ADP3303
THEORY OF OPERATION
This is no longer true with the ADP3303 anyCAP™ LDO. It
can be used with virtually any capacitor, with no constraint on
the minimum ESR. The innovative design allows the circuit to
be stable with just a small 0.47 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 ±1.4% accuracy is
guaranteed over line, load and temperature.
The new anyCAP™ LDO ADP3303 uses a single control loop
for regulation and reference functions. The output voltage is
sensed by a resistive voltage divider consisting of R1 and R2,
which is varied to provide the available output voltage options.
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.
Additional features of the circuit include current limit, thermal
shutdown and noise reduction. Compared to standard solutions
that give warning after the output has lost regulation, the
ADP3303 provides improved system performance by enabling the
ERR Pin to give warning before the device loses regulation.
OUTPUT
R1
INPUT
Q1
COMPENSATION
CAPACITOR
ATTENUATION
BANDGAP OUT
(V
/V
)
D1
R3
PTAT
As the chip’s temperature rises above 165°C, the circuit
activates a soft thermal shutdown, indicated by a signal low on
the ERR Pin, to reduce the current to a safe level.
(a)
NONINVERTING
WIDEBAND
DRIVER
V
OS
Gm
PTAT
R
LOAD
CURRENT
R2
R4
C
LOAD
To reduce the noise gain of the loop, the node of the main
divider network (a) is made available at the noise reduction (NR)
pin, which can be bypassed with a small capacitor (10 nF–100 nF).
ADP3303
APPLICATION INFORMATION
Capacitor Selection: anyCAP™
Figure 20. Functional Block Diagram
Output Capacitors: as with any micropower device, output
transient response is a function of the output capacitance. The
ADP3303 is stable with a wide range of capacitor values, types
and ESR (anyCAP™). A capacitor as low as 0.47 µF is all that is
needed for stability; larger capacitors can be used if high output
current surges are anticipated. The ADP3303 is stable with
extremely low ESR capacitors (ESR ≈ 0), such as Multilayer
Ceramic Capacitors (MLCC) or OSCON.
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 bandgap” 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 flexibil-
ity on the trade-off of noise sources that leads to a low noise
design.
Input Bypass Capacitor: an input bypass capacitor is not
required; for applications where the input source is high
impedance or far from the input pins, a bypass capacitor is
recommended. Connecting a 0.47 µF capacitor from the input
pins (Pins 7 and 8) to ground reduces the circuit’s sensitivity to
PC board layout. If a bigger output capacitor is used, the input
capacitor should be 1 µF minimum.
The R1, R2 divider is chosen in the same ratio as the bandgap
voltage to the output voltage. Although the R1, R2 resistor
divider is loaded by the diode D1, and a second divider consist-
ing of R3 and R4, the values are chosen to produce a tempera-
ture 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.
Noise Reduction
A noise reduction capacitor (CNR) can be used to further reduce
the noise by 6 dB–10 dB (Figure 21). Low leakage capacitors in
the 10 nF–100 nF range provide the best performance. Since
the noise reduction pin (NR) is internally connected to a high
impedance node, any connection to this node should be carefully
done to avoid noise pickup from external sources. The pad
connected to this pin should be as small as possible. Long PC
board traces are not recommended.
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.
3
NR
Most LDOs place 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 resis-
tance. 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
NR
ADP3303-5.0
10nF
1
2
7
8
OUT
V
= 5V
IN
V
OUT
IN
R1
330kΩ
+
+
C2
10µF
C1
1µF
ERR
4
6
E
OUT
5
ON
OFF
GND
SD
Figure 21. Noise Reduction Circuit
REV. 0
–6–
5秒后页面跳转
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