FAN6862, 283xxx, AU501: 6-Pin PWM IC Pinout FB CS OUT RT GND VDD for SMPS Repair

User question

why RT left not connected in one design

Artificial Intelligence Response

Direct answer to the question

• In many PWM controllers, the RT pin is left unconnected because either (a) the IC provides a valid internal default switching frequency when RT floats, or (b) the particular, pin‑compatible variant makes Pin 4 a “no connect” or repurposes it (e.g., NTC/OTP input) that the design chose not to use. Designers exploit this to save cost/parts, reuse a universal PCB, or because precise frequency/OTP isn’t required for that product.
• Key points:
  • Behavior is IC‑specific; some controllers require RT, others allow it to float safely.
  • Fixed‑frequency or repurposed‑pin variants exist; the same footprint may have different Pin‑4 functions.
  • Leaving RT open can be acceptable only if the datasheet explicitly permits it.

Detailed problem analysis

• Internal default oscillator (RT optional)

  • Several low‑cost current‑mode flyback controllers implement an internal timing network and accept a floating RT pin as “use default fSW.” Internally, a trimmed current source charges an internal capacitor; if RT is high‑impedance, the device selects a factory‑set frequency (often around 60–70 kHz). Advantages:
  • Eliminates one precision resistor and its tolerance/temperature drift.
  • Reduces BOM and assembly time on high‑volume consumer SKUs.
  • Trade‑offs:
  • Wider frequency tolerance versus an external RT resistor.
  • Less control over EMI spectral placement and transformer optimization.

• Fixed‑frequency or repurposed Pin 4

  • Pin‑compatible families often offer “fixed‑frequency” versions where Pin 4 is NC internally. On a universal PCB, the RT footprint is simply left unpopulated when that IC is used.
  • Some families rename RT to NTC/OTP (thermistor input for over‑temperature protection). If OTP is not implemented, designers may leave the pin open or tie it to a safe level per datasheet (e.g., via divider to an internal reference). Caution: for these parts, a floating pin is acceptable only if the datasheet states an internal bias that defines a safe default state.

• Universal PCB strategy

  • Manufacturers commonly design one board to accept multiple controller options. Depending on what part is stuffed in production:
  • If a tunable‑frequency IC is installed, the RT resistor is populated.
  • If a fixed‑frequency/NC/OTP variant is installed, the RT pad is left empty.

• ICs that must not have RT floating

  • Classic controllers (e.g., SG3525/TL494‑style, many current‑mode PWM controllers) require RT (and often CT) to define the oscillator. Leaving RT open prevents oscillation or leads to undefined behavior. Always check the exact datasheet—pin names can coincide while behavior differs.

• EMI, acoustic noise, and magnetics implications

  • Frequency sets transformer core loss, copper loss skin effect, diode/MOSFET switching loss, and EMI spectral lines.
  • A floating‑RT default may pass in a low‑power “always‑on”/standby rail, but higher‑power adapters often need a defined frequency for EMI and magnetics sizing.
  • If audible noise occurs due to harmonics landing near the acoustic band, designers often pick a specific RT to shift fSW.

Current information and trends

• Modern offline controllers increasingly:
  • Integrate frequency jitter/spread‑spectrum and HV startup, reducing the need for external RT in some SKUs.
  • Offer pin‑compatible subsets (fixed‑frequency, OTP‑on‑pin4, brown‑in/OUT variants) to ease second‑sourcing and supply continuity.
• Market trend: universal PCBs plus multi‑sourcing. It’s common to see the same board shipped with different controllers across lots, with RT populated or not depending on the part installed.

Supporting explanations and details

• Oscillator principle (why a default exists): f ≈ Icharge/(Cint·ΔV). With known internal current and thresholds, the vendor can factory‑trim a nominal fSW. An external RT simply alters Icharge to tune fSW.
• Why cost matters: a single 1% SMD resistor plus placement time is a non‑trivial cost at millions of units; fewer parts also improve yield.
• Repurposed‑pin behavior: when Pin 4 is an NTC/OTP input, the IC commonly biases it with an internal current source. If unused, the datasheet typically instructs tying it to a defined level (e.g., via resistor to a reference) or permits it to float if an internal pull makes the “safe” state.

Ethical and legal aspects

• Safety: If Pin 4 is an OTP input and you leave it floating contrary to the datasheet, you may unintentionally disable thermal protection, risking overheating and noncompliance with product safety standards (e.g., IEC/UL 62368‑1).
• EMC/EMI: Undefined switching frequency can jeopardize regulatory compliance (FCC/EN 55032). Production must validate EMI with the chosen RT strategy.
• Reliability: Removing OTP or using a floating control pin without explicit allowance can reduce MTBF and violate internal design standards in medical/industrial/automotive contexts.

