ADP3333
THEORY OF OPERATION
The ADP3333 anyCAP LDO uses a single control loop for
regulation and reference functions (see Figure 22). The output
voltage is sensed by a resistive voltage divider consisting of R1
and R2 that is varied to provide the available output voltage
option. Feedback is taken from this network by way of a series
diode (D1) and a second resistor divider (R3 and R4) to the
input of an amplifier.
temperature stable output. This unique arrangement specifically
corrects for the loading of the divider so that the error resulting
from base current loading in conventional circuits is avoided.
The patented amplifier controls a new and unique noninverting
driver that drives the pass transistor, Q1. The use of this special
noninverting driver enables the frequency compensation to
include the load capacitor in a pole splitting arrangement to
achieve reduced sensitivity to the value, type, and ESR of the
load capacitance.
INPUT
OUTPUT
ATTENUATION
BAND GAP OUT
Q1
(V /V
)
R1
(a)
COMPENSATION
CAPACITOR
Most LDOs place very strict requirements on the range of ESR
values for the output capacitor because they are difficult to
stabilize due to the uncertainty of load capacitance and resistance.
Moreover, the ESR value required to keep conventional LDOs
stable changes depending on load and temperature. These ESR
limitations make designing with LDOs more difficult because
of their unclear specifications and extreme variations over
temperature.
C
L
D1
R3
PTAT
OS
NONINVERTING
WIDEBAND
DRIVER
FB
V
g
m
PTAT
CURRENT
R
L
R4
R2
ADP3333
GND
Figure 22. Functional Block Diagram
A very high gain error amplifier is used to control this loop.
The amplifier is constructed in such a way that at equilibrium it
produces a large, temperature-proportional input offset voltage
that is repeatable and very well controlled. The temperature
proportional offset voltage is combined with the complementary
diode voltage to form a virtual band gap voltage, implicit in the
network, although it never appears explicitly in the circuit.
Ultimately, this patented design makes it possible to control the
loop with only one amplifier. This technique also improves the
noise characteristics of the amplifier by providing more flexibility
on the trade-off of noise sources and leads to a low noise design.
With the ADP3333 anyCAP LDO, this is no longer true. This
device can be used with virtually any good quality capacitor,
with no constraint on the minimum ESR. Its innovative design
allows the circuit to be stable with just a small 1.0 μF capacitor
on the output. Additional advantages of the pole splitting
scheme include superior line noise rejection and very high
regulator gain, which leads to excellent line and load regulation.
An impressive 1.8% accuracy is guaranteed over line, load, and
temperature.
Additional features of the circuit include current limit and
thermal shutdown.
The R1, R2 divider is chosen in the same ratio as the band gap
voltage to the output voltage. Although the R1/R2 resistor
divider is loaded by the diode, D1, and a second divider
consisting of R3 and R4, the values can be chosen to produce a
Rev. B | Page 9 of 12
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