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| PRELIMINARY AIC3842 Current-Mode PWM Controller n FEATURES l Low Start-Up Current (300A Typical) l Internal Precision Reference. l 500KHz Current-Mode Operation. l Pulse-by-Pulse Current Limiting. l Automatic Feed-Forward Compensation. l Optimized for Off-Line and DC/DC Converters. l Undervoltage Lockout with Hysteresis. l Double Pulse Suppression. l High Current Totem-Pole Output. n DESCRIPTION The AIC3842 control IC provides the features that are necessary to implement off-line or DC/DC Converter fixed-frequency current-mode schemes with a minimum number of external components. This integrated circuits features an undervoltage lockout (UVLO) with approximately 300A start-up current, a precision reference trimmed for accuracy at the error amplifier input, high gain error amplifier, current sensing comparator, logic to insure latched operation, and a totem-pole output stage designed to source or sink high peak current. The output stage, suitable for driving N channel MOSFETs, is low in the off state. n APPLICATIONS l Off-Line Converter. l DC/DC Converter. n TYPICAL APPLICATION CIRCUIT 2 C2 R3 51K FEEDBACK SIGNAL 300pF C3 1000P 6 OUTPUT GND 5 VFB VCC 7 + C1 47F VCC INDUCTOR M1 R2 20K R5 C6 300P 390 C7 1nF RS 0.33 1 8 COMP VREF R1 10 R4 7.5K C5 0.1F 4 RT/CT ISENSE 3 AIC3842 C4 2.2nF Current Mode PWM Control Circuit n ORDERING INFORMATION AIC3842CX PACKAGE TYPE N: PLASTIC DIP S: SMALL OUTLINE ORDER NUMBER AIC3842CN (PLASTIC DIP) COMP PIN CONFIGURATION 1 8 VREF 7 VCC 6 OUTPUT 5 GND VFB 2 ISENSE 3 AIC3842CS (PLASTIC SO) RT/CT 4 Analog Integrations Corporation DS-3842-P1 May 5, 00 4F, 9, Industry E. 9th Rd, Science Based Industrial Park, Hsinchu Taiwan, ROC TEL: 886-3-5772500 FAX: 886-3-5772510 www.analog.com.tw 1 AIC3842 n ABSOLUTE MAXIMUM RATINGS Supply Voltage (Low Impedance Source ) Supply Voltage (ICC <300mA) ... ............. ... ... ... ................ ... ... ........................ 30V ...................... ..... ... ... ... .... ... ... ..... ................... ... ... .......... Self Limiting ...................... ........... ... ... ... ... .... ............ ... ... .... ...................... 5J Output Current ................. ... ... ... ... ... ... ... ... ....... ... ... ... ....................... ... ... .... ... ... ............ ......... 1A Output Energy (Capacitive Load) Analog Inputs (Pins 2, 3) ... ........ ... ... ... ... ... ... ... ...... ......... ... ... ... .................... ... ... .... -0.3V to +6.3V Error Amp Output Sink Current ......... ....... .... ... ...... ... ... ... ................... ... ... .... ... ... ... ............... 10mA Operation Temperature Range ......... ....... .... ... ...... ... ... ... .................... ... ... ... ............... -40C~85C ......... ... ... ... ... ... ... ... ... ....... ... ... ... ..... ... ............... 1W ............ ... .... ..... ........ ... ... ... ... ... ... ............. 725mW -65C to +150C SOIC Package Storage Temperature Range Note 1: Power Dissipation at TA 25C DIP Package ........ ... ...... ... ... ... .............................. ... ......... ... ... ... Lead Temperature (Soldering, 10 seconds) ........ ... ...... ... ... ... ..... ... ... ... ... .... ... ....... ... ........... 300C All voltages are with respect to Pin 5. All currents are positive into the specified terminal. n TEST CIRCUIT VREF RT R5 4.7K Q2 2N2222 COMP Error Amp. Adjust R7 4.7K R4 100K VFB R6 10K ISENSE Adjust R3 5K ISENSE RT/CT AIC3842 GROUND CT VCC OUTPUT VREF C2 0.1F R1 1K 1W C1 0.1F OUTPUT GND A S1 VCC 2 AIC3842 n ELECTRICAL CHARACTERISTICS {V specified.