -5 fosc=1MHz fosc=185kHz fosc=50 kHz -40 100 FB terminal voltage vs. Duty cycle FA7700 80 80 Duty cycle [％] Duty cycle [％] 90 fosc=1MHz 60 50 40 30 20 60 80 100 fosc＝1MHz 60 50 40 30 fosc=185kHz 10 0 0 0.5 0.7 0.9 1.1 FB terminal voltage [V] 0.5 1.3 FB terminal voltage vs. Duty cycle FA7701 100 100 90 90 80 80 70 fosc=1MHz 60 0.7 0.9 1.1 CS terminal voltage [V] 1.3 CS terminal voltage vs. Duty cycle FA7701 Duty cycle [％] Duty cycle [％] 40 70 20 fosc=185kHz 10 20 CS terminal voltage vs. Duty cycle FA7700 100 90 70 0 Ambient temperature Ta [℃] Timing resistor R T ［kΩ］ 100 -20 50 40 30 70 fosc＝1MHz 60 50 40 30 20 20 fosc=185kHz 10 10 fosc=185kHz 0 0 0.5 0.7 0.9 1.1 FB terminal voltage [V] 1.3 0.5 0.7 0.9 1.1 CS terminal voltage [V] 9 1.3 Quality is our message Maximum Duty cycle vs. ambient temperature FA7700 Error Amp. Reference voltage vs. ambient temperature 0.90 94 92 Reference voltage [V] Maximum Duty cycle [％] fosc=50 kHz 90 88 fosc=185kHz 86 fosc=1MHz 84 0.89 0.88 0.87 82 0.86 80 -40 -20 0 20 40 60 Ambient temperature Ta [℃] 80 -40 100 Internal bias voltage vs. ambient temperature 80 100 2.20 VCC terminal ON/OFF threshold Internal bias voltage [V] 0 20 40 60 Ambient temperature Ta [℃] Under voltage lock-out vs. ambient temperature 2.28 2.26 2.24 2.22 2.20 2.18 2.15 V CCON 2.10 2.05 2.00 1.95 V CCOFF 1.90 1.85 1.80 -40 -20 0 20 40 60 Ambient temperature Ta [℃] 80 100 -40 CS terminal ON/OFF threshold vs. ambient temperature 0.40 -20 0 20 40 60 Ambient temperatureTa [℃] 80 100 CS terminal voltage vs.CS terminal sink current 200 CS terminal sink current [uA] CS terminal ON/OFF threshold [V] -20 0.35 0.30 0.25 0.20 0.15 -40 -20 0 20 40 60 Amient temperature Ta [℃] 80 100 180 Ta=-30℃ 160 Ta=85℃ 140 120 Ta=25℃ 100 80 60 FB<1.35V FB>1.65V 40 20 0 0 1 2 3 4 5 CS terminal voltage [V] 10 6 7 Quality is our message Vcc vs. Operating mode supply current Duty=50% IN(-)-FB:shorted 3.0 Operating mode supply current [mA] 2.0 Operating mode supply current [mA] Vcc vs. operating mode supply current fosc=1MHz 1.5 1.0 fosc=185kHz 0.5 Duty=50% IN(-)-FB:shorted 2.5 fosc=1MHz 2.0 fosc=185kHz 1.5 1.0 0.5 0.0 0.0 0 0.5 1 1.5 Vcc [V] 2 2.5 4 3 CS=0V 10 12 14 Vcc [V] 16 18 20 RT=22kΩ 1.5 Operating mode supply current [mA] OFF mode supply current [uA] 60 55 Vcc=20V 45 40 Vcc= 6V 35 Vcc=20V (Duty=50%) 1.4 1.3 1.2 1.1 Vcc= 6V (Duty=50%) 1.0 0.9 0.8 Vcc= 6V (Duty=0%) 0.7 0.6 30 -40 -20 0 20 40 60 Temperature Ta [℃] 80 -40 100 Vcc=6V RT=22kΩ CS>2.35V 0.95 0.90 0.85 0.80 0.75 3.0 Operating mode supply current [mA] 1.00 -20 0 20 40 60 Temperature Ta [℃] Oscillation frequency vs. operating mode supply current Latched mode supply current vs. temperature Latched mode supply current [mA] 8 Operating mode supply current vs. temperature OFF mode supply current vs. temperature 50 6 80 100 VCC=6V Duty=50% 2.5 2.0 1.5 1.0 0.5 0.0 0.70 -40 -20 0 20 40 60 Temperature Ta [℃] 80 100 10 100 Oscillation frequency [kHz] 11 1000 Quality is our message OUT terminal High side voltage vs. source current OUT terminal Low side voltage vs. sink current 200 400 350 OUT terminal sink current [mA] OUT terminal source current [mA] 450 Vcc=20V 300 Vcc=12V 250 Vcc= 6V 200 150 100 Vcc=2.5V 50 0 0 5 10 15 20 OUT terminal voltage [V] 25 150 100 50 0 0 0.5 1 OUT terminal voltage [V] Error Amplifier Gain and Phase vs. frequency 12 1.5 Quality is our message 8． ． Description of each circuit (1) Reference Voltage Circuit This circuit consists of the reference voltage circuit using band gap reference, and also serves as the power supply of the internal circuit. The precision of output is 2.23V±3%. It is stabilized under the supply voltage of 2.5V or over. The precision of reference voltage of error amplifier circuit is 0.88V±2%, and the reference voltage circuit