UC3845BD1

UC2842B/3B/4B/5B UC3842B/3B/4B/5B HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER . TRIMMED OSCILLATOR FOR PRECISE FRE- QUENCY CONTROL OSCILLATOR FREQUENCY GUARANTEED . AT 250kHz . CURRENT MODE OPERATION TO 500kHz .AUTOMATIC FEED FORWARD COMPENSA- TION Minidip SO8 .LATCHING PWM FOR CYCLE-BY-CYCLE CURRENT LIMITING .INTERNALLY TRIMMED REFERENCE WITH UNDERVOLTAGE LOCKOUT comparator which also provides current limit control, HIGH CURRENT TOTEM POLE OUTPUT . and a totem pole output stage designed to source UNDERVOLTAGE LOCKOUT WITH HYSTER- . or sink high peak current. The output stage, suitable ESIS for driving N-Channel MOSFETs, is low in the off- . LOW START-UP AND OPERATING CURRENT state. Differences between members of this family are the DESCRIPTION under-voltage lockout thresholds and maximum duty The UC384xB family of control ICs provides the nec- cycle ranges. The UC3842B and UC3844B have essary features to implement off-line or DC to DC UVLO thresholds of 16V (on) and 10V (off), ideally suited off-line applications The corresponding thresh- fixed frequency current mode control schemes with a minimal external parts count. Internally imple- olds for the UC3843B and UC3845B are 8.5 V and 7.9 mented circuits include a trimmed oscillator for pre- V. The UC3842B and UC3843B can operate to duty cise DUTY CYCLE CONTROL under voltage lock- cycles approaching 100%. A range of the zero to < out featuring start-up current less than 0.5mA, a pre- 50 % is obtained by the UC3844B and UC3845B by cision reference trimmed for accuracy at the error the addition of an internal toggle flip flop which blanks amp input, logic to insure latched operation, a PWM the output off every other clock cycle. BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B) 7 Vi UVLO 34V 8 5V VREF 5 S/R REF 5V 50mA GROUND INTERNAL BIAS 2.50V VREF GOOD LOGIC 6 OUTPUT 4 RT/CT OSC T ERROR AMP. 2R + S 2 VFB - R PWM R1V LATCH 1 COMP CURRENT SENSE 3 UC3842B CURRENT COMPARATOR SENSE D95IN331 March 1999 1/15UC2842B/3B/4B/5B - UC3842B/3B/4B/5B ABSOLUTE MAXIMUM RATINGS Symbol Parameter Value Unit V Supply Voltage (low impedance source) 30 V i V Supply Voltage (Ii < 30mA) Self Limiting i I Output Current A O 1 EO Output Energy (capacitive load) 5 J Analog Inputs (pins 2, 3) 0.3 to 5.5 V Error Amplifier Output Sink Current 10 mA P 1.25 W tot Power Dissipation at T 25 C (Minidip) amb P 800 mW tot Power Dissipation at Tamb 25 C (SO8) T Storage Temperature Range 65 to 150 C stg T Junction Operating Temperature 40 to 150 C J T Lead Temperature (soldering 10s) 300 L C * All voltages are with respect to pin 5, all currents are positive into the specified terminal. PIN CONNECTION (top view) Minidip/SO8 COMP 1 8 V REF V 2 7 Vi FB I 3 6 OUTPUT SENSE R /C 4 5 GROUND T T D95IN332 PIN FUNCTIONS No Function Description 1 COMP This pin is the Error Amplifier output and is made available for loop compensation. 2V This is the inverting input of the Error Amplifier. It is normally connected to the switching FB power supply output through a resistor divider. 3I A voltage proportional to inductor current is connected to this input. The PWM uses this SENSE information to terminate the output switch conduction. 4R /C The oscillator frequency and maximum Output duty cycle are programmed by connecting T T resistor R to Vref and cpacitor C to ground. Operation to 500kHz is possible. T T 5 GROUND 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 1A are sourced and sunk by this pin. 7V This pin is the positive supply of the control IC. CC 8V This is the reference output. It provides charging current for capacitor C through resistor R . ref T T ORDERING NUMBERS SO8 Minidip UC2842BD1 UC3842BD1 UC2842BN UC3842BN UC2843BD1 UC3843BD1 UC2843BN UC3843BN UC2844BD1 UC3844BD1 UC2844BN UC3844BN UC2845BD1 UC3845BD1 UC2845BN UC3845BN 2/15