50/100mA SOT-23 CMOS RF LDO™ Regulators
Operation
The ILC7080/81 LDO design is based on an advanced cir- the regulator to have gain out to the frequency where the
cuit configuration for which patent protection has been output pole continues to reduce the gain to unity. The con-
applied. Typically it is very difficult to drive a capacitive out- figuration of the poles and zero are shown in figure 1.
put with an amplifier. The output capacitance produces a Instead of powering the critical circuits from the unregulat-
pole in the feedback path, which upsets the carefully tai- ed input voltage, the CMOS RF LDO powers the internal
lored dominant pole of the internal amplifier. Traditionally circuits such as the bandgap, the error amplifier and most
the pole of the output capacitor has been “eliminated” by of the transconductance amplifier from the boot strapped
reducing the output impedance of the regulator such that regulated output voltage of the regulator. This technique
the pole of the output capacitor is moved well beyond the offers extremely high ripple rejection and excellent line tran-
gain bandwidth product of the regulator. In practice, this is sient response.
difficult to do and still maintain high frequency operation. A block diagram of the regulator circuit used in the
Typically the output impedance of the regulator is not sim- ILC7080/81 is shown in figure 2, which shows the input-to-
ply resistive, such that the reactive output impedance inter- output isolation and the cascaded sequence of amplifiers
acts with the reactive impedance of the load resistance and that implement the pole-zero scheme outlined above.
capacitance. In addition, it is necessary to place the domi- The ILC7080/81 were designed in a CMOS process with
nant pole of the circuit at a sufficiently low frequency such some minor additions, which allow the circuit to be used at
that the gain of the regulator has fallen below unity before input voltages up to 13V. The resulting circuit exceeds the
any of the complex interactions between the output and the frequency response of traditional bipolar circuits. The
load occur. The ILC7080/81 does not try to eliminate the ILC7080/81 is very tolerant of output load conditions with
output pole, but incorporates it into the stability scheme. the inclusion of both short circuit and thermal overload pro-
The load and output capacitor forms a pole, which rolls off tection. The device has a very low dropout voltage, typical-
the gain of the regulator below unity. In order to do this the ly a linear response of 1mV per milliamp of load current,
output impedance of the regulator must be high, looking like and none of the quasi-saturation characteristics of a bipolar
a current source. The output stage of the regulator output device. All the good features of the frequency
becomes a transconductance amplifier, which converts a response and regulation are valid right to the point where
voltage to a current with a substantial output impedance. the regulator goes out of regulation in a 4mV transition
The circuit which drives the transconductance amplifier is region. Because there is no base drive, the regulator is
the error amplifier, which compares the regulator output to capable of providing high current surges while remaining in
the band gap reference and produces an error voltage as regulation. This is shown in the high peak current of 500mA
the input to the transconductance amplifier. The error ampli- which allows for the ILC7080/81 to be used in systems that
fier has a dominant pole at low frequency and a “zero” require short burst mode operation.
which cancels out the effects of the pole. The zero allows
DOMINANT POLE
85 dB
OUTPUT POLE
COMPENSATING
ZERO
UNITY GAIN
FREQUENCY
Figure 1: ILC7080/81 RF LDO frequency response
Impala Linear Corporation
ILC7080/81 1.1
www.impalalinear.com
(408) 574-3939
Sept. 1998
5
IMPALA [ Impala Linear Corporation ]
