When designing embedded microcontroller applications, one of thegreatest challenges can be the creation of the power supply for themicrocontroller, particularly when the only supply voltage available issignificantly higher than the microcontroller's maximum VDD .
Such situations arise in a variety of applications including whitegoods appliances, automotive applications and intelligent Point Of Loadswitching power supplies.
In an applianceor white goods environmen t in which atransformerless powersupply is often used, the typical approach to powering amicrocontroller off of AC is to step down the voltage to 8-10 VAC using a transformer, then rectify, filter, and regulate to the required5 VDC .
Unfortunately, cost restrictions prevent the use of the transformer,and linear regulators cannot withstand 100+ volts, so the reduction ofthe AC is handled by a pair of series resistors. The AC is thenrectified, regulated with a zener diode and filtered. Using a shuntregulator as the zener diode eliminates the cost of one more componentin the transformerless power-supply design, and the regulator is alsomore accurate than the zener.
In automotivedesigns , the battery voltage generally runs a 10-13 VDC,withload-dump spikes as high as 40-50V. Load dump is the voltage spike outof the alternator, when the headlights or some other high-draw load isturned off.
There are regulators that are designed to handle this high voltagespike, but they are not cheap, and they are typically designed for 1 ormore Amps of load current.
Using a shunt regulator, two equal-value resistors are placed inseries between the battery voltage and the regulator. At the junctionbetween the two resistors, a zener diode is placed as a voltage clamp.Typically, the zener voltage is chosen to be 2-3 Volts higher than thenormal voltage at the resistor junction.
Now, when the load dump spike comes through, the zener clamps thecenter point and prevents the spike from passing through to theregulator. This is less expensive solution to a high voltage spike inthe supply.
Another good example of an application in which a significantlyhigher supply voltage is used to power a microcontroller is the biasvoltage generator in an intelligent Point Of Load switchingpower supply .
Here, the switching power supply needs to have some bias current topower both the PWM circuitry and the microcontroller, so it can acceptcontrol communications prior to powering up the main power-supplyoutput, and for the initial switching to generate the output voltage.
Given that the intermediate voltage is typically in the range of 24to 48 VDC, a simple linear regulator can't handle the job. As themicrocontroller is already present, it can handle soft start, power upsequencing, error handling and communications. Combining the twofunctions together saves component cost, and expands the feature set ofthe POL regulator. Typically the best and quickest way to solve suchproblems is through the use of a shunt regulator (Figure 1, below ).
|Figure1: A typical shunt regulator design using a Texas Instruments TL431|
Current is supplied through the +24V input, and load current leavesthrough the +5V output. R1 is chosen such that, at the maximum 5V load,the drop across R1 is approximately 19V or a little less. If the 5Vload is not drawing its full current, the voltage drop across R1 issmaller and the +5V output increases in voltage.
The shunt regulator senses the risein the 5V voltage through theresistor divider formed by R2 and R3, and shunts current from the 5Vline to ground to compensate. Using this system, the regulator acts asa variable resistor, adjusting its own value, such that the voltageoutput at 5V remains constant.
The regulator's ability to shunt current to ground only limits thevariations in the system. If the +24V increases, the regulator willhave to shunt additional current to increase the voltage drop acrossR1. If the load current on +5V decreases, the shunt regulator will haveto shunt additional current to maintain the voltage drop across R1.
Both changes result in variations in the shunt current. If bothvariations are sufficiently restricted such that the shunt currentremains within the regulators capabilities, then the actual magnitudeof the load current is not important. However, regulating more than acouple of 100 milli-Amps would require both the +24V and the loadcurrent to remain relatively constant.
A simpler version of the circuit can also be built with a zenerdiode (Figure 2, below ),whichtakes over the function of the shunt regulator, shorting current toground whenever the voltage across the diode exceeds its zener voltage.
|Figure2. Zener diode replacing the function of the shunt regulator.|
However, in many designs a more useful and flexible way to solvesuch problems is to integrate such shunt regulator circuitry onto thesame die as the microcontroller that is typically also used in suchapplications. The advantages of a shunt regulator include:
1. Simpledesign, all that is needed is a resistor and a bypass capacitor.
2. No additionalpins on the microcontroller, just power and ground.
3. Operating fromvoltages greater than 20 VDC is possible without special (andexpensive) regulator circuits.
4. The resistorand bypass capacitor form an RC low pass filter,which helps to limitnoise from the source and conducted noise from the microcontroller.
5. It eliminatesone or more components from the power-supply design.
6. The supplyvoltage can be used to power other components in the circuit.
7. The amount ofcurrent available from the supply is not limited by the capabilities ofthe regulator. The regulator's current capability only limits thevariations in supply voltage and load current.
8. For designswith large variations in supply voltage or load current, additionaltricks can be used to keep the shunt regulator within its specifiedlimits.
In certain of itsmicrocontroller offerings, Microchip, for example, has incorporatedan on-chip 5V shunt regulator. When compared to the use of a circuitsuch as the TL431 or a zener circuit, the advantages stack up prettyquickly in favor of a high voltage microcontroller incorporating theshunt regulator.
