Data

®

UC3842T UC3843T UC3844T UC3845T

HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

. . . . . . . . .

TRIMMED OSCILLATOR FOR PRECISE FREQUENCY CONTROL OSCILLATOR FREQUENCY GUARANTEED AT 250kHz CURRENT MODE OPERATION TO 500kHz AUTOMATIC FEED FORWARD COMPENSATION LATCHING PWM FOR CYCLE-BY-CYCLE CURRENT LIMITING INTERNALLY TRIMMED REFERENCE WITH UNDERVOLTAGE LOCKOUT HIGH CURRENT TOTEM POLEOUTPUT UNDERVOLTAGE LOCKOUT WITH HYSTERESIS LOW START-UP AND OPERATING CURRENT

Minidip

SO8

DESCRIPTION The UC384XT family of control ICs provides the necessary features to implement off-line or DC to DC fixed frequency current mode control schemes with a minimal external parts count. Internally implemented circuits include a trimmed oscillator for precise DUTY CYCLE CONTROL under voltagelockout featuring start-up current less than 0.5mA, a precision reference trimmed for accuracy at the error amp input, logic to insure latched operation, a PWM

comparator which also provides current limit control, 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 offstate. Differences between membersof this family are the under-voltage lockout thresholds and maximum duty cycle ranges. The UC3842T and UC3844T have UVLO thresholds of 16V (on) and 10V (off), ideally suited to off-line applications The corresponding thresholds for the UC3843T and UC3845T are 8.5 V and 7.9 V. The UC3842T and UC3843T can operate to duty cycles approaching 100%. A range of zero to < 50 % is obtained by the UC3844Tand UC3845T by the addition of an internal toggle flip flop which blanks the output off every other clock cycle.

BLOCK DIAGRAM (toggle flip flop used only in UC3844T and UC3845T)
Vi 7 34V GROUND 5 UVLO S/R 5V REF INTERNAL BIAS VREF GOOD LOGIC RT/CT 4 OSC ERROR AMP. 2R R 1V T

8

VREF 5V 50mA

2.50V

6

OUTPUT

VFB COMP CURRENT SENSE

2 1 3

+ -

S R CURRENT SENSE COMPARATORD95IN331

PWM LATCH

September 2001

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ABSOLUTE MAXIMUM RATINGS
Symbol Vi Vi IO EO Parameter Supply Voltage (low impedance source) Supply Voltage (Ii < 30mA) Output Current Output Energy (capacitive load) Analog Inputs (pins 2, 3) Error Amplifier Output Sink Current Ptot Ptot Tstg TL Power Dissipation at Tamb ≤ 25 °C (Minidip) Power Dissipationat Tamb ≤ 25 °C (SO8) Storage Temperature Range Lead Temperature (soldering 10s) Value 30 Self Limiting ±1 5 – 0.3 to 5.5 10 1.25 800 – 65 to 150 300 A µJ V mA W mW °C °C Unit V

* All voltages are with respect to pin 5, all currents are positive into the specified terminal.

PIN CONNECTION (top view) Minidip/SO8

COMP VFB ISENSE RT/CT

1 2 3 4
D95IN332

8 7 6 5

VREF Vi OUTPUT GROUNDPIN FUNCTIONS
No 1 2 3 4 5 6 7 8 Function COMP VFB ISENSE RT/CT GROUND OUTPUT VCC Vref Description This pin is the Error Amplifier output and is made available for loop compensation. This is the inverting input of the Error Amplifier. It is normally connected to the switching power supply output through a resistor divider. A voltage proportional to inductor current is connected to this input.The PWM uses this information to terminate the output switch conduction. The oscillator frequency and maximum Output duty cycle are programmed by connecting resistor RT to Vref and cpacitor CT to ground. Operation to 500kHz is possible. This pin is the combined control circuitry and power ground. This output directly drives the gate of a power MOSFET. Peak currents up to 1A are sourced and sunkby this pin. This pin is the positive supply of the control IC. This is the reference output. It provides charging current for capacitor C T through resistor RT.

ORDERING NUMBERS
SO8 UC3842TD UC3843TD UC3844TD UC3845TD Minidip UC3842TN UC3843TN UC3844TN UC3845TN

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THERMAL DATA
Symbol Rth j-amb Description Thermal Resistance Junction-ambient....