Linear Regulaor Design

 
Linear Regulator Design Key Notes
James Zou
03/10/2005
The basic properties about linear regulator electric components are discussed in these notes. An electrolytic capacitor model is shown in Figure 1. In this model, it shows the effective series inductance LESL, the effective series resistance RESR, the capacitance C, the dielectric leakage resistance Rleakage.

 
                                            Figure 1. Electrolytic Capacitor Model

Effects of Capacitor Parasitic Parameter
ESR:  The ESR (Equivalent Series Resistance) causes internal heating due to power dissipation as the ripple current flows into and out of the capacitor.  The capacitor can fail if ripple current exceeds maximum ratings. Excessive outputvoltage ripple will result from high ESR, and regulator loop instability is also possible.  ESR is highly dependent on temperature, increasing very quickly at temperatures below about 10 C.
The ESR is the primary cause of regulator loop instability in both linear LDO regulators and switching regulators.

ESL:  The ESL (Effective Series Inductance) limits the high frequency effectiveness of the capacitor.  High ESL is the reason electrolytic capacitors need to be bypassed by film or ceramic capacitors to provide good high frequency performance. The ESR, ESL and C within the capacitor form a resonant circuit, whose frequency of resonance should be as high as possible.  Switching regulators generate ripple voltages on their outputs with very high frequency (>1 MHz) components, which can cause ringing on the output voltage
if the capacitor resonant frequency is low enough to be near these frequencies.
The  ESL limits a capacitors effectiveness at high frequencies, and is the primary reason electrolytic capacitors must be bypassed by good RF capacitors in switching regulator applications (ceramic and film types are often used).

Output Capacitance Affecting Regulator Loop Stability Too
The output capacitor used on an LDO linear regulator can make it oscillate if the
capacitor is not selected correctly.
INPUT CAPACITOR ESL EFFECTS

All of the switching regulators operate as DC-DC converters at a very high frequency.  As the converter switches, it has to draw current pulses from the input source.  The source impedance is extremely important, as even a small amount of inductance can cause significant ringing and spiking on the voltage at the input of the converter. The best practice is to  always provide adequate capacitive bypass as near as possible to the switching converter input.  For best results, an electrolytic is used with a film capacitor (and possibly a ceramic capacitor) in parallel for optimum high frequency bypassing.

OUTPUT CAPACITOR ESR EFFECTS
The primary function of the output capacitor in a switching regulator is filtering.  As the converter operates, current must flow into and out of the output filter capacitor. The ESR of the output capacitor directly affects the performance of the switching regulator.  ESR is specified by the manufacturer on good quality capacitors, but be certain that it is specified at the frequency of intended operation. General-purpose electrolytics usually only specify ESR at 120 Hz, but capacitors intended for high-frequency switching applications will have the ESR guaranteed at high frequency (like 20 kHz to 100 kHz).Some ESR dependent parameters are:
Ripple Voltage:  In most cases, the majority of the output ripple voltage results from the ESR of the output capacitor.  If the ESR increases (as it will at low operating temperatures) the output ripple voltage will increase accordingly.
Efficiency:  As the switching current flows into and out of the capacitor (through the ESR), power is dissipated internally.  This “wasted” power reduces overall regulator efficiency, and can also cause the capacitor to fail if the ripple current exceeds the maximum allowable specification for the capacitor.
Loop Stability:  The ESR of the output capacitor can affect regulator loop stability. Products such as the LM2575 and LM2577 are compensated for stability assuming the ESR of the output capacitor will stay within a specified range.Keeping the ESR within the “stable” range is not always simple in designs that must operate over a wide temperature range.   The ESR of a typical aluminum electrolytic may increase by 40X as the temperature drops from 25C to -40C. In these cases, an aluminum electrolytic must be paralleled by another type of capacitor with a flatter ESR curve (like Tantalum or Film) so that the effective ESR (which is the parallel value of the two ESR’s) stays within the allowable range.
Note:  if operation  below -40C is necessary, aluminum electrolytics are probably not feasible for use.

BYPASS CAPACITORS
High-frequency bypass capacitors are always recommended on the supply pins of IC devices, but if the devices are used in assemblies near switching converters bypass capacitors are absolutely required. The components which perform the high-speed switching (transistors and rectifiers) generate significant EMI that easily radiates into PC board traces and wire leads. To assure proper circuit operation, all IC supply pins must be bypassed to a clean, low-inductance ground (for details on grounding, see next section).