is connected to the non-inverting input of the error amplifier circuit. (2) Oscillator The oscillator generates a triangular waveform by charging and discharging the built-in capacitor. A desired oscillation frequency can be determined by the value of the resistor “RT”connected to the RT terminal (Fig. 1). The built-in capacitor voltage oscillates between approximately 0.66V and 1.1V with almost the same charging and discharging gradients. You can set the desired oscillation frequency by changing the gradients using the resistor connected to the RT terminal. (Large RT: low frequency, small RT: high frequency) The oscillator waveform cannot be observed from the outside because a RT value : small 1.1V terminal for this purpose is not provided. The oscillator output is connected to the PWM comparator. 0.66V OSC 1 RT RT Fig.1 RT value : large fig.2 (3) Error Amplifier Circuit The IN(-) terminal (Pin3) is an inverting input terminal. The nonInverting input is internally connected to the reference voltage (0.88V±2%; 25℃). The FB terminal (Pin4) is the output of the error amplifier. Gain setting and phase compensation setting is done by connecting a capacitance and a resistor between the FB terminal and the IN(-) terminal. Vout which is the output voltage of DC to DC converter can be calculated by: Vout = VB × Vout RNF R1 Er.AMP 3 4 IN(-) FB + R2 R1 + R 2 R2 VB (0.88V) PWM fig.3 Gain AV between the Vout and the FB terminal can be calculated by: AV = − RNF R1 (4) PWM comparator The PWM comparator has 4 input terminals. (Fig. 4) The oscillator output ① is compared with the CS terminal voltage ② , and the error amplifier voltage ③ ,then, the lower voltage between ② and ③ is preferred. While the preferred voltage is lower than the oscillator output, the PWM comparator output is LOW. While the preferred voltage is higher than the oscillator output, the PWM comparator output is HIGH(Fig. 5). When the IC starts, the capacitor connected to the ①Oscillation output ②CS terminal voltage ③Error Amplifier output ④DT voltage + + + ③Error Amplifier output ①Oscillation output PWM output fig.4 ②CS terminal voltage ④DT voltage PWM output pulse 13 fig.5 Quality is our message CS terminal is charged through the resistor connected to the power supply , and then the output pulses begin to widen gradually as the operation of soft start. In steady operation, the pulse width is determined based on the voltage of the error amplifier③, and then the output voltage is stabilized. The Dead Time control voltage (④DT voltage) of FA7700 and FA7701 has different characteristics to adjust the ICs to various types of power supply circuits being controlled and also to reduce external discrete components as many as possible. FA7700 is developed for fly-back circuits, and boost circuits, and the DT voltage is set in the IC so that the maximum output duty cycle is fixed to 80%(min.). (Maximum output duty cycle changes according to operation frequencies.— See P10 “Maximum output duty vs. temperature”.) It prevents magnetic saturation of the transformer or the like when a short-circuit in the output circuit occurs. FA7701 is developed for buck circuits, and it is designed for the maximum output duty cycle of 100%. The timing chart of PWM comparator is described in Fig. 5. VCC REF OFF + Rcs C3 0.3 V CS 5.5V C1 1.5V FB Output off S.C.P + Cs ON/OFF 8 2.2V C2 + (5) Soft start function As described in Fig. 6, RCS is connected between CS terminal and VCC terminal, and Cs is connected between CS terminal and GND. The voltage of CS terminal rises when starting the power supply, because Cs is charged by Vcc through Rcs. The soft start function starts by charging a capacitor Cs connected to PWM comparator. To estimate the soft start period, the