LM3842A中文资料

CURRENT MODE PWM CONTROLLER LM3842A/3A/4A/5A 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 ORDERING INFORMATION Device Package DESCRIPTION LM3842A/3A/4A/5A D 8 SOP LM3842A/3A/4A/5A N 8 DIP The LM3842A is fixed frequency current-mode PWM controller. It is specially designed for Off-Line and DC-to-DC converter applications with minimal external components. This integrated circuit 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 ABSOLUTE MAXIMUM RATINGS (TA= 25℃) Characteristic Symbol Value Unit Power Supply Voltage Vcc 30 V Output Current I ± 1 A Analog Inputes Voltage VIN -0.3 to 5.5V V Error Amp Output Sink Current ISINK 10 mA Power Dissipation PD 1 W Storage Temperature Range Tstg -65 to 150 ℃ Lead Temperature (soldering 5 sec) T 260 ℃ HTC 1 5V Reference Vcc Undervoltage Lockout Latching PWM Oscillator VREF Undervoltage Lockout +- ⑦ Vcc ⑥ Output ⑤ Ground ③ Current Sense Input VREF ⑧ RT/CT ④ VFB ② COMP ① Error Amplifie r R R 1 2 3 4 8 7 6 5 Voltage Feedback Current Sense RT/CT Vcc OUTPUT GND VREFCompen sation 8 SOP/ 8 DIP PIN Configulation 元器件交易网www.cecb2b.com CURRENT MODE PWM CONTROLLER LM3842A/3A/4A/5A ELECTRICAL CHARACTERISTIC (Vcc=15V(Note 1), RT = 10kΩ, CT=3.3nF 0 ≤ TA ≤ 70℃ ; unless otherwise specified) Characteristic Symbol Test Condition Min Typ Max Unit REFERENCE SECTION Reference Output Voltage VREF Tj = 25℃, IO=1 mA 4.90 5.00 5.10 V Line Regulation Vo 12V ≤ Vcc ≤ 25V 2 20 mV Load Regulation Vo 1mA ≤ Io ≤ 20mA 3 2.5 mV Output Short Circuit Current Isc TA = 25℃ -85 -180 mA OSCILLATOR SECTION Normal Frequency FOSC Tj = 25℃ 47 52 57 kHz Voltage Stability Sv 12V ≤ Vcc ≤ 25V 0.2 1 % Amplitude Vosc 1.6 Vp-p ERROR AMPLIFIER SECTION Input B Current I BI -0.1 -2 μA Feedback Input Voltage VFB Vo=2.5V 2.42 2.50 2.58 V Open Loop Voltage Gain AVOL 2V≤ Vo ≤ 4V 65 90 dB Power Supplier Rejection Ratio PSRREA 12V ≤ Vcc ≤ 25V 60 70 dB Output Sink Current ISI VFB=2.7V, Vo - 1.1V 2 7 mA Output source Current ISO V = 2.3V, Vo=5V -0.5 -1.0 mA Output Voltage High VOH VFB=2.7V, RL=15kΩ to GND 5 6 V Output Voltage Low VOL VFB=2.7V, RL=15kΩ to VRGR 0.8 1.1 V CURRENT SENSE SECTION Input Voltage Gain Av (Note 2 & 3) 2.85 3 3.15 V/V Maximum Input Signal VMAX Vo=5V (Note 2) 0.9 1 1.1 V Power Supply Rejection Ratio PSRRSC 12V ≤ Vcc ≤ 25V 70 dB Input Bias Current I BI -2 -1.0 μA OUTPUT SECTION Output Voltage Low VOL Isink = 20mA 0.1 0.4 V I inks = 20mA 1.5 2.2 V Output Voltage High VOH Isource = 20mA 13 13.5 V I ources = 20mA 12 13.0 V Rise Time T Tj = 25℃, CL=1nF 45 150 nS Fail Time T Tj = 25℃, CL=1nF 35 150 nS UNDERVOLTAGE LOCKOUT SECTION Start-up Threshold Vth 3842A / 3844A 14.5 16 17.5 V3843A / 3845A 7.8 8.4 9 V Minimum Operating Voltage VCC(MIN) 3842A / 3844A 8.5 10 11.5 V(After Turn-On ) 3843A / 3845A 7.0 7.6 8.2 V TOTAL STANDBY CURRENT Start-up Current I ts Vcc=14V 0.17 0.3 mA Operating Supply Current I CC 13 mA17 Zener Voltage V Icc = 25mA 30 38 V 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: HTC 2 ΔV Output Compensation(pin FB) ΔV Current Sanseinput(pin CS) A V = ; 元器件交易网www.cecb2b.com CURRENT MODE PWM CONTROLLER LM3842A/3A/4A/5A Fig.1 Open Loop Test Circuit 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 LM3842A LM3843A/5A VON 16V 8.4V VOFF 10V 7.6V 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. Fig.3 