LM3842A

CURRENT MODE PWM CONTROLLER FEATURES ● ● ● ● ● ● ● ● ● Automatic feed forward compensation Optimized for offline converter Double pulse suppression Current mode operation to 500 KHz High gain totem pole output Internally trimmed bandgap reference Undervoltage lockout with hysteresis Low start up current :＜ 0.3 mA Moisture Sensitivity Level 3 LM3842A/3A/4A/5A 8 SOP/ 8 DIP PIN Configulation Compen sation 8 7 6 5 VREF 1 2 3 4 SOP-8 PKG Voltage Feedback Current Sense Vcc OUTPUT GND DIP-8 PKG RT/CT ORDERING INFORMATION Device LM3842A/3A/4A/5A D Package 8 SOP 8 DIP DESCRIPTION LM3842A/3A/4A/5A N The LM384xA are fixed frequency current-mode PWM controller. They are specially designed for Off-Line and DC-to-DC converter applications with minimal external components. These integrated circuits features a trimmed oscillator for precise duty cycle control, a temperature compensated reference, high gain error amplifier, current sensing comparator, and a high current totempole output ideally suited for driving a power MOSFET. Protection circuitry includes built in under-voltage lockout and current limiting. SIMPLIFIED BLOCK DIAGRAM VREF ⑧ R R RT/CT ④ VFB ② COMP ① + VREF Undervoltage Lockout Oscillator 5V Reference Vcc Undervoltage Lockout ⑦ Vcc Latching PWM ⑥ Output ⑤ Ground ③ Current Sense Input Error Amplifie r ABSOLUTE MAXIMUM RATINGS (TA= 25℃) Characteristic Power Supply Voltage Output Current Analog Inputes Voltage Error Amp Output Sink Current Power Dissipation Storage Temperature Range Lead Temperature (soldering 5 sec) 2008 - Ver. 1.0 Symbol Vcc Io VIN ISINK PD Tstg TL Value 30 ±1 -0.3 to 5.5V 10 1 -65 to 150 260 Unit V A V mA W ℃ ℃ −1− HTC CURRENT MODE PWM CONTROLLER ELECTRICAL CHARACTERISTIC LM3842A/3A/4A/5A Typ 5.00 2 3 -85 52 0.2 1.6 -0.1 2.50 90 70 (Vcc=15V(Note 1), RT = 10kΩ, CT=3.3nF 0 ≤ TA ≤ 70℃ ; unless otherwise specified) Characteristic Symbol Test Condition Min REFERENCE SECTION Tj = 25℃, IO=1 mA VREF 4.90 Reference Output Voltage Line Regulation Vo 12V ≤ Vcc ≤ 25V Load Regulation Vo 1mA ≤ Io ≤ 20mA TA = 25℃ Output Short Circuit Current Isc OSCILLATOR SECTION Tj = 25℃ FOSC 47 Normal Frequency Voltage Stability Sv 12V ≤ Vcc ≤ 25V Amplitude Vosc ERROR AMPLIFIER SECTION IIB Input B Current VFB Vo=2.5V 2.42 Feedback Input Voltage AVOL 2V≤ Vo ≤ 4V 65 Open Loop Voltage Gain PSRREA 60 12V ≤ Vcc ≤ 25V Power Supplier Rejection Ratio VFB=2.7V, Vo - 1.1V ISI 2 Output Sink Current ISO Output source Current V = 2.3V, Vo=5V -0.5 VFB=2.7V, RL=15kΩ to GND VOH 5 Output Voltage High VFB=2.7V, RL=15kΩ to VRGR VOL Output Voltage Low CURRENT SENSE SECTION Input Voltage Gain Av (Note 2 & 3) 2.85 VMAX Vo=5V (Note 2) 0.9 Maximum Input Signal PSRRSC 12V ≤ Vcc ≤ 25V Power Supply Rejection Ratio IIB Input Bias Current OUTPUT SECTION Isink = 20mA VOL Output Voltage Low Isink = 20mA Isource = 20mA 13 VOH Output Voltage High Isource = 20mA 12 Tr Tj = 25℃, CL=1nF Rise Time Tt Tj = 25℃, CL=1nF Fail Time UNDERVOLTAGE LOCKOUT SECTION 3842A/3844A 14.5 Vth Start Threshold 7.8 3843A/3845A 3842A/3844A 8.5 Minimum Operating Voltage (After Turn-on VCC(MIN) 7.0 3843A/3845A PWM SECTION 3842A/3843A 14.5 Maximum Duty Cycle Dmax 3844A/3845A 7.8 TOTAL STANDBY CURRENT Vcc=14V for 3842A/3844A Ist Start-up Current Vcc=6.5V for 3843A/3845A ICC Vpin2=Vpin3=0V Operating Supply Current Viz Zener Voltage Ii = 25mA 30 Max 5.10 20 25 -180 57 1 Unit V mV mV mA kHz % Vp-p μA V dB dB mA mA V V V/V V dB μA V V V V nS nS V V V V V V mA mA mA V -2 2.58 7 -1.0 6 0.8 3 1 70 -3 0.1 1.5 13.5 13.0 45 35 16 8.4 10 7.6 16 8.4 0.17 0.17 14 38 1.1 3.15 1.1 -10 0.4 2.2 150 150 17.5 9 11.5 8.2 17.5 9 0.3 0.3 17 HTC −2− CURRENT MODE PWM CONTROLLER LM3842A/3A/4A/5A Note: 1. Adjust Vcc above the start threshould before setting at 15V. 2. Parameter measured at trip point of latch with VFB - 0. 