REGULATOR LOOP RESPONSE
The loop response of a typical regulator is shown in Figure 2.  The most important point to realize is that for a stable loop, the gain must cross below 0 dB before the phase angle reaches 180.  A phase angle of 180 means that the signal being fed back around the loop is actually positive feedback, and will cause oscillations to occur.
(Note:  In reality, a phase margin of 45 is usually required for good stability, which means it is advisable to get a 0 dB crossover before the phase angle reaches 135).
In an LDO regulator, the output capacitor is required to force the gain to roll off fast enough to meet the stability requirements (a standard NPN regulator is internally compensated, and usually needs no output capacitor for stability).
As shown in Figure 2, the ESR of the output capacitor causes an unwanted “zero” in the response, which delays the 0 dB crossover point.  If the ESR is large enough, the “zero frequency” gets low enough to cause regulator instability.
The stability requirements for a specific regulator will be listed on the data sheet for the part.  In some cases, a range is given which requires that the ESR be within the minimum and maximum limits.  In the newer parts, only a maximum limit must be met (which makes selecting a capacitor much easier).
 

Figure 2. Loop Gain Plot

TEMPERATURE DEPENDENCE OF ESR
Having now established the necessity of controlling the ESR of the output capacitor on an LDO regulator (to keep the regulator from oscillating), we need to point out one very important thing:  ESR is not constant with temperature.
Figure 3 shows a plot of ESR versus temperature for a typical aluminum electrolytic capacitor.  The most important point to observe is how fast the ESR increases at low temperatures.
In cases where an LDO regulator must be operated below about -10  C, it is sometimes not possible to find an aluminum electrolytic capacitor that can maintain an ESR within the acceptable range.  Also, it is essential that the capacitor is specified to operate over the full temperature range:  some aluminum electrolytics are not usable below -20C (because their electrolyte freezes). If the regulator has only a  maximum limit which the ESR must not exceed, the aluminum electrolytic capacitor can be paralleled with a solid tantalum capacitor (which has a much lower ESR).  When two capacitors are in parallel, the effective ESR is the parallel of the two ESR values, which means the tantalum will help suppress the low-temperature ramp up seen in Figure 3.  As a good rule, the tantalum should be selected so that its capacitance is about 20% of the aluminum electrolytic. If the regulator has both a maximum and minimum limit (the ESR must stay in a specified range), it may be necessary to use a low value carbon film resistor placed in series with a low ESR capacitor (tantalum, film, or ceramic will work). The best type of capacitor to use will depend upon how much total capacitance is required.

Figure 3. Aluminum Electrolytic Capacitor ESR VS Temperature

The Carrot in LDO Regulator Ground Pin Current
Many (but not all) LDO regulators have a characteristic in their ground pin current referred to as the “carrot”.  The carrot is a point in the ground pin current that spikes up as the input voltage is reduced (see Figure 4). The error amplifier in a regulator always tries to force the output to be the right voltage by adjusting the current through the pass device (in this case, the PNP transistor). As the input voltage is reduced (and the voltage across the pass transistor decreases) the current gain of the PNP begins to drop.  To maintain the correct output voltage, the error amplifier has to drive the base of the PNP harder to supply the same load current.  The PNP base drive current leaves the regulator as ground
pin current. As the input voltage drops further, the regulator will approach dropout, causing the error amplifier to drive the PNP base with maximum current (this is the top of the carrot).  This value of current may be 3 or 4 times the maximum ground pin current that is required to drive full rated load current with 5V across the pass transistor.
The carrot is recognized as an undesirable characteristic, since the additional ground pin current must be supplied by the source, but does not power the load (it just heats up the regulator). In the newer LDO regulators, circuitry was built in to prevent this ground pin spike from occurring.  For example, the  ZP2321 (and all of the products in that family) have only a negligible increase in ground pin current as the input voltage crosses through the range where dropout is occurring.
 

Figure 4. LDO regulator with “carrot”

Proper Ground and “skin effect”
The “ground” in a circuit is supposed to be at one potential, but in real life it is not. When ground currents flow through traces which have non-zero resistance, voltage differences will result at different points along the ground path. In DC or low-frequency circuits, “ground management” is comparatively simple:  the only parameter of critical importance is the  DC resistance of a conductor, since that defines the voltage drop across it for a given current.  In high-frequency circuits, it is the inductance of a trace or conductor that is much more important. In switching converters, peak currents flow in high-frequency (> 50 kHz) pulses, which can cause severe problems if trace inductance is high.  Much of the “ringing” and “spiking” seen on voltage waveforms in switching converters is the result of high current being switched through parasitic trace (or wire) inductance. 
Current switching at high frequencies tends to flow near the surface of a conductor (this is called “skin effect”), which means that ground traces must be very wide on a PC board to avoid problems. It also means that even you increase the thickness, it is useless.  It is usually best (when possible) to use one side of the PC board as a ground plane.

PCB Layout

Which one is good?
 


Figure 5. Eliminating Load Regulation Effects
  Selecting the Best Regulator For Your Application

 Maximum Load Current
 Type of Input Voltage Source (Battery or AC)
 Output Voltage Precision (Tolerance)
 Quiescent (Idling) Current
 Special Features (Shutdown Pin, Over current, under voltage, over voltage, timer, retry, etc.)

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About superjameszou

Hardware Design Engineer
This entry was posted in LDO Regulator Design. Bookmark the permalink.

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