Error Amp Configuration HTC 3 1 2 3 4 8 7 6 5 COMP VREF VFB VC OUTPUT GND ISENSE RT/CT LM3842A 0.1 0.1 A CT OUTPUT 1K/1W VREF V1 RT 100K 2N 22224.7K 1K 5KISENSE ADJUST 4.7K E/A ADJUST Error amp can source or sink up to 0.5mA 2 1 - + Zi Zf COMP VFB 0.5mA2.5V 7 ON/OFF COMMAND TO REST OF IC < 15mA < 1mA VOFF VON VCC ICC 元器件交易网www.cecb2b.com CURRENT MODE PWM CONTROLLER LM3842A/3A/4A/5A Fig.4 Current Sense Circuit Peak current (IS) is determined by the formula: IS(MAX) A small RC filter may be required to suppress switch transients. Fig.5 Oscillator Waveforms and Maximum Duty Cycle Oscillator timing capacitor, CT, is charged by VREF through RT, and discharged by an internal current source. During the discharge time, the internal clock signal blanks the output to the low state. Selection of RT and CT therefore determines both oscillator frequency and maximum duty cycle. Charge and discharge times are determined by the formulas: tc 0.55 RT CT td RT CT ∫n( ) Frequency, then, is: f = (tc + td) -1 For RT>5KΩ, f HTC 4 1 3 5 IS ERROR AMP 2R R 1V CURRENT SENSE COMPARATOR COMP CURRENT SENSE GND C RS R ~~ 1.0VRS ~~ 0.0063 RT - 2.7 0.0063 RT - 4~~ 1.8 RT CT ~~ 8 4 5 VREF RT/CT GND LARGE RT SMALL CT V4 INTERNAL CLOCK LARGE RT SMALL CT V4 INTERNAL CLOCK RT CT 元器件交易网www.cecb2b.com CURRENT MODE PWM CONTROLLER LM3842A/3A/4A/5A Fig.6 Shutdown Techniques 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 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. HTC 5 1 SHUTDOWN COMP 8 3 VREF ISENSE 1K 330Ω 500Ω SHUTDOWN TO CURRENT SENSE-RESISTOR 8 4 3 ISENSE R2 VREF RT/CT 0.1μF RT CT R1 RSENCEC ISENSE LM3842A 元器件交易网www.cecb2b.com CURRENT MODE PWM CONTROLLER Fig.1 Output Dead Time Fig.2 Timing Resistor vs Frequency Fig.3 Output Saturation Characteristics Fig.4 Error Amplifier Open Loop Gain and Phase Frequency PIN FUNCTION DESCRIPTION Pin No. Function Description 1 Compensation This pin is the Error Amplifier output and is made available for loop compensation. 2 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. 3 Current Sense A voltage proportional to inductor current is connected to this input. The PWM uses this information to terminate the output switch conduction. 4 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. 5 GND This pin is the combined control circuitry and power ground. 6 Output This output directly drives the gate of a power MOSFET. Peak currents up to 1.0A are sourced and sunk by this pin. 7 Vcc This pin is the positive supply of the control IC. 8 VREF This is the reference output. It provides charging current for capacitor CT through resistor RT. HTC 6 LM3842A/3A/4A/5A 10-3 2 4 6 8 10-1 2 4 86 4 2 0 1 3 SINK (VOL) VCC=15V TA=25℃ SOURCE (VCC-VOH) V S AT , O ut pu t S at ur at ion V olt ag e (V Io, Output Load Current (A) 80 50 20s ist er (k 8.0 5.0 1.0M R T , T im ing R e 2.0 0.8 10k 20k 50k100k 500k200k Ω fosc, Frequency (kHz) CT=10nF CT=5.0nF CT=2.0nF CT=1.0nF CT=500pF CT=200pF CT=100pF 100 30) 10R T(k Ω 3 1M100k10k1k100 fosc, Frequency(Hz) CT=100nF CT=47nF CT=22nF CT=10nF CT=4.7nF CT=2.2nF CT=10nF 80 60 40 10K 100K 1M1K100 0 -135 10 -180 0 -45 20 Fosc, Frequency (Hz) -90 10M A V OL , O pe n- Lo op V olt ag e Ga in (d Φ, E xc es s P ha se D eg re eGain Phase 元器件交易网www.cecb2b.com 元器件交易网www.cecb2b.com