150mA SOT-23 Ultra Low Noise CMOS RF-LDO™ Regulator
ELECTRICAL CHARACTERISTICS ILC7082AIM5 (continued)
OPERATION
The ILC7082 LDO design is based on an advanced circuit
configuration for which patent protection has been applied.
Typically it is very difficult to drive a capacitive output with
an amplifier. The output capacitance produces a pole in the
feedback path, which upsets the carefully tailored dominant
pole of the internal amplifier. Traditionally the pole of the
output capacitor has been “eliminated” by reducing the out-
put impedance of the regulator such that the pole of the
output capacitor is moved well beyond the gain bandwidth
product of the regulator. In practice, this is difficult to do and
still maintain high frequency operation. Typically the output
impedance of the regulator is not simply resistive, such that
Figure 1: ILC7082 RF LDO frequency response
the reactive output impedance interacts with the reactive
impedance of the load resistance and capacitance. In addi-
tion, it is necessary to place the dominant pole of the circuit
at a sufficiently low frequency such that the gain of the reg-
ulator has fallen below unity before any of the complex
interactions between the output and the load occur. The
ILC7082 does not try to eliminate the output pole, but incor-
porates it into the stability scheme. The load and output
capacitor forms a pole, which rolls off the gain of the regu-
lator below unity. In order to do this the output impedance
of the regulator must be high, looking like a current source.
The output stage of the regulator becomes a transconduc-
tance amplifier, which converts a voltage to a current with a
substantial output impedance. The circuit which drives the
transconductance amplifier is the error amplifier, which
compares the regulator output to the band gap reference
and produces an error voltage as the input to the transcon-
ductance amplifier. The error amplifier has a dominant pole
at low frequency and a “zero” which cancels out the effects
of the pole. The zero allows the regulator to have gain out
to the frequency where the output pole continues to reduce
the gain to unity. The configuration of the poles and zero are
shown in figure 1. Instead of powering the critical circuits
from the unregulated input voltage, the CMOS RF LDO
powers the internal circuits such as the bandgap, the error
amplifier and most of the transconductance amplifier from
the boot strapped regulated output voltage of the regulator.
This technique offers extremely high ripple rejection and
excellent line transient response.
VIN
INTERNAL VDD
CNOISE
BANDGAP
REFERENCE
VREFD
ERROR
AMPLIFIER
TRANS-
CONDUCTANCE
AMPLIFIER
VOUT
FEEDBACK
GND
ON/OFF
Figure 2: ILC7082 RF LDO regulator block diagram
The ILC7082 is designed in a CMOS process with some
minor additions, which allow the circuit to be used at input
voltages up to 13V. The resulting circuit exceeds the fre-
quency response of traditional bipolar circuits. The ILC7082
is very tolerant of output load conditions with the inclusion
of both short circuit and thermal overload protection. The
device has a very low dropout voltage, typically a linear
response of 1mV per milliamp of load current, and none of
the quasi-saturation characteristics of a bipolar output
devices. All the good features of the frequency response
and regulation are valid right to the point where the regula-
tor goes out of regulation in a 4 millivolt transition region.
Because there is no base drive, the regulator is capable of
providing high current surges while remaining in regulation.
This is shown in the high peak current of 500mA which
allows for the ILC7082 to be used in systems that require
short burst mode operation.
A block diagram of the regulator circuit used in the ILC7082
is shown in figure 2 (following page), which shows the input-
to-output isolation and the cascaded sequence of amplifiers
that implement the pole-zero scheme outlined above.
Impala Linear Corporation
(408) 574-3939
ILC7082 1.3
www.impalalinear.com
April 1999
5
IMPALA [ Impala Linear Corporation ]
