New Triac dimmable LED Controller IC is a really an Old Dog with new circuit tricks!

Dec 14, 2010   //   LED News

One of the hottest “new” HB triac dimmable off-the-line HB LED drivers is an oldie but goody dressed up with some clever circuit design tricks to make it one of the most cost effective dimming solutions on the market. I am speaking about the “new” Texas Instruments TPS92001 ( tps92001), which is advertised as a “General Purpose LED Lighting Controller” made using the BCDMOS process.


  • Ideal for Single Stage Designs
  • Supports Isolated and Non-Isolated Topologies
  • Phase-Cut TRIAC Dimmable
  • Few External Components Mode Operation
  • Wide Duty Cycle Range for Wide-Input Voltage or Dimming Range
  • Convenient 5-V Reference Output
  • Undervoltage Lockout for Safe Operation
  • Operation to 1-MHz
  • 0.4-A Source/0.8-A Sink FET Driver
  • Low 100-μA Startup Current

But wait! This part is really an old Unitrode UCC1809/280/3809 device that was released in the early 2000 era described THEN as an “Economy Primary Side Controller” for current control applications. Unitrode Semiconductor was a power management IC company purchased by TI back in 1999. But with some REALLY clever external circuit design tricks, it makes one of the best and most cost effective controllers on the market, more than competitive with the industry standard National Semiconductor LM3445 device at a CONSIDERABLE cost savings.

Take at look at the relative simplicity of a non-isolated off-the line triac dimmable circuit that can supply about 700 ma at 40 VDC (about 30 W) shown here from the datasheet (PMP5924RevC_sch). The device functions as a negative Buck converter that generates a negative voltage output to the LED pins. By using the negative buck converter configuration, N-Channel Mosfet Q2 can switch the ground side of the circuit, eliminating the need for an expensive charge-pump gate driver circuit on the IC U1. Current sensing is performed on the low side by resistors R8/R10 in series to allow low cost SMT components. This signal is feed back to the FB pin of U1 to enable closed loop control.

Now the cleverness of circuit design comes in: Op amp U2 (TL331KBDV) is used as a comparator to detect the zero cross of the current and to turn on Q4. Q4 puts a “dummy load” (R20 1K/2W) across the line input around the ever zero cross of the AC waveform, 120 times for second for a 60 HZ power input. Its purpose is to keep the triac CONDUCTING with a minimum holding current. So the triac turns ON and OFF at about a 10V level but sees sufficient current to insure it does not SHUT off or HALF cycle on AC waveform.

  • Transistor Q3 proves a little feed-forward compensation for the current control of the IC, thus accomplishing a poor-mans Power Factor Correction (PFC) WITHOUT a boost converter. CLEVER!
  • Transistor Q1 and Zener Z5 function as a simple shunt regulator to supply the low voltage Vdd of the chip of about 12 VDC.
  • Inductors L1 and L3 perform a dual function of EMI/EMC. They help reduce the radiated EMI generated by the 1 MHZ switching buck converter. But also it acts an EMC compatibility by provided an impedance for the triac circuitry to work into.
  • The end result is a triac phase-cut dimmable circuit that operates down to roughly a 1% duty cycle with NO hysteresis.
  • The Power Factor for this circuit is approximately 0.9, which is great for Energy Star requirements!

How does it compare with the National Semi LM3445 “industry standard” device?

The National Reference design board has a LOT of hysteresis. Granted you can dim down pretty low, but if the power is removed, the control will come up OFF until the dimming level is increased to about 75% level. Thus the circuit has no “memory” for a set and forget dimmer.

Secondly, the author has seen some 120 HZ flicker in the LEDS at very LOW  levels of dimming with LM3445, which could indicate a slight variance in triac triggering level at low levels of dimming.

Finally, the 5k price of the TPS92001 is listed at $0.45 at the 1K level versus the $1.55 of LM3445 at the same 1K level! That is some serious money for products which find themselves in very cost sensitive, high volume consumer products like LED light bulbs. For volume customer this about a 30 cent part.

You really CAN teach an “old dog” new tricks!


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