} PARAMETERS CONDITIONS CC = 15V (see Note 2), Ta=25C, unless otherwise MIN. TYP. MAX. UNIT Reference Section Output Voltage Line Regulation Load Regulation Temperature Coefficient of Output Voltage Output Noise Voltage Output Voltage Long-Term Drift Short-Circuit Output Current Oscillator Section Oscillator Frequency (see Note 3) Frequency Change with Supply Voltage Frequency Change with Temperature Peak-to-Peak Amplitude at RT/CT Error Amplifier Section Feedback Input Voltage Input Bias Current Open-Loop Voltage Amplification Gain-Bandwidth Product Supply Voltage Rejection Ratio Output Sink Current Output Source Current High-Level Output Voltage Low-Level Output Voltage VFB at 2.7V, COMP at 1.1V VFB at 2.3V, COMP at 5V VFB at 2.3V, RL=15K to GND VFB at 2.7V, RL=15 to VREF 10 1 6.2 0.8 mA mA V V VCC =12V to 25V 1 70 MHz dB VO=2V to 4V COMP at 2.5V 2.42 2.50 -0.3 90 2.58 V A dB VCC=12V to 25V 47 52 0.2 57 KHz % 85 mA f=10Hz to 10KHz After 1000H at TA=25C 50 5 V mV IO=1mA VCC=12V to 25V IO=1mA to 20mA 4.9 5 5 5 0.2 5.1 20 25 V mV mV mV/C TA=TLOW to THIGH 5 1.7 % V 3 AIC3842 n ELECTRICAL CHARACTERISTICS (Continued) PARAMETERS Current Sense Section Voltage Amplification Current Sense Comparator Threshold Supply Voltage Rejection Ratio Input Bias Current Delay Time to Output Output Section High-Level Output Voltage ISOURCE=20mA ISOURCE =200mA Low-Level Output Voltage ISINK=20mA ISINK=200mA Rise Time Fall Time Undervoltage Lockout Section Start Threshold Voltage Minimum Operating Voltage after Start-Up Pulse-Width-Modulator Section Maximum Duty Cycle Minimum Duty Cycle Supply Voltage Start-Up Current Operating Supply Current Limiting Voltage VFB and ISENSE at 0V ICC =25mA 0.3 12 34 0.5 17 mA mA V 96 0 % % 14.5 8.5 16 10 17.5 11.5 V V CL=1nF CL=1nF 13.5 13.4 0.1 1.5 50 50 V V V V nS nS -2 150 A nS VCC =12V to 25V, See Note 3 70 dB See Note 3 and 4 COMP at 5V, See Note 3 3 1 V/V V CONDITIONS MIN. TYP. MAX. UNIT Note: 2: Adjust VCC above the start threshold before setting it to 15V. 3. These parameters are measured at the trip point of the latch w VFB at 0V. ith 4. Voltage amplification is measured between ISENSE and COMP with the input changing from 0V to 0.8V. 4 AIC3842 n BLOCK DIAGRAM VCC 7 ZD 34V UVLO S/R 5V REF Internal Bias 2.50V 4 RT/CT OSC Error Amp. 2 VFB 1 COMP 3 ISENSE D1 D2 R2 Z1 1V PWM Latch Q2 VREF Good Logic 8 VREF 5V/50mA 5 GND Q1 6 OUTPUT S R Current Sense comparator n PIN DESCRIPTIONS PIN 1: COMP - This pin is the error amplifier output and is made available for loop compensation. PIN 2: VFB - This is the inverting input of the error amplifier. It is normally connected to the switching PIN 7: VCC power supply output through a resistor divider. PIN 3: ISENSE - A voltage proportional to PIN 5: GNDThis pin is the combined control circuitry and power ground. PIN 6: OUTPUT- This output directly drives the gate of a power MOSFET. Peak currents up to 1A are sourced and sunk by this pin. This pin is the positive supply of the control IC. PIN 8: VREF This is the reference output. It provides charging current for capacitor CT through resistor RT. inductor current is connected to this input. The PWM uses this information to terminate the output switch conduction. PIN 4: RT/CT - The oscillator frequency by and maximum output duty cycle are programmed CT to connecting Operation to resistor RT to VREF and capacitor ground. 500KHz is feasible. 