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CS-8156-CS8156YTHA5-CS8156YTVA5
12 V, 5.0 V Low Dropout Dual Regulator with ENABLE
SR003AN/D
Compensation for
Linear Regulators
Kieran O’Malley
ON Semiconductor
2000 South County Trail
East Greenwich, RI 02818In the majority of low drop and quasi low drop (composite NPN–PNP) regulators, the pass device or the pass device driver, is
a lateral PNP transistor. The lateral PNP transistor is inherently a low frequency cutoff device with a poor transient response.
An external capacitor must be connected between the output and ground to provide filtering and compensation.
This application note discusses the role of the output capacitor in loop stability. Guidelines for selecting an appropriate capacitor
are provided.
IntroductionThere is little specific information available on choosing
the correct type and value of compensation capacitor for low
and quasi low drop out linear regulators. Many designers
simply use the one shown in the application diagram of the
device data sheet without worrying about how and why that
value was selected.
The Role of the Output CapacitorThe compensation capacitor value cited in most linear
regulator data sheets has a large safety factor built in. It is not
necessarily the cheapest or the best solution for a particular
application.The compensation capacitor determines three
key characteristics of a linear regulator’s performance:
start–up delay, load transient response and loop stability.
Start–up delay and transient response can be considered
together since they are so closely related.
A large capacitor takes a long time to charge up. The
charging period TStartup, is:
TStartup� CVOUT
ILIMIT
where VOUT is the regulator’s output voltage, ILIMIT is the
current limit of the regulator and C is the value of the
capacitor. If the capacitor is completely discharged before
the regulator is powered up, the regulator will current limit
during start–up and the rise in the output voltage will be
delayed. The larger the value of the capacitor, the longer it
will take the output voltage to reach its nominal value.
Conversely, if the capacitor is too small, the output
voltage will overshoot its nominal value during startup. The
regulator’s internal control loop can’t respond quickly
enough to limit the output voltage. If the overshoot is large
enough, it may damage sensitive electronics tied to the
regulator’s output.
Since it is impossible to investigate all three key
achieving a stable loop design first and then check the start
up delay and the load transient response.
Loop StabilityA simplified block diagram of a low drop linear regulator
is shown in Figure 1. It consists of three basic parts: A voltage reference, VREF, which is independent of
temperature and supply voltage. An error amplifier that compares the voltage
reference to a sample of the output voltage and
generates a signal proportional to their difference. A series pass device that provides the output current
to the load and regulates the output voltage.
Figure 1. Simplified Low Dropout Linear RegulatorVIN
A well designed feedback system has a closed loop
response that ensures unconditional stability. An ideal
system (Figure 2) has a single pole response that determines
the system’s crossover frequency. Its crossover frequency
(fC) is chosen to ensure that the system can quickly respond
to load transients without undue ringing (oscillation). To