Practical guidelines

• Identify the exact controller:
  • Read the top‑mark, check the manufacturer’s datasheet for Pin‑4 function and recommended unused‑pin handling.
• Inspect the PCB:
  • Is there an unpopulated resistor footprint from Pin 4 to GND? If yes, the board supports tunable frequency; an installed fixed‑frequency/OTP variant explains the empty pad.
  • If Pin 4 pad has no trace (island), it’s likely an NC in this build.
• Measurement checks:
  • Scope switching node to see if fSW is near a common “default” (often ~65 kHz) and remains stable across load.
  • If substituting a tunable‑frequency part, temporarily tack a known RT (e.g., 100 kΩ to GND) and verify the expected frequency shift.
• Repairs/replacements:
  • Replacing a fixed‑frequency/OTP variant with a “standard” RT‑required IC will fail unless you add the proper RT network.
  • If Pin 4 is an NTC/OTP input and you are not using OTP, follow the datasheet’s instruction (usually tie to a defined level) rather than leaving it floating unless explicitly permitted.
• Documentation:
  • Record the exact IC PN/suffix installed per lot; pin‑function changes are common between close part numbers.

Possible disclaimers or additional notes

• Without the specific IC part number and schematic, the above are best‑practice scenarios; the correct action depends on the exact datasheet guidance for that controller.
• Some clones change pin functions versus “reference” parts despite identical footprints and similar markings.

Suggestions for further research

• Obtain and compare datasheets for the exact controller marking and its pin‑compatible variants to confirm Pin‑4 role and “unused pin” recommendations.
• Review vendor app notes on frequency selection, EMI, and OTP/NTC implementation for the controller family you have.
• If designing new hardware, run quick EMI pre‑scans with both floating‑default and RT‑defined frequency to quantify margin.

Brief summary

• RT is left unconnected in some designs because the controller either supports a valid default frequency with RT floating, or Pin 4 is NC/repurposed (e.g., OTP) and not used in that product. This saves parts and supports universal PCBs, but it is only correct if the datasheet for the specific IC allows it. For repairs or redesigns, always identify the exact part and follow its guidance; if you swap to an RT‑required device, you must populate the proper RT network. If you can share the IC marking and a photo/schematic snippet around Pin 4, I can confirm the intended behavior and recommend the exact RT value or tie‑off.

User question

sot23-6 PWM control IC with PIN 1 GND PIN2 FB PIN3 RT PIN4 CS PIN5 VDD PIN6 GDRV The printed code 01N541 .PIN3 NC .What could be the type of this IC .Thanks

Artificial Intelligence Response

Direct answer to the question

• Most probable type: a primary‑side flyback PWM controller in SOT‑23‑6, very likely a Richtek RT7736x (SmartJitter PWM flyback controller) variant.
• Why: Your pin map (1‑GND, 2‑FB/COMP, 3‑NC, 4‑CS, 5‑VDD, 6‑GDRV) matches RT7736 pin functions; RT7736’s Pin‑3 is a PRO/OTP/OVP input that may be left unconnected (seen as “NC”). Richtek top‑marks for RT7736 variants include two‑character product codes such as “0N” followed by a date/lot code, which fits your reading “01N541” (often read as “0N 541”). (digikey.kr)

Detailed problem analysis

• Functional class:
  • The presence of a dedicated “GDRV” pin (Pin‑6) that drives an external MOSFET gate, plus “CS” (current‑sense) and “VDD” in the 9–20 V range, identifies an offline flyback PWM controller rather than a buck regulator. Buck controllers in SOT‑23‑6 typically expose SW/BOOT pins instead of a high‑current gate driver for an external HV MOSFET.
• Pin compatibility:
  • RT7736x pinout (top view): 1‑GND, 2‑COMP (feedback via optocoupler), 3‑PRO (external OVP/OTP or ON/OFF), 4‑CS, 5‑VDD, 6‑GATE. If the design does not use external OVP/OTP or remote on/off, Pin‑3 is legitimately left open (“NC”), exactly as you observe. (digikey.kr)
• Frequency/UVLO hallmarks to confirm on the bench:
  • RT7736 nominal fSW ≈ 65 kHz with Richtek’s “SmartJitter” spread; UVLO ≈ 14.5 V (turn‑on) and ≈ 9 V (turn‑off). If you scope the gate (Pin‑6) you should see ~60–70 kHz at normal load and a characteristic burst/green mode at light load; VDD will start switching near 14.5 V. These are strong signatures to distinguish RT7736‑class parts. (digikey.kr)
• About the top mark:
  • Richtek shows two‑character product codes printed on SOT‑23‑6 (e.g., “00”, “09”, “0N”, “0F”, “0P”), followed by date/lot characters. It’s common for “0N541” (or similar) to be read as “01N541” under magnification. The “0N” product‑code is explicitly listed for RT7736DGE; other variants use “00”, “09”, etc. (digikey.kr)
• Why alternative suggestions (buck AP398x, CT5601, “1A541W”) are unlikely:
  • AP398x and similar synchronous bucks don’t provide a primary‑side flyback gate driver arrangement and usually expose SW/BOOT rather than a high‑current totem‑pole “GDRV”.
  • Primary‑side CV/CC charger chip families (e.g., CT560x) often repurpose pins, but their published pin functions and typical application circuits diverge from your map; no reliable top‑mark correlation to “0N/01N” exists.
  • “1A541W” claims online lack authoritative datasheet backing and typically trace to marketplace listings rather than primary documentation.

