ADP3303A Analog Devices, ADP3303A Datasheet - Page 6

• Manufacturer Part Number: ADP3303A
• Description: High Accuracy anyCAP Adjustable 200 mA Low Dropout Linear Regulator
• Manufacturer: Analog Devices

ADP3303A
THEORY OF OPERATION
The new anyCAP LDO ADP3303A 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.
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.
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.
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.
Most LDOs place strict requirements on the range of ESR val-
ues for the output capacitor because they are difficult to sta-
bilize 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.
This is no longer true with the ADP3303A 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 1 F capacitor on the output. Addi-
tional advantages of the pole splitting scheme include superior line
INPUT
ADP3303A
NONINVERTING
WIDEBAND
DRIVER
Q1
Figure 20. Functional Block Diagram
COMPENSATION
CAPACITOR
g
m
PTAT
V
OS
R4
GND
(V
ATTENUATION
CURRENT
BANDGAP
R3
PTAT
OUTPUT
/V
D1
OUT
)
R1
R2
(a)
C
R
LOAD
LOAD
–6–
noise rejection and very high regulator gain, which leads to excel-
lent line and load regulation. An impressive 1.4% accuracy is
guaranteed over line, load and temperature.
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
ADP3303A provides improved system performance by enabling
the ERR Pin to give warning before the device loses regulation.
As the chip’s temperature rises above 165 C, the circuit acti-
vates a soft thermal shutdown, indicated by a signal low on the
ERR Pin, to reduce the current to a safe level.
APPLICATION INFORMATION
The ADP3303A is very easy to use. The only external compo-
nent required for stability is a small 1 F bypass capacitor on the
output. If the shutdown feature is not used, the shutdown pin
(Pin 8) should be tied to the input pin.
CAPACITOR SELECTION
Bypass Capacitor (C1): connecting a 0.47 F capacitor from the
IN pins (Pins 10 and 11) to ground greatly improves its line
transient response and reduces the circuit’s sensitivity to PC
board layout. A larger capacitor could be used if line transients
of longer duration are expected.
Output Capacitor (C2): as will all members of the anyCAP low
dropout regulator family, the ADP3303A is stable with any type
of output capacitor down to zero ESR. A small 1 F output
capacitor is required for stability. Larger capacitors with low
ESR are recommended for improved load transient response.
For space limited applications, Multilayer Ceramic Capacitors
(MLCC) are a good choice. For low temperature operations
OS-CON capacitors offer better performance.
Noise Reduction Capacitor (CNR): to reduce the ADP3303A’s
low output noise by 6 dB–10 dB, a noise gain limiting capacitor
can be connected between the feedback (FB) pin (Pin 6) and
the OUT pins as shown in Figure 21. Low leakage capacitors
in the 100 pF–500 pF range provide the best performance.
Larger capacitors will slow down the output transient response.
CNR is not needed in low noise applications where fast load
transients are not expected.
OUTPUT VOLTAGE SELECTION
The ADP3303A is characterized by having the output voltage
divider placed externally. The output voltage will be divided by
R1 and R2 and fed back to the FB pin.
In order to have the lowest possible sensitivity of output voltage
versus any temperature variation, it is important that the parallel
resistance of R1 and R2 is always 44 k .
V
IN
C1
Figure 21. Noise Reduction Circuit
10
11
IN
ADP3303A
SD
8
OUT
FB
7
GND
9
4
5
6
R1
R2
R3
330k
C
NR
ERR
1 F
V
OUT
REV. A
= +5V