The TL431 requires two resistors, in addition to the TL431 itself,whie the zener circuit requires the zener diode. The HVmicrocontroller, on the other hand, only requires R1 and C1. It is moreaccurate than the zener circuit and it takes fewer parts than zener orthe TL431. Plus, it doesn't require any additional pins on themicrocontroller, the only pins used are ground and power.
The inclusion of an on-chip regulator allows the microcontroller tooperate from a wide variety of supply voltages. As an added bonus, theshunt regulator topology also allows the connection of other circuitry,external to the microcontroller, to be powered by the VDD pin.
Whether externally, or as a part of the microcontroller's internalcircuitry, the inclusion of a shunt regulator can simplify the designof control circuits which must operate from voltages above the normalrange of the microcontroller's supply voltage. The circuit can even actas a supply for other active devices in the circuit. All that isrequired is a little careful design and component selection.
The basics of a shunt regulator
A shunt regulator generates a specific supply voltage by creating avoltage drop across a pass resistor RSER . The voltage at theVDD pin of the microcontroller is monitored and compared toan internal voltage reference (See Figure3, below )
The current through the resistor is then adjusted, based on theresult of the comparison, to produce a voltage drop equal to thedifference between the supply voltage VUNREG and the VDD of the microcontroller.
The advantage to a shunt regulator is that the supply voltage, VUNREG ,is only limited by the power dissipation and breakdown voltage of theexternal resistor, RSER , not the power or breakdowncharacteristics of the regulator. The challenge in designing a shuntregulator circuit is choosing an appropriate value for the resistorsuch that the range of currents over which the regulator has controlwill produce the correct voltage drop needed to produce a 5.0 VDCsupply.
So, all we really need to know to design with a shunt regulator isOhm's Law. Theproblem is that the supply voltage, VUNREG ,is not constant and neither is the load current. In addition, the rangeof current over which the regulator has control is also limited. Sothe choice of RSER really becomes a balancing act, trying tofind a resistance that will meet all three requirements.
|Figure3: Shunt Regulator Block Diagram|
The best place to start in the design process is to catalog thevariations possible in the supply voltage and the load current. For ourpurposes, the following definitions will be used:
VU_MIN is the minimum supplyvoltage to the system.
VU_MAX is the maximum supply voltageto the system.
ILOAD_MIN is the minimum loadcurrent, excluding the regulator.
ILOAD_MAX is the maximum loadcurrent, excluding the regulator.
Given these values, it is now possible to determine the minimum andmaximum pass resistor values for the circuit. Equation 1 and Equation 2, below ,are used to calculate these values.
These values, RMIN and RMAX , represent thelimits for the resistance of the pass resistor. The constant 5.0 Vreferred to in the equations is to the VDD voltage of theregulator, the 4 mA constant is the minimum regulation current for theregulator and the 50 mA constant is the maximum regulation current forthe regulator.
If the minimum value is less than the maximum value, a final passresistor value can be chosen between the two limits. Good designpractice is to then check the minimum and maximum regulator currents. Equation 3 and Equation 4, below, show how these values are calculated.
The minimum regulator current must be less than the maximum loadcurrent and the difference must be less than the maximum regulatorcurrent of 50 mA. If not, then check the calculations for the passresistor value.The minimum power rating of the pass resistor can now be calculatedusing Equation 5, below. Remember to allow for adequate cooling and anappropriate amount of margin when deciding on the final power rating.
Don't Bypass The Bypass Capacitor
The next step is to determine the appropriate size bypass capacitor forthe design. While most microcontroller applications can use”rule-of-thumb” values for their bypass capacitors, the unique natureof the shunt regulator complicates the selection.
First of all, the combination of the pass resistor and the bypasscapacitor form an unintended RC time constant that limits the rise timeof the microcontroller VDD. Therefore, it is necessary tolimit the size of the capacitor such that the resulting rise time forthe VDD supply is faster than the specified minimum risetime for the microcontroller's VDD .
For example, the minimum rise time for a Microchip microcontroller'sVDD is 0.5V/mS, until the supply voltage exceeds 2.1V(Power-on Reset trip point). So, the supply must exceed 2.1 voltswithin 42 mS, (2.1V/0.5V/ms). Using this information and Equation 6 below , the maximumcapacitor value can be determined.
The bypass capacitor must be less than the value specified byEquation 6 to meet thepower-up requirements of the Power-on Reset, andgreater than 0.1/.047 microfarads for noise suppression. Typically, thevalue is chosen to be closer to the 0.1 microfarad value forconvenience.
Next in Part 2: Tips and tricks for implementing shuntregulators into high voltage MCU designs
Keith Curtis is PrincipalApplications Engineer in the Security, Microcontroller and TechnologyDevelopment Division at MicrochipTechnology Inc. In this role, Keith develops training and referencedesigns for incorporating microcontrollers in intelligent power supplydesigns. Keith also sits on the PMBus development committee, andischair of the PMBus development tools subcommittee.
1) AN1035 Designing with HVMicrocontrollers
2) AN786, “Considerations forDriving MOSFETs in High-Current, Switch Mode Regulators” (DS00786).
3) AN898, “Determining MOSFETDriver Needs for Motor Drive Applications” (DS00898).