5 AIC3842 n APPLICATION INFORMATIONS l Undervoltage Lockout There are two separate undervoltage lockout comparators incorporated to make sure that the IC is fully functional before the output stage is enabled. One is for power supply voltage (VCC) and the other is for reference output voltage (VREF). Each has a built in hysteresis to prevent erratic output behavior when their respective thresholds are crossed. For VCC comparator the upper and lower thresholds are 16V and 10V, respectively. For VREF compactor the upper and lower threshold are 3.6V and 3.4V, respectively. The large hysteresis and low start up current (0.3mA) of the AIC3842 make it ideally suited in off-line converter applications where efficient bootstrap startup techniques are required. A 34V zener is connected as a shunt regulator from VCC to ground. Its purpose is to protect the IC from excessive voltage that can occur during system start-up. Oscillator The oscillator frequency can be programmed through the setting of timing components RT and CT. Capacitor CT is charged from the 5.0V reference output through R to about 2.8V and T discharged to about 1.2V by the internal discharge current. When CT is discharged the output (pin 6) must be in the low state, thus producing a controlled amount of output deadtime. Note that many values of R and C T T can produce the same frequency but only one combination will yield a specific output deadtime at a given frequency. Current Sense Comparator and PWM Latch The output switch of AIC3842 is initiated by the oscillator and terminated when the peak inductor current reaches the threshold level established by the error amplifier output (pin 1). The AIC3842 is operated at a current mode since the inductor current is monitored cycle-by-cycle and decides the duty cycle. The inductor current is converted to a voltage by inserting the ground referenced sense resistor RS in series with the source of output switch M1. This voltage is monitored by the current sense input (pin 3) and is compared to a level derived from the error amplifier output. In the normal operating conditions the peak inductor current is controlled by the voltage at pin 1 where IPK = V ( pin 1)- 1. 4V 3RS Reference Output The 5.0V reference output is trimmed to 2.0% tolerance at T =25C. It supplies charging current A to the oscillator timing capacitor and is capable of providing current in excess of 20mA for powering additional control system circuitry. In case of overload, the reference is short-circuit protected at about 85mA. Error Amplifier A fully compensated error amplifier is provided with inverting input and output externally accessible. The noninverting input is internally biased at 2.5V. The converter output voltage is usually divided down and connected to the inverting input. The output of the error amplifier is accessible for external loop compensation, with an offset at two diode drops ( 1.4V) and divided by three, before connected to the inverting input of the current sense comparator. This guarantees that no drive pulse appears at the output (pin 6). PWM Latch is used to ensure that only a single pulse appears at the output during any given oscillator cycle. However, a narrow spike on the leading edge of the current waveform can usually be observed and may cause the power supply to exhibit an instability when the output is tightly loaded. Output Switch The AIC3842 contains a single totem-pole output stage that was specifically designed for direct drive of power MOSFET. If any undervoltage lockout is deetected, internal circuitry will keep the output switch in a sinking current mode, no external pull down resistor is needed. 6 AIC3842 n PHYSICAL DIMENSIONS l 8 LEAD PLASTIC SO (unit: mm) D SYMBOL A A1 H E MIN 1.35 0.10 0.33 0.19 4.80 3.80 MAX 1.75 0.25 0.51 0.25 5.00 4.00 B C D e A C A1 E e H L L 1.27(TYP) 5.80 0.40 6.20 1.27 B l 8 LEAD PLASTIC DIP (unit: mm) D SYMBOL A1 E1 MIN 0.381 2.92 0.35 0.20 9.01 7.62 6.09 -- 2.92 MAX -- 4.96 0.56 0.36 10.16 8.26 7.12 10.92 3.81 A2 b C E D E C A2 A1 L E1 e eB L 2.54 (TYP) eB b e 7 |
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