The LM4041 has been round for over 20 years. Throughout these many years, whereas primarily marketed as a precision adjustable shunt regulator, this basic system additionally discovered its method into various purposes. These embrace voltage comparators, overvoltage protectors, voltage limiters, and so on. Voltage, voltage, voltage, should it all the time be voltage? It will get tedious. Absolutely this standard precision chip, whereas admittedly fairly—um—“mature”, should have untapped potential for doing one thing that doesn’t begin with voltage.
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
The Design Thought (DI) introduced in Determine 1 provides the 4041 a typical, probably bizarre, possibly even new position to play. It’s a precision present supply.
Determine 1 Weirdly, the “CATHODE” serves because the sense pin for lively present supply regulation.
The above block diagram reveals how the 4041 works at a conceptual degree:
Sourced present = Is = (V+ – (Vc + 1.24v))/R1
Is > 0, V+ < 15v, Is < 20 mA
The sequence connection subtracts an inner 1.24-V precision reference from the exterior voltage enter on the CATHODE pin. The inner op-amp subtracts the voltage enter on the FB pin from that distinction, then amplifies and applies the outcome to the move transistor. If it’s constructive [(V+ – 1.24) > Vc], the transistor activates and shunts present from CATHODE to ANODE. In any other case, it turns off.
When a 4041 is linked within the conventional vogue (FB linked to CATHODE and ANODE grounded), the scheme works like a shunt voltage regulator ought to, forcing CATHODE to the inner 1.24-V reference voltage. However what is going to occur if the FB pin is linked to a relentless management voltage [Vc < (V+ – 1.24v)] and CATHODE—and as an alternative of being linked to FB—floats freely on present sensing resistor R1?
What occurs is the present will get regulated as an alternative of the voltage. As a result of Vc is mounted and might’t be pulled up to make FB = CATHODE – 1.24, CATHODE should be pulled down till equality is achieved. For this to occur, a programmed present, Is, should be handed that’s given by:
Is = (V+ – (Vc – 1.24))/R1.
Determine 2 illustrates how this relationship can be utilized (assuming a 5-V rail that’s correct sufficient) to make a floated-cathode 4041 regulate a relentless present supply of:
Is = (5v – 2.5 – 1.23)/R1 = 1.27v /R1
It additionally illustrates how including a booster transistor Q1 can accommodate purposes needing present or energy past Z1’s modest limits. Discover that Z1’s accuracy will probably be unimpaired as a result of, with no matter fraction of Is that Q1 causes to bypass, Z1 is summed again in earlier than passing by way of R1.
Determine 2 The booster transistor Q1 can deal with present past 4041 max Is and dissipation limits.
Determine 3 reveals how Is could be digitally linearly programmed with PWM.
Determine 3 Schematic displaying the DAC management of Is. Is = Df amps, the place Df = PWM obligation issue. The asterisked resistors needs to be 1% or higher.
Incoming 5-Vpp, 10-kHz PWM causes Q2 to change R5, making a variable common resistance = R5/Df. Due to the two.5-V Z1 reference, the result’s a 0 to 1.22 mA present into Q1’s supply. That is summed with a relentless 1.22 mA bias from R4 and degree shifted by Q1 to make a 1.22 to 2.44 V management voltage, Vc, for present supply Z2.
The result’s a linear 0- to 1-A output present, Is, right into a grounded load the place Is = Df amps. Voltage compliance is 0 to 12 V. The 8-bit suitable PWM ripple filtering is 2nd order utilizing “Cancel PWM DAC ripple with analog subtraction.”
R3C1 gives the first-stage ripple filter and R7C2 the second. The C1 and C2 values proven are scaled for Fpwm = 10 kHz to supply an 8-bit settling time of 6 ms. If a special PWM frequency is used, scale each capacitors by 10kHz/Fpwm.
A scorching subject is that This fall could be known as on to dissipate greater than 10 W, so don’t skimp on heatsink capability.
Q3 is a security shutdown function. It removes Q1 gate drive when +5 falls beneath about 3 V, shutting off the present supply and defending the load when controller logic is powered down.
Determine 4 provides zero and span pots to implement a single-pass calibration for greatest accuracy:
- Set Df = 0% and alter single flip ZERO trim for zero output present
- Set Df = 100% and alter single flip CAL trim for 1.0 A output
- Executed.
Determine 4 Further zero and span pots to implement a single-pass calibration for greatest accuracy.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- PWM-programmed LM317 fixed present supply
- Low-cost precision adjustable present reference and utility
- A adverse present supply with PWM enter and LM337 output
- A high-performance present supply
- Easy, exact, bi-directional present supply
The submit LM4041 voltage regulator impersonates precision present supply appeared first on EDN.
