ADP1109

–5–

REV. 0

APPLICATION INFORMATION

THEORY OF OPERATION

The ADP1109 is a flexible, low power switch-mode power sup-

ply (SMPS) controller for step-up dc/dc converter applications.

This device uses a gated-oscillator technique to provide very

high performance with low quiescent current. For example,

more than 2 W of output power can be generated from a +5 V

source, while quiescent current is only 450

µA.

A functional block diagram of the ADP1109 is shown on page 1.

The internal 1.25 V reference is connected to one input of the

comparator, while the other input is externally connected (via

the FB pin) to a feedback network connected to the regulated

output. When the voltage at the FB pin falls below 1.25 V, the

120 kHz oscillator turns on. A driver amplifier provides base

drive to the internal power switch, and the switching action

raises the output voltage. When the voltage at the FB pin ex-

ceeds 1.25 V, the oscillator is shut off. While the oscillator is off,

the ADP1109 quiescent current is only 450

µA. The comparator

includes a small amount of hysteresis, which ensures loop stabil-

ity without requiring external components for frequency com-

pensation.

A shutdown feature permits the oscillator to be shut off. Hold-

ing

SHUