Data Sheet

ADP3339

Rev. C | Page 9 of 12

THEORY OF OPERATION

The ADP3339 anyCAP LDO 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 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.

A very high gain error amplifier is used to control this loop. The

amplifier is constructed in such a way that equilibrium produces a

large, temperature-proportional input offset voltage that is repeata-

ble and very well controlled. The temperature-proportional

offset voltage is combined with the complementary diode volt-

age to form a virtual band gap voltage that is 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 band gap

voltage to the output voltage. Although the R1/R2 resistor

divider is loaded by Diode D1 and a second divider consisting

of R3 and R4, the values can be chosen to produce a temperature-

stable output. This unique arrangement specifically corrects for

the loading of the divider, thus avoiding the error resulting from

base current loading in conventional circuits.

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 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 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.

With the ADP3339 anyCAP LDO, this is no longer true. The

ADP3339 can be used with virtually any good quality capacitor,

with no constraint on the minimum ESR. This innovative

design allows the circuit to be stable with just a small 1 μF

capacitor on the output. Additional advantages of the pole-

splitting scheme include superior line noise rejection and very

high regulator gain, which lead to excellent line and load

regulation. An impressive ±1.5% accuracy is guaranteed over

line, load, and temperature.

Additional features of the circuit include current limit and

thermal shutdown.

OUT

ADP3339

C1

1

μF

C2

1

μF

GND

IN

Figure 20. Typical Application Circuit

PTAT

NONINVERTING

WIDEBAND

DRIVER

INPUT

Q1

ADP3339

COMPENSATION

CAPACITOR

ATTENUATION

R1

D1

R2

R3

R4

OUTPUT

PTAT

CURRENT

(a)

GND

Figure 21. Functional Block Diagram