Overview

The UC3842 (commonly UC3842N or UC3842B in an 8-pin DIP package) is the industry-standard fixed-frequency current-mode PWM controller IC, manufactured by Texas Instruments, STMicroelectronics, and ON Semiconductor. Designed for off-line AC-to-DC converters and high-voltage DC-DC flyback/forward power supplies, it directly drives high-voltage power MOSFETs.

Featuring high Undervoltage Lockout (UVLO) thresholds (16.0 V16.0\text{ V} turn-on / 10.0 V10.0\text{ V} turn-off), the UC3842 can self-start directly from rectified AC mains via a high-value trickle-charge resistor, then run off an auxiliary transformer winding.

Quick reference

Controller TypeFixed-Frequency Current-Mode PWM Controller
Package8-Pin DIP (Through-Hole) / SOIC-8
Supply Voltage Range (VCCV_{CC})12.0 V12.0\text{ V} to 30.0 V30.0\text{ V} DC (30 V30\text{ V} max)
UVLO ThresholdsTurn-ON: 16.0 V16.0\text{ V} / Turn-OFF: 10.0 V10.0\text{ V} (6V6\text{V} hysteresis)
Peak Gate Output Current±1.0 A\pm 1.0\text{ A} (Totem-Pole Driver for Power MOSFETs)
Max Duty CycleUp to 100%100\%
Oscillator FrequencyProgrammable up to 500 kHz500\text{ kHz} via external RT/CTR_T / C_T
Internal Reference+5.0 V±1%+5.0\text{ V} \pm 1\% trimmed internal reference voltage (VREF)

Pinout (8-Pin DIP Package)

text
        ┌──────────┐
  COMP ─│ 1      8 │─ VREF
   VFB ─│ 2      7 │─ VCC
ISENSE ─│ 3      6 │─ OUTPUT
 RT/CT ─│ 4      5 │─ GND
        └──────────┘
PinNameDescription
1COMPError amplifier output pin (connect loop compensation network to VFB)
2VFBVoltage feedback sense input pin (2.5V2.5\text{V} internal reference)
3ISENSECurrent sense input pin (monitors primary MOSFET source resistor voltage drop)
4RT/CTOscillator timing pin (resistor RTR_T to VREF, capacitor CTC_T to GND)
5GNDGround reference (0 V)
6OUTPUTHigh-current totem-pole gate output (drives external N-channel MOSFET gate)
7VCCPower supply voltage input (+12V to +30V DC)
8VREF+5.0V+5.0\text{V} precision reference voltage output

Specifications

ParameterSymbolMinTypMaxUnitConditions
Startup Voltage ThresholdVTH(ON)V_{TH(ON)}14.516.017.5VVCCV_{CC} rising
Shutdown Voltage ThresholdVTH(OFF)V_{TH(OFF)}8.510.011.5VVCCV_{CC} falling after startup
Feedback Reference VoltageVFBV_{FB}2.452.502.55VTJ=25∘CT_J = 25^\circ\text{C}
Gate Peak Output CurrentIOUT_PEAKI_{OUT\_PEAK}±0.8\pm 0.8±1.0\pm 1.0—ASourcing / Sinking peak
Current Sense ThresholdVCS_MAXV_{CS\_MAX}0.91.01.1VMaximum current limit voltage
Startup Trickle CurrentISTARTI_{START}—0.51.0mAVCC=14VV_{CC} = 14\text{V} (before startup)
Operating Supply CurrentICCI_{CC}—1117mANormal switching state

Typical Application Circuit (Flyback SMPS Controller)

text
        +300V DC Rectified Mains
           │
        [Primary Winding] ─── [MOSFET Drain]
           │                        │
       [100kΩ Startup]      [Pin 6: OUTPUT] ─── [10Ω] ──> MOSFET Gate
           │                        │
        [Pin 7: VCC]             UC3842
           │                        │
        [10µF Cap]         [Pin 3: ISENSE] ─── [MOSFET Source] ─── [0.47Ω Sense Resistor] ─── GND
           │                        │
          GND                      GND

fOSC=1.72RT×CTf_{OSC} = \frac{1.72}{R_T \times C_T}

Common mistakes

  • Attempting to run off a 12V rail without reaching 16V startup: The UC3842 requires 16.0 V16.0\text{ V} minimum on VCCV_{CC} to turn ON initially. Connecting it to a fixed 12V supply means it will NEVER start up. Use the UC3843 (8.4 V8.4\text{ V} turn-on threshold) for 12V systems instead!
  • Omitting RC filter on ISENSE pin: Switching spikes on the MOSFET source resistor can prematurely trigger current limiting. Add a small RC filter (1 kΩ1\text{ k}\Omega and 470 pF470\text{ pF}) between the current sense resistor and Pin 3 (ISENSE).

Notes

  • UC3842 vs UC3843 vs UC3844 vs UC3845: UC3842 has 16V/10V UVLO (100% duty cycle); UC3843 has 8.4V/7.6V UVLO (100% duty cycle); UC3844/45 limit max duty cycle to 50%.

Assets & downloads