Fairchild Semiconductor KA3842ACS
Full Technical Specifications
11 Parameters| Parameter / Attribute | Specification / Value |
|---|---|
| Mounting | Through Hole |
| Voltage - start up | 16 V |
| Voltage - supply (Vcc (Vdd)) | 10V ~ 30V |
| Frequency - switching | Up to 500kHz |
| Duty cycle | 96% |
| Operating temperature | 0°C ~ 70°C (TA) |
| Package | Tube |
| Case | 8-DIP (0.300\", 7.62mm) |
| Control features | Frequency Control |
| Fault protection | Current Limiting |
| Internal switch | No |
Need Stock or a Drop-in Second-Source for KA3842ACS?
Package: DIP-8 / SOIC-8 · Pin-compatible replacements with immediate Shenzhen spot stock
The UC384x family is the current-mode PWM controller inside a generation of offline switching supplies, adapters, chargers and ATX standby rails - an 8-pin part that appears in repair in every country, and the most-fitting-wrong-part in power-supply benches. The four suffixes are not interchangeable decorations: the start-up threshold is either 16V (3842/3844, offline rails) or 8.4V (3843/3845, low-voltage rails), and the maximum duty cycle is either full (3842/3843) or 50 percent (3844/3845, for half-bridge and push-pull topologies). The wrong suffix shows up as a supply that will not start, or a transformer that saturates.
Verified Pin-Compatible Replacements (4 verified alternatives)
The Low-Voltage Start Sibling
Shenzhen Spot Stock (24-48h Dispatch)
Identical current-mode PWM engine with the UVLO window shifted down: 8.4V turn-on / 7.6V turn-off against 16V / 10V for the UC3842. Full duty cycle, 500kHz class oscillator, 5V reference, totem-pole output.
Works from lower VCC rails - the part for DC-DC converters running from 12V and 24V buses.
The 50% Duty Offline Sibling
Shenzhen Spot Stock (24-48h Dispatch)
Same 16V UVLO as the 3842 with the output limited to 50 percent maximum duty by an internal toggle flip-flop - the part for half-bridge and push-pull offline converters.
16V turn-on / 10V turn-off; the offline start-up window.
Low Start and 50% Duty Combined
Shenzhen Spot Stock (24-48h Dispatch)
The fourth quadrant of the matrix: 8.4V UVLO with the 50 percent duty flip-flop - low-voltage push-pull and half-bridge converters.
8.4V turn-on / 7.6V turn-off.
The Industrial-Temperature Grade
Shenzhen Spot Stock (24-48h Dispatch)
Electrically the same controller in the UC284x (industrial temperature) grade against the UC384x (commercial) - same thresholds and duty per suffix digit.
Same UVLO scheme; the 2 in the prefix is temperature range, not function.
Buyer & Quality Checklist for Sourcing Alternatives
- Read two numbers before any substitution: UVLO start threshold (3842/3844 = 16V, 3843/3845 = 8.4V) and maximum duty (3842/3843 = full, 3844/3845 = 50 percent). Wrong threshold = will not start or starts on brown-out; wrong duty = underperformance or transformer saturation.
- On the 50-percent parts (3844/3845) the oscillator runs at twice the switching frequency by design - when repairing, set RT/CT for the doubled value or everything downstream shifts.
- The start-up network is part of the chip choice: offline designs charge VCC through a resistor from the high rail, and the aux winding must take over before the 10V turn-off. Fitting a lower-threshold suffix changes when that handover happens.
- Current-sense input tolerances and leading-edge blanking are fixed in silicon - if a repair needed a snubber change to behave, suspect the wrong suffix or a clone die before redesigning the sense network.
- The clone market here is deep: relabeled no-name dies pass DC tests and drift on frequency. For fleet repairs, keep verified lots and photograph the markings.
- UC184x / UC284x / UC384x are temperature grades of the same function - use the industrial grade for hot enclosures rather than substituting down.
Frequently Asked Technical Questions
Q: What is the difference between UC3842 and UC3843?
A: The start-up threshold: UC3842 turns on at 16V (offline supplies), UC3843 at 8.4V (low-voltage DC-DC). Everything else - including full duty capability - is the same. Match the original suffix so the start-up network behaves as designed.
Q: Can I use a UC3842 where a UC3844 was fitted?
A: No - the 3844 is the 50-percent-duty part used in push-pull and half-bridge designs. A full-duty part in that socket can saturate the transformer and destroy the switching devices. The last digit encodes the duty limit; respect it.
Q: Why does my supply not start after a PWM chip replacement?
A: Check the suffix first: a 3843/3845 fitted into an offline design starts at 8.4V and the bias winding handover may misbehave, and a 16V part in a low-voltage design never reaches its threshold. Wrong-threshold swaps are the most common self-inflicted repair failure on this family.