CS3845B CS3845B Current Mode PWM Control Circuit with 50% Max Duty Cycle Description Features exceeds 50%. The undervoltage lockout feature ensures that VREF is stabilized within specification before the output stage is enabled. The CS3845B has been optimized for lower start up current (500µA max). The CS3845B provides all the necessary features to implement off-line fixed frequency current-mode control with a minimum number of external components. The CS3845B incorporates a precision temperature-controlled oscillator to minimize variations in frequency. An internal toggle flip-flop, which blanks the output off every other clock cycle, ensures that the duty-cycle never Other features include 1% trimmed band gap reference, pulse-by-pulse current limiting, and a high-current totem pole output for driving capacitive loads. Absolute Maximum Ratings Supply Voltage (ICC<30mA) ..........................................................Self Limiting Supply Voltage (Low Impedance Source)...................................................30V Output Current ...............................................................................................±1A Output Energy (Capacitive Load) .................................................................5µJ Analog Inputs (VFB, VSENSE) ..........................................................–0.3V to 5.5V Error Amp Output Sink Current...............................................................10mA Lead Temperature Soldering Wave Solder (through hole styles only) ..........10 sec. max, 260°C peak Reflow (SMD styles only) ...........60 sec. max above 183°C, 230°C peak ■ Optimized for Off-line Use ■ Temperature Compensated Oscillator ■ 50% Maximum Duty-cycle Clamp ■ Low Start-up Current (500µA max) ■ Pulse-by-pulse Current Limiting ■ Undervoltage Active Pull Down ■ Double Pulse Suppression ■ 1% Trimmed Bandgap Reference ■ High Current Totem Pole Output Package Options Block Diagram 8 Lead PDIP & SO V CC Undervoltage Lock-out V CC 34V V CC Pwr Set/ 5.0 Volt Reset Reference Gnd V REF 8.4V/7.6V V FB OSC R + 2.50V Oscillator R Toggle Flip-Flop NOR VREF 7 VCC Sense 3 6 VOUT OSC 4 5 Gnd COMP 1 14 VREF NC 2 13 NC VFB 3 12 VCC NC 4 11 VCC Pwr Sense 5 10 VOUT NC 6 9 Gnd OSC 7 8 Pwr Gnd V OUT Pwr Gnd R Sense 8 2 14 Lead SO Narrow V REF Undervoltage Lockout S 2 R R 1 VFB Internal Bias Error Amplifier COMP COMP 1V Current PWM Sensing Latch Comparator ON Semiconductor 2000 South County Trail, East Greenwich, RI 02818 Tel: (401)885–3600 Fax: (401)885–5786 N. American Technical Support: 800-282-9855 Web Site: www.cherry–semi.com December, 1999 - Rev. 6 1 CS3845B Electrical Characteristics: 0≤TA≤70°C, VCC=15V; RT=10kΩ, CT=3.3nF for sawtooth mode, unless otherwise stated PARAMETER TEST CONDITIONS MIN TYP MAX UNITS 4.90 5.00 5.10 V 6 20 mV ■ Reference Section Output Voltage TJ = 25°C, IREF=1mA Line Regulation 12 ≤ VCC ≤ 25V Load Regulation 1 ≤ IREF ≤ 20mA Temperature Stability (Note 1) Total Output Variation Line, Load, Temp. (Note 1) 6 25 mV 0.2 0.4 mV/°C 5.18 V 4.82 Output Noise Voltage 10Hz ≤ f ≤ 10kHz, TJ = 