The LM4041 has been round for over 20 years. Throughout these many years, whereas primarily marketed as a precision adjustable shunt regulator, this basic machine additionally discovered its means into various functions. These embrace voltage comparators, overvoltage protectors, voltage limiters, and so forth. Voltage, voltage, voltage, should it at all times be voltage? It will get tedious. Absolutely this widespread precision chip, whereas admittedly reasonably—um—“mature”, should have untapped potential for doing one thing that doesn’t begin with voltage.
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
The Design Concept (DI) introduced in Determine 1 presents the 4041 a regular, probably bizarre, possibly even new function to play. It’s a precision present supply.
Determine 1 Weirdly, the “CATHODE” serves because the sense pin for energetic present supply regulation.
The above block diagram reveals how the 4041 works at a conceptual stage:
Sourced present = Is = (V+ – (Vc + 1.24v))/R1
Is > 0, V+ < 15v, Is < 20 mA
The collection connection subtracts an inner 1.24-V precision reference from the exterior voltage enter on the CATHODE pin. The interior op-amp subtracts the voltage enter on the FB pin from that distinction, then amplifies and applies the outcome to the cross transistor. If it’s optimistic [(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 style (FB linked to CATHODE and ANODE grounded), the scheme works like a shunt voltage regulator ought to, forcing CATHODE to the interior 1.24-V reference voltage. However what’s going to occur if the FB pin is linked to a continuing management voltage [Vc < (V+ – 1.24v)] and CATHODE—and as a substitute of being linked to FB—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 mounted and may’t be pulled up to make FB = CATHODE – 1.24, CATHODE have to be pulled down till equality is achieved. For this to occur, a programmed present, Is, have to 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 continuing present supply of:
Is = (5v – 2.5 – 1.23)/R1 = 1.27v /R1
It additionally illustrates how including a booster transistor Q1 can accommodate functions needing present or energy past Z1’s modest limits. Discover that Z1’s accuracy might 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 via 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 exhibiting the DAC management of Is. Is = Df amps, the place Df = PWM responsibility issue. The asterisked resistors needs to be 1% or higher.
Incoming 5-Vpp, 10-kHz PWM causes Q2 to modify 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 continuing 1.22 mA bias from R4 and stage 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 offers the first-stage ripple filter and R7C2 the second. The C1 and C2 values proven are scaled for Fpwm = 10 kHz to offer an 8-bit settling time of 6 ms. If a distinct PWM frequency is used, scale each capacitors by 10kHz/Fpwm.
A scorching matter is that This fall could be referred to as on to dissipate greater than 10 W, so don’t skimp on heatsink capability.
Q3 is a security shutdown characteristic. It removes Q1 gate drive when +5 falls under 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 modify single flip ZERO trim for zero output present
- Set Df = 100% and modify single flip CAL trim for 1.0 A output
- Accomplished.
Determine 4 Extra 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 good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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