3. Comparator Gain defined as: A = ΔV Output Compensation(pin FB) ; ΔV Current Sanseinput(pin CS) Fig.1 Open Loop Test Circuit VREF RT 4.7K 2N 2222 A V1 1 E/A ADJUST 1K 100K 2 ISENSE ADJUST 4.7K CT 5K 3 COMP VREF 8 0.1 VFB VC 7 1K/1W 0.1 OUTPUT LM3842A ISENSE RT/C T OUTPUT 6 5 4 GND High peak currents associated with capacitive loads necessitate careful grounding techniques Timing and bypass capacitors should be connected close to pin 5 in a single point ground. The transistor and 5KΩ potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to pin 3. Fig.2 Under Voltage Lockout 7 ON/OFF COMMAND TO REST OF IC ICC LM3842A VON VOFF 16V 10V LM3843/5A 8.4V 7.6V < 15mA < 1mA VOFF VON VCC During Under-Voltage Lock-Out, the output driver is biased to a high impedance state. Pin 6 should be shunted to ground with a bleeder resistor to prevent activating the power switch with output leakage current. HTC −3− CURRENT MODE PWM CONTROLLER Fig.3 Error Amp Configuration 2.5V LM3842A/3A/4A/5A 0.5mA + VFB Zi Zf COMP 2 1 - Error amp can source or sink up to 0.5mA Fig.4 Current Sense Circuit ERROR AMP IS 2R 1 R COMP CURRENT SENSE R 1V CURRENT SENSE COMPARATOR 3 C RS GND 5 Peak current (IS) is determined by the formula: IS(MAX) ~ 1.0V RS A small RC filter may be required to suppress switch transients. Fig.5 Oscillator Waveforms and Maximum Duty Cycle VREF 8 RT RT/CT INTERNAL CLOCK 4 CT GND 5 INTERNAL CLOCK LARGE RT SMALL CT V4 LARGE RT SMALL CT V4 HTC −4− CURRENT MODE PWM CONTROLLER duty cycle. Charge and discharge times are determined by the formulas: tc ~ 0.55 RT CT td ~ RT CT ∫n( 0.0063 RT - 2.7 ) 0.0063 RT - 4 LM3842A/3A/4A/5A Frequency, then, is: f = (tc + td) -1 For RT>5KΩ, f ~ 1.8 ~R C T T Fig.6 Shutdown Techniques 1K 8 VREF 1 COMP 330Ω 500Ω SHUTDOWN 3 ISENSE SHUTDOWN TO CURRENT SENSE-RESISTOR Shutdown of the LM3842A can be accomplished by two methods; either raise pin 3 above 1V or pull pin 1 below a voltage two diode drops above ground. Either method causes the output of the PWM comparator to be high (refer to block diagram). The PWM latch is reset dominant so that the output will remain low until the next clock cycle after the shoutdown condition at pins 1 and/or 3 is removed. In one example, an externally latched shutdown may be accomplished by adding an SCR which will be reset by cycling VCC below the lower UVLO threshold. At this point the reference turns off, allowing the SCR to reset. Fig.7 Slope Compensation VREF 8 0.1μF RT LM3842A RT/CT 4 CT R1 R2 ISENSE ISENSE 3 C RSENCE HTC −5− CURRENT MODE PWM CONTROLLER LM3842A/3A/4A/5A A fraction of the oscillator ramp can be resistively summed with the current sense signal to provide slope compensation for converters requiring duty cycles over 50%. Note that capacitor, C, forms a filter with R2 to suppress the leading edge switch spikes. Fig.1 Output Dead Time 80 RT, Timing Resister (kΩ 50 CT=10nF 20 8.0 5.0 2.0 0.8 10k 20k 50k 100k 200k 500k 1.0M CT=5.0nF CT=2.0nF CT=1.0nF CT=500pF CT=200pF CT=100pF Fig.2 Timing Resistor vs Frequency 100 CT=100nF CT=47nF 30 RT(kΩ) CT=22nF CT=10nF 10 CT=4.7nF CT=2.2nF CT=10nF 3 100 1k 10k 100k 1M fosc, Frequency(Hz) fosc, Frequency (kHz) Fig.3 Output Saturation Characteristics Fig.4 Error Amplifier Open Loop Gain and Phase Frequency VSAT, Output Saturation Voltage (V 4 VCC=15V TA=25℃ 3 AVOL, Open-Loop Voltage Gain (d 80 Gain 60 0 Φ, Excess Phase Degree -45 2 40 Phase -90 1 SOURCE (VC-VOH) C SINK (V ) OL 20 -135 0 -180 0 10-3 2 4 6 8 10-1 2 4 68 10 100 1K 10K 100K 1M 10M Io, Output Load Current (A) Fosc, Frequency (Hz) PIN FUNCTION DESCRIPTION Pin No. 1 2 3 4 Function Description Compensation This pin is the Error Amplifier output and is made available for loop compensation. Voltage This is the inverting input of the Error Amplitier. It is normally connected Feedback to the switching power supply output through a resister divider. Current Sense A voltage proportional to inductor current is connected to this input. The PWM uses this information to terminate the output switch conduction. RT/CT The Oscillator frequency and maximum Output duty cycle are programmed by connecting resistor RT to VREF and capacitor CT to ground. Operation to 500kHz is possible. GND This pin is the combined control circuitry and power ground. Output This output directly drives the gate of a power MOSFET. Peak currents up to 1.0A are sourced and sunk by this pin. Vcc This pin is the positive supply of the control IC. VREF This is the reference output. It provides charging current for capacitor CT through resistor RT. 5 6 7 8 HTC −6−