06-10-2012, 02:21 PM
Linear Regulators: Theory of Operation and Compensation
Linear Regulators.pdf (Size: 228.22 KB / Downloads: 23)
Introduction
The explosive proliferation of battery powered equipment in
the past decade has created unique requirements for a
voltage regulator that cannot be met by the industry standards
like the LM340 or the LM317. These regulators use an
NPN Darlington pass transistor (Figure 1), and will be referred
to in this document as NPN regulators. The demand
for higher performance is being met by the newer
low-dropout (LDO) regulators and quasi-LDO regulators.
The Quasi-LDO Regulator
Another regulator configuration that is becoming very popular
in certain applications (like 5 - 3.3V conversion) is the
quasi-LDO regulator (Figure 3). The quasi-LDO is so named
because it is ’half way’ between the NPN Darlington and the
true LDO. The pass transistor is made up of a single NPN
transistor being driven by a PNP. As a result, the dropout
voltage is less than the NPN Darlington regulator, but more
than an LDO.
Regulator Operation
All three of these regulator types regulate the output voltage
to a fixed (constant) value using the same
technique (Figure 4).
The output voltage is sampled (measured) through a resistive
divider which is fed into the inverting input of the error
amplifier. The non-inverting input is tied to a reference voltage,
which is derived from an internal bandgap reference.
The error amplifier will always try to force the voltages at it’s
input to be equal. To do this, it sources current as required to
provide sufficient load current to maintain the output voltage
at the regulated value
Performance Comparison
The primary differences in performance between the NPN,
LDO, and quasi-LDO are in the parameters of dropout
voltage (previously defined) and ground pin current. For
this analysis, we will define ground pin current (IGND) as
shown in Figure 4, neglecting the small IC bias currents
which also flow to ground. It can be seen that the value of
IGND is the load current IL divided by the gain of the pass
transistor.
The high gain of the Darlington in an NPN regulator means it
requires very little drive to source IL, so it’s ground pin
current is very low (typically a few mA). The quasi-LDO also
has very good performance, with products like National’s
LM1085 being able to source more than 3A with less than 10
mA of ground pin current.
The ground pin current of an LDO is typically much higher. At
full load current, PNP beta values of 15 - 20 are not unusual,
which means the LDO ground pin current can be as high as
7% of the load current.
A big advantage of NPN regulators is that they are unconditionally
stable (most require no external capacitors). An LDO
does require at least one external capacitor on the output to
reduce the loop bandwidth and provide some positive phase
shift. Quasi-LDOs typically require some output capacitance,
but much less than an LDO and with less restrictive limits on
its performance characteristics.
NPN Regulator Compensation
The pass transistor of the NPN regulator (see Figure 1) is
connected in a circuit configuration known as common collector.
An important characteristic of all common collector
circuits is low output impedance, which means the pole from
the power stage that it places in the loop gain occurs at a
very high frequency.
The NPN regulator uses a technique called dominant pole
compensation because it has no inherent low-frequency
poles. In this case, a capacitor is built into the IC which
places a pole in the loop gain at a low frequency (Figure 12).