The TL431 has been round for practically 50 years. Throughout these many years, whereas primarily marketed as a precision adjustable shunt regulator, this legacy machine additionally discovered its manner into different functions. These embody voltage comparators, audio amplifiers, present sources, overvoltage protectors, and many others. Sadly, in nearly each instance from this mighty menagerie of circuits, the 431’s “anode” pin sinks to the identical lowly destiny. It will get grounded.
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
Present sink regulation
The design thought offered right here gives that poor persecuted pin a extra buoyant function to play, Determine 1.
Determine 1 The floated anode serves as a way pin for energetic present sink regulation.
The Determine 1 block diagram reveals how the 431 works at a conceptual stage the place:
Sink present = Is = (Vc – 2.5v)/R1 = 0 to 1/R1 as Vc = 2.5v to three.5v
Vs < 37v, Is < 100mA, Is(Vs – R1Is) < 500mW @ 50oC ambient
Collection connection provides an inner 2.5-V precision reference to exterior voltage enter on the ANODE pin. The op-amp subtracts this sum from the voltage enter on the REF pin, then amplifies and applies the distinction to the go transistor. If the distinction is optimistic (sum < REF), the transistor activates and shunts present from CATHODE to ANODE. In any other case (sum > REF), it turns off.
If the 431 is linked within the conventional vogue (REF linked to CATHODE and ANODE grounded). In that case, the scheme works like a shunt voltage regulator ought to, forcing CATHODE to a resistor-string-programmed a number of of the inner 2.5-V reference voltage. However what’s going to occur if the REF pin is linked to a relentless management voltage (Vc > 2.5 V); and the ANODE, as a substitute of being grounded, floats freely on present sensing resistor R1?
What occurs is the present will get regulated as a substitute of the voltage. As a result of Vc is fastened and might’t be pulled down to make REF = ANODE + 2.5, ANODE have to be pulled up till equality is achieved. For this to occur:
Is = (Vc – 2.5v)/R1
Fixed present sink regulation of 1/R1
Determine 2 illustrates how a hard and fast voltage divider is perhaps used (assuming a 5-V rail that’s correct sufficient) to make use of a floated-anode Z1 to control a relentless sink present of:
Is = (3.5v – 2.5v)/R1 = 1/R1
It additionally illustrates including a booster transistor Q1 to accommodate functions needing present or energy past Z1’s modest TO92ish limits. Discover that Z1’s accuracy will likely be unimpaired as a result of no matter fraction of Is that Q1 causes to bypass Z1 is summed again in earlier than passing via R1.
Determine 2 Booster transistor Q1 can deal with present and voltage past 431 max Ic and dissipation limits, whereas the three.5-V voltage divider applications a relentless Is.
Programming sink present with DAC
Determine 3 reveals how Is is perhaps digitally programmed with a 2.5-V DAC sign. Observe the DAC sign is inverted (Is = max when Vx = 0) whereas Z2 gives the mandatory stage shift:
Is = (2.5v – Vx)/(2.5R1) = 0 to 1/R1 as Vx = 2.5v to 0
Determine 3 DAC management of Is, the DAC sign is inverted, whereas Z2 gives the mandatory stage shift.
Programming sink present to Df/R1 with DAC
Determine 4 reveals an alternate programming technique utilizing PWM with Is = Df /R1 the place Df equals the 0 to 1 (0% to 100%) PWM responsibility issue:
Is = (2.5R2/(R3/Df))/R1 as Df = 0 to 1
Df = IsR1R3/(2.5R2)
Df = Is R1
Determine 4 PWM management of Is, the place Is is the ratio of the PWM responsibility issue and R1.
The 8-bit PWM decision and 10-kHz frequency are assumed. The R2C1 single-pole ripple filter has a time fixed of roughly 64x the PWM interval (10 kHz = 100 µs assumed) for 1-lsb peak-to-peak max ripple and 38-ms max settling time.
Rushing up settling time
One shortcoming of Determine 4 is the lengthy settling time (~40 ms to eight bits) imposed by the single-pole R1C12 ripple filter. If an additional resistor and capacitor gained’t break the financial institution, that may be sped up by an element of about 5 (~8 ms) with Determine 5’s R5C2 offering 2nd-order analog-subtraction filtration.
Determine 5 The addition of R5 and C2 gives quicker settling occasions with a 2nd-order ripple filter.
Programmable present sink utility circuit
Lastly, Determine 6 reveals the Determine 4 circuit mixed with an affordable 24-W AC adapter and a 5-V regulator to energy a small digital testing system. Remember to adequately heatsink Q1.
Determine 6 The mixed present sink and small system energy provide the place the max Is is 1 A, Max Vs is 20 V, and Is = Df.
Thanks for the implicit suggestion, Ashutosh!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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