time(ts) between the start and the moment when the width of output pulse reaches 50% is calculated by: S.C.DET 1.5V Vcc ts [ms]≒Cs × Rcs × ln( ) Vcc − 0.88 fig.6 Cs: Capacity of Cs [μF] Rcs: resistance of Rcs [kΩ] Vcc: supply voltage [V] The maximum current flowing in Rcs should be within the recommended value(50μA max.). VCC − 1.5 VCC − 1.5 < RCS[ MΩ] < 50uA + IL 1uA + IL (IL: leak current of capacitor Cs) Note) This IC operates ON/OFF function by the CS terminal(CS<0.3V typ. :OFF), then it turns off the internal bias voltage VREF (off mode). Therefore, you can not connect the resistor “Rcs” between CS terminal and REF terminal, and can connect the resistor only to VCC terminal. (6) ON/OFF circuit Vcc The ON/OFF function can be controlled by external signal to the CS terminal,the IC becomes off mode .When the CS terminal voltage is below 0.30V(typ.), the output of ON/OFF comparator C3 is set to LOW, CS and the internal power source VREF is shut off, then the IC is switched ON/OFF to the off mode. The power consumption in the off mode is 40μA(typ.). Cs A sample circuit is given in Fig. 7. fig.7 14 Quality is our message CS terminal voltage [V] (7) Timer latch short-circuit protection circuit The short-circuit protection 6 circuit consists of two Lower value of either 5.5V or Vcc terminal voltage comparators C1, C2(Fig. 6). In 5 steady operation, the output of S.C.DET comparator C2 is 4 set to High, and the CS terminal Start-up 3 is clamped by the 1.5V Zener 2.2V diode,because the output of 2 error amplifier is about 1V. If the 1.5V short circuit protection tp converter output voltage drops 1 momentary short circuit due to a short-circuit etc, when short circuit soft start 0 the output voltage of error Time amplifier rises. excesses 1.5V, fig.8 the output of S.C.DET comparator C2 is set to Low, and then the clamp of Zener diode is turned off. As a result, the voltage of CS terminal rises up to the lower value of either 5.5 V or the voltage of VCC terminal. If the voltage of CS terminal excesses 2.2V, the output of S.C.P comparator C1 is set to High,and the circuit shuts down the output circuit of the IC. When it occurs, the current consumption of the IC is 0.9mA(typ.) because the IC is set to OFF latch mode. The period (tp) between the occurrence of a short-circuit in the converter output and the triggering of the short-circuit protection function can be calculated by the following expression: Vcc − 1.5 tp [ms]≒Cs × Rcs × ln( ) Vcc − 2.2 Cs: capacitance of Cs[μF] Rcs: Resistance of Rcs[kΩ] Vcc: supply voltage [V] Note) When the IC is used in a product with low VCC voltage, the period (tp) of the triggering of the shortcircuit protection described above fluctuates significantly. Therefore, sufficient care should be taken in such cases. Ex.) When Rcs=750kΩ, Cs=0.1μF: Vcc=2.5V: tp≒90ms Vcc=3.6V: tp≒30ms You can reset the off latch mode operation of the short-circuit protection by either of the following ways: lowering the CS voltage below 2.03V(typ.); lowering the Vcc voltage below the OFF threshold voltage of Under voltage Lock out ; 1.93V(typ.); lowering the voltage of FB terminal below 1.5V(typ.) The off latch mode action cannot be triggered by externally applying voltage of over 2.2 V forcibly to the CS terminal (1.5V,ZD clamped) Characteristics of the current and the voltage of CS terminal is shown in the characteristics curve [CS terminal voltage vs. CS terminal sink current] in page 10. Be sure to use the IC up to the recommended CS terminal current of 50μA. 15 Quality is our message (8) Output circuit The IC contains a push-pull output stage and can directly drive MOSFETs (FA7700: N ch, FA7701: P ch). The maximum peak current of the output stage is a sink current of +150mA, and a source current