Current information and trends

• Green‑mode flyback controllers in SOT‑23‑6 (Richtek RT7736, Leadtrend LD5535E1, onsemi NCP1250/1251, On‑Bright OB2268, etc.) remain widely used for 5–30 W offline adapters and standby supplies. Many now integrate spread‑spectrum/jitter and flexible protection pins (OTP/OPP/OVP) that designers may leave unused (NC) to reduce BOM. (alldatasheet.com)
• Richtek’s RT7736 family continues to be available and documented; its pin‑3 usage (PRO or BNO on certain options) explains why you see “RT/NC” silkscreen carried over from a generic footprint. (digikey.kr)

Supporting explanations and details

• How your pins map to RT7736:
  • Pin‑1 GND → controller ground.
  • Pin‑2 FB → COMP (opto‑coupler to error amp).
  • Pin‑3 RT/NC → PRO/BNO (unused = NC allowed).
  • Pin‑4 CS → primary current sense.
  • Pin‑5 VDD → bias supply (~9–27 V, UVLO 9/14.5 V).
  • Pin‑6 GDRV → MOSFET gate driver. (digikey.kr)
• Board‑level tells:
  • A high‑value startup chain from the rectified HV bus into VDD, an auxiliary winding diode to VDD, an optocoupler from secondary to Pin‑2, and a low‑ohmic sense resistor from MOSFET source to CS confirm a flyback controller. (digikey.kr)

Ethical and legal aspects

• Safety: If this is on the primary (mains) side, replacement and probing must follow safe‑work practices (isolation transformer, differential probing, proper CAT‑rated instruments). Using non‑equivalent controllers can compromise OVP/OTP and creepage/clearance compliance (UL/IEC 62368‑1). Always verify protective functions after repair. (General regulatory practice; consult your local safety standards.)

Practical guidelines

• How to positively confirm identity in‑circuit:
  • With an isolation transformer and series bulb/VARIAC, monitor VDD: start switching near 14.5 V, stop near 9 V (UVLO window). Observe ~65 kHz switching at Pin‑6 with jitter at normal load. If both match, RT7736‑class is very likely. (digikey.kr)
• Replacement parts (check details before substituting):
  • Exact family: Richtek RT7736x (choose the option letter matching required protections on Pin‑3: PRO or BNO). (digikey.kr)
  • Close alternatives with same basic pin order (verify Pin‑3 function and UVLO!):
  • Leadtrend LD5535E1 (Pin‑2=COMP, Pin‑3=OTP/RI depending variant). (alldatasheet.com)
  • onsemi NCP1250/NCP1251 (Pin‑2=FB, Pin‑3=OPP/OVP; leaving Pin‑3 open disables OPP—use with caution). (digikey.in)
  • On‑Bright OB2268 (current‑mode PWM controller in SOT‑23‑6; verify CS/FB/RI mapping). (alldatasheet.es)
• Before swapping:
  • Check that your board does not use Pin‑3 for brown‑in/out, OTP or OVP. If it is NC on the PCB (no pad trace), the RT7736“G/R/L/E” subfamily (Pin‑3=PRO) can be drop‑in. If the footprint or BOM was meant for a “RI/RT” pin, you may still run on the internal fixed 65 kHz if supported, but verify EMI. (digikey.kr)

Possible disclaimers or additional notes

• The top‑mark alone is not definitive; different fabs use short codes. Use both marking AND electrical signatures (UVLO thresholds, fSW, gate waveform) to avoid mis‑ID.
• Some online claims (e.g., “1A541W”) lack primary datasheet support—treat with caution.

Suggestions for further research

• Under a microscope, re‑read the first two characters. If it’s “0N” (zero‑N), it matches a documented RT7736 product code. Cross‑check with your VDD/UVLO and fSW measurements. (digikey.kr)
• If you can share a clear photo of the chip area and surrounding opto/aux winding parts, we can verify whether the exact RT7736 option (B/D/F vs G/R/L/E) is needed (Pin‑3=BNO vs PRO). (digikey.kr)

Brief summary

• Given your pin map and “Pin‑3 = NC”, the device is almost certainly a SOT‑23‑6 flyback PWM controller; Richtek RT7736x fits both the electrical functions and the known two‑character top‑marking scheme (e.g., “0N DNN”). Confirm by checking ~65 kHz switching and 14.5/9 V UVLO; if confirmed, replace with the appropriate RT7736 variant or a pin‑compatible controller after verifying Pin‑3’s intended function. (digikey.kr)