REV. A

ADP3331

–6–

THEORY OF OPERATION

The ADP3331 anyCAP LDO uses a single control loop for both

regulation and reference functions, as shown in Figure 2. The

output voltage is sensed by an external resistive voltage divider

consisting of R1 and R2. 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.

PTAT

NONINVERTING

WIDEBAND

DRIVER

INPUT

Q1

ADP3331

COMPENSATION

CAPACITOR

ATTENUATION

R1

D1

R2

R3

R4

OUTPUT

PTAT

CURRENT

(a)

GND

Figure 2. 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. Ulti-

mately, 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, which leads to a low noise design.

The R1, R2 divider is chosen in the same ratio as the band gap

voltage to 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 are chosen to produce a temperature stable

output. This unique arrangement specifically corrects for the

loading of the divider so that the error resulting from the 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 capacitor.

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 the load capacitance and resistance. More-

over, the ESR value required to keep conventional LDOs stable

changes, depending on load and temperature. These ESR limita-

tions make designing with LDOs more difficult because of their

unclear specifications and extreme variations over temperature.

The ADP3331 solves this problem. It can be used with any good

quality capacitor, with no constraint on the minimum ESR. The

innovative design allows the circuit to be stable with just a small

0.47

mF capacitor on the output. Additional advantages of the

pole-splitting scheme include superior line noise rejection and

very high regulator gain. The high gain leads to excellent regula-

tion, and

±1.4% accuracy is guaranteed over line, load, and

temperature.

Additional features of the circuit include current limit, thermal

shutdown, and an error flag. Compared to standard solutions that

give a warning after the output has lost regulation, the ADP3331

provides improved system performance by enabling the

ERR pin

to give a warning just before the device loses regulation.

As the chip’s temperature rises above +165

∞C, the circuit acti-

vates a soft thermal shutdown to reduce the current to a safe

level. The thermal shutdown condition is indicated by the

ERR

signal going low.

APPLICATION INFORMATION

Capacitor Selection

Output Capacitor: The stability and transient response of the

LDO is a function of the output capacitor. The ADP3331 is stable

with a wide range of capacitor values, types, and ESR (anyCAP).

A capacitor as low as 0.47

mF is all that is needed for stability;

larger capacitors can be used if high current surges on the output

are anticipated. The ADP3331 is stable with extremely low ESR

capacitors (ESR

ª 0), such as multilayer ceramic capacitors

(MLCC) or OSCON. Note that the effective capacitance of some

capacitor types falls below the minimum over temperature or

with dc voltage.

Input Capacitor: An input bypass capacitor is not strictly required

but is recommended in any application involving long input

wires or high source impedance. Connecting a 0.47

mF capacitor

from the input to ground reduces the circuit’s sensitivity to

PC board layout and input transients. If a larger output capacitor

is necessary, a larger value input capacitor is also recommended.

Noise Reduction Capacitor: A noise reduction capacitor can be

used to reduce the output noise by 6 dB to 10 dB. This capaci-

tor limits the noise gain when connected between the feedback

pin (FB) and the output pin (OUT), as shown in Figure 3. Low

leakage capacitors in the 10 pF to 500 pF range provide the best

performance. Since FB is internally connected to a high imped-

ance 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. When adding a noise reduction capacitor,

use the following guidelines:

∑ Maintain a minimum load current of 1 mA when not in

shutdown.

∑ For CNR values greater than 500 pF, add a 100 kW series

resistor (RNR).

It is important to note that as CNR increases, the turn-on time

will be delayed. With CNR values greater than 1 nF, this delay

may be on the order of several milliseconds.