of - 400mA. The IC can also drive NPN, and PNP transistors. The maximum peak current in such cases is ± 50mA. Be sure to design the output current considering the rating of power dissipation. (9) Power good signal circuit/ Undervoltage lockout circuit The IC contains a protection circuit against Undervoltage malfunctions to protect the circuit from the damage caused by malfunctions when the supply voltage drops. When the supply voltage rises from 0V, the circuit starts to operate at VCC of 2.07V(typ.) and outputs generate pulses. If a drop of the supply voltage occurs, it stops output at VCC of 1.93V(typ.). when it occurs, the CS terminal is turned to LOW level and then it is reset. The power good signal circuit monitors the voltage of REF terminal, and stops output until the voltage of REF terminal excesses approximately 2V to prevent malfunctions. 16 Quality is our message 9． ． Design Advice (1) Setting the oscillation frequency As described in Section 8(1), “Description of Each Circuit,” a desired oscillation frequency can be determined by the value of the resistor connected to the RT terminal. When designing an oscillation frequency, you can set any frequency between 50kHz and 1MHz. You can roughly obtain the oscillation frequency from the characteristic curve “Oscillation frequency(fosc) vs. timing resistor resistance(RT)” or the value can be calculated by the following expression. fosc = 3000 × RT −0.9 3000 RT = fosc 1.11 fosc: oscillation frequency [kHz] RT: timing resistor [kΩ] This expression, however, can be used for rough calculation, the value obtained is not guaranteed. The operation frequency varies due to the conditions such as tolerance of the characteristics of the ICs, influence of noises, or external discrete components. When determining the values, be sure to verify the effectiveness of the values of the components in an actual circuit. (2) Operation around the maximum or the minimum output duties As described in P9 of characteristic curves of “FB terminal voltage (VFB) vs.output duty cycle” and “CS terminal voltage (Vcs) vs. output duty cycle”, the linearity of the output duty of this IC drops around the minimum output duty and the maximum output duty(FA7701 only). This phenomena are conspicuous when operating in a high frequency(when the pulse width is narrow). Therefore be careful when using high frequency. (3) Restriction of external discrete components To achieve a stable operation of the ICs, the value of external discrete components connected to Vcc, REF, CS, FB terminals should be within the recommended operational conditions. (4) Loss Calculation Since it is difficult to measure IC loss directly, the calculation to obtain the approximate loss of the IC connected directly to a MOSFET is described below. When the supply voltage is Vcc, the current consumption of the IC is Icc, the total input gate charge of the driven MOSFET is Qg, the switching frequency is fsw, the total loss Pd of the IC can be calculated by: Pd ≒ Vcc×(Icc+Qg×fsw). The values in this expression is influenced by the effects of the dependency of supply voltage, the characteristics of temperature, or tolerance. Therefore,be sure to verify appropriateness of the value considering the factors above under all applicable conditions. Example) When VCC=6V, in the case of a typical IC, from the characteristic curve, Icc=1.2mA. When operating in Qg=6nC, fsw=500kHz, Pd should be: Pd≒6×(1.2mA+6nC×500kHz)≒25.2mW 17 Quality is our message 10． ．Application Vin 2.5∼11V Vout 12V/0.2A 8 CS 7 VCC ON /OFF 6 OUT 5 GND FA7700 RT 1 REF 2 IN3 FB 4 Vin Vout 8 CS ON /OFF 7 VCC 6 OUT 5 GND FA7700 RT 1 REF 2 IN3 FB 4 18 Quality is our message Vin 7∼18V Vout 5V/0.5A 8 CS ON /OFF 7 VCC 6 OUT 5 GND FA7701 RT 1 REF 2 IN3 FB 4 19