25°C (Note 1) 50 Long Term Stability TA = 125°C, 1000 Hrs. (Note 1) 5 25 mV Output Short Circuit TA = 25°C –30 –100 –180 mA Initial Accuracy Sawtooth Mode, TJ = 25°C 47 52 57 kHz Voltage Stability 12 ≤ VCC ≤ 25V 0.2 1.0 % Temperature Stability Sawtooth Mode TMIN ≤ TA ≤ TMAX (Note 1) VOSC (peak to peak)(Note 1) 5 1.7 µV ■ Oscillator Section Amplitude Discharge % V TJ = 25°C; (Note 1) TMIN ≤ TA ≤ TMAX (Note 1) 7.5 7.2 8.3 9.3 9.5 mA mA VCOMP = 2.5V 2.42 2.50 2.58 V –0.3 –2.0 µA ■ Error Amp Section Input Voltage Input Bias Current VFB = 0V AVOL 2 ≤ VOUT ≤ 4V 65 90 dB Unity Gain Bandwidth (Note 1) 0.7 1.0 MHz PSRR 12 ≤ VCC ≤ 25V 60 70 dB Output Sink Current VFB = 2.7V, VCOMP = 1.1V 2 6 mA Output Source Current VFB = 2.3V, VCOMP = 5V –0.5 –0.8 mA VOUT HIGH VFB = 2.3V, RL15kΩ to Gnd 5 6 V VOUT LOW VFB = 2.7V, RL = 15kΩ to VREF 0.7 1.1 V ■ Current Sense Section Gain (Notes 2&3) 2.85 3.00 3.15 V/V Maximum Input Signal VCOMP = 5V (Note 2) 0.9 1.0 1.1 V PSRR 12 ≤ VCC ≤ 25V (Note 2) 70 dB Input Bias Current VSense = 0V –2 –10 µA Delay to Output TJ = 25°C (Note 1) 150 300 ns Output Low Level ISINK = 20mA ISINK = 200mA 0.1 1.5 0.4 2.2 V V Output High Level ISOURCE = 20mA ISOURCE = 200mA ■ Output Section 13.0 12.0 13.5 13.5 V V Rise Time TJ = 25°C, CL = 1nF (Note 1) 50 150 ns Fall Time TJ = 25°C, CL = 1nF (Note 1) 50 150 ns 2 PARAMETER TEST CONDITIONS MIN TYP MAX UNITS 300 500 µA 17 mA ■ Total Standby Current Start-Up Current Operating Supply Current VFB = VSense = 0V RT = 10kΩ, CT = 3.3nF 11 VCC Zener Voltage ICC=25mA 34 V ■ PWM Section Maximum Duty Cycle 46 48 Minimum Duty Cycle 50 % 0 % ■ Under-Voltage Lockout Section Start Threshold Min. Operating Voltage Notes: After Turn On 1. These parameters, although guaranteed, are not 100% tested in production. 7.8 8.4 9.0 V 7.0 7.6 8.2 V 3. Gain defined as: A = 2. Parameter measured at trip point of latch with VFB=0 ∆VCOMP ∆VSense ; 0 ≤ VSense ≤ 0.8V. Package Pin Description PACKAGE PIN # PIN SYMBOL FUNCTION 8L PDIP & SO 14L SO 1 1 COMP 2 3 VFB 3 5 Sense Noninverting input in Current Sense Comparator 4 7 OSC Oscillator timing network with Capacitor to Gnd, resistor to VREF 5 9 Gnd Ground 6 10 VOUT Output drive pin 7 12 VCC Positive power supply 8 14 VREF Output of 5V internal reference 8 Pwr Gnd Output driver Gnd 11 VCC Pwr Output driver positive supply 2, 4, 6, 13 NC Error amp output, used to compensate error amplifier Error amp inverting input No Connection 3 CS3845B Electrical Characteristics: continued CS3845B Test Circuit V REF RT 2N2222 V CC A 100kΩ COMP 4.7kΩ 1kΩ Error Amp Adjust V REF V FB V CC CS3845B 0.1µF 5kΩ 4.7kΩ V OUT Sense Sense Adjust 0.1µF OSC 1kΩ 1W V OUT Gnd Gnd CT Circuit Description V CC Undervoltage Lockout During Undervoltage Lockout (Figure 1), the output driver is biased to sink minor amounts of current. The output should be shunted to ground with a resistor to prevent activating the power switch with extraneous leakage currents. ON/OFF Command to reset of IC CSX845B V ON V OFF PWM Waveform To generate the PWM waveform, the control voltage from the error amplifier is compared