UC3843 — Current-Mode PWM Controller (8.4V/7.6V UVLO)

Overview

The UC3843 (commonly UC3843N, UC3843AN, or UC3843B in an 8-pin DIP package) is a fixed-frequency current-mode PWM controller IC manufactured by Texas Instruments, ON Semiconductor, and STMicroelectronics. Optimized for low-voltage power supplies, automotive converters, and 12V DC-DC switch-mode power supplies (SMPS), it directly drives high-power N-channel MOSFET gates.

Unlike the high-voltage UC3842 (16V startup), the UC3843 features lower Undervoltage Lockout (UVLO) thresholds ( $8.4\text{ V}$ turn-on / $7.6\text{ V}$ turn-off), allowing it to operate reliably from nominal $12\text{ V}$ power rails or battery supplies.

Quick reference


Controller Type	Low-Voltage Current-Mode PWM Controller
Package	8-Pin DIP (Through-Hole) / SOIC-8
Supply Voltage Range ( $V_{CC}$ )	$7.6\text{ V}$ to $30.0\text{ V}$ DC ( $30\text{ V}$ max)
UVLO Thresholds	Turn-ON: $8.4\text{ V}$ / Turn-OFF: $7.6\text{ V}$ ( $0.8\text{V}$ hysteresis)
Peak Gate Output Current	$\pm 1.0\text{ A}$ (Totem-Pole Driver for Power MOSFETs)
Max Duty Cycle	Up to $100\%$
Oscillator Frequency	Programmable up to $500\text{ kHz}$ via external $R_T / C_T$
Internal Reference	$+5.0\text{ V} \pm 1\%$ 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
        └──────────┘

Pin	Name	Description
1	`COMP`	Error amplifier output pin (connect loop compensation network to VFB)
2	`VFB`	Voltage feedback sense input pin ( $2.5\text{V}$ internal reference)
3	`ISENSE`	Current sense input pin (monitors primary MOSFET source resistor voltage drop)
4	`RT/CT`	Oscillator timing pin (resistor $R_T$ to VREF, capacitor $C_T$ to GND)
5	`GND`	Ground reference (0 V)
6	`OUTPUT`	High-current totem-pole gate output (drives external N-channel MOSFET gate)
7	`VCC`	Power supply voltage input (+7.6V to +30V DC)
8	`VREF`	$+5.0\text{V}$ precision reference voltage output

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Startup Voltage Threshold	$V_{TH(ON)}$	7.8	8.4	9.0	V	$V_{CC}$ rising
Shutdown Voltage Threshold	$V_{TH(OFF)}$	7.0	7.6	8.2	V	$V_{CC}$ falling after startup
Feedback Reference Voltage	$V_{FB}$	2.45	2.50	2.55	V	$T_J = 25^\circ\text{C}$
Gate Peak Output Current	$I_{OUT\_PEAK}$	$\pm 0.8$	$\pm 1.0$	—	A	Sourcing / Sinking peak
Current Sense Threshold	$V_{CS\_MAX}$	0.9	1.0	1.1	V	Maximum current limit voltage
Operating Supply Current	$I_{CC}$	—	11	17	mA	Normal switching state

Typical Application Circuit (12V DC-DC Converter Controller)

text

        +12V DC Input Rail
           │
        [10Ω Resistor]
           │
        [Pin 7: VCC]             UC3843 ───── [Pin 6: OUTPUT] ─── [10Ω] ──> Power MOSFET Gate
           │                        │
        [10µF Cap]         [Pin 3: ISENSE] ─── [MOSFET Source] ─── [0.22Ω Sense Resistor] ─── GND
           │                        │
          GND                      GND

$f_{OSC} = \frac{1.72}{R_T \times C_T}$

Common mistakes

Exceeding 30V on VCC: While the UC3843 turns on at 8.4V, its absolute maximum supply voltage is $30.0\text{ V}$ . Operating above 30V without a zener clamp will destroy the chip.
Omitting decoupling on VREF: Pin 8 (VREF) supplies 5.0V to internal comparator blocks and external timing resistors. Always bypass Pin 8 to GND with a $100\text{ nF}$ ceramic capacitor.

Notes

UC3843 vs UC3842: UC3843 starts up at $8.4\text{ V}$ (ideal for 12V supplies and DC-DC converters), whereas UC3842 requires $16.0\text{ V}$ (designed for AC mains offline supplies).

Interface	Gate Output, Current Sense ISENSE, Feedback VFB, Error Amp COMP, RT/CT
Supply	7.6–30 V
Package	8-pin PDIP / SOIC-8