to a current sense signal which represents the peak output inductor current (Figure 2). An increase in VCC causes the inductor current slope to increase, thus reducing the duty cycle. This is an inherent feed-forward characteristic of current mode control, since the control voltage does not have to change during changes of input supply voltage. When the power supply sees a sudden large output current increase, the control voltage will increase allowing the duty cycle to momentarily increase. Since the duty cycle tends to exceed the maximum allowed to prevent transformer saturation in some power supplies, the internal oscillator waveform provides the maximum duty cycle clamp as programmed by the selection of OSC components. 8.4V 7.6V I CC <15mA <1mA V CC V ON V OFF Figure 1: Startup voltage for CS3845B. 4 Setting the Oscillator The parameters Tc and Td can be determined as follows: VOSC OSC RESET t c = RTCT ln Toggle F/F Output t d = RTCT ln EA Output Switch Current ( ( VREF - Vlower VREF - Vupper ) VREF - IdRT - Vlower VREF - IdRT - Vupper ) Substituting in typical values for the parameters in the above formulas: VREF = 5.0V, Vupper = 2.7V, Vlower = 1.0V, Id = 8.3mA, then tc ≈ 0.5534RTCT VCC IO VO td = RTCT ln Figure 2: Timing Diagram ( 2.3 - 0.0083 RT 4.0 - 0.0083 RT ) For better accuracy RT should be ≥10kΩ. Vupper Grounding High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected close to Gnd in a single point ground. The transistor and 5kΩ potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to Sense. Vlower ton toff td tC ton = tC toff = tC+2td Figure 3: Timing Parameters. 5 CS3845B Circuit Description: continued CS3845B Package Specification PACKAGE THERMAL DATA PACKAGE DIMENSIONS IN mm (INCHES) D Lead Count Metric 8 Lead PDIP 8 Lead SO Narrow 14 Lead SO Narrow Max 10.16 5.00 8.75 Thermal Data 8L PDIP 52 100 English Min 9.02 4.80 8.55 Max .400 .197 .344 RQJC RQJA Min .355 .189 .337 typ typ 8L SO 45 165 14 L SO 30 125 °C/W °C/W PDIP: 300 mil wide 7.11 (.280) 6.10 (.240) 8.26 (.325) 7.62 (.300) 1.77 (.070) 1.14 (.045) 2.54 (.100) BSC 3.68 (.145) 2.92 (.115) 0.39 (.015) MIN. .356 (.014) .203 (.008) .558 (.022) .356 (.014) REF: JEDEC MS-001 Some 8 and 16 lead packages may have 1/2 lead at the end of the package. All specs are the same. D SO Narrow; 150 mil wide 4.00 (.157) 3.80 (.150) 6.20 (.244) 5.80 (.228) 0.51 (.020) 0.33 (.013) 1.27 (.050) BSC 1.75 (.069) MAX 1.57 (.062) 1.37 (.054) 1.27 (.050) 0.40 (.016) 0.25 (.010) 0.19 (.008) D 0.25 (0.10) 0.10 (.004) REF: JEDEC MS-012 Ordering Information Part Number CS3845BGN8 CS3845BGD8 CS3845BGDR8 CS3845BGD14 CS3845BGDR14 Description 8L PDIP 8L SO 8L SO (tape & reel) 14L SO 14L SO (tape & reel) ON Semiconductor and the ON Logo are trademarks of Semiconductor Components Industries, LLC (SCILLC). ON Semiconductor reserves the right to make changes without further notice to any products herein. For additional information and the latest available information, please contact your local ON Semiconductor representative. 6 © Semiconductor Components Industries, LLC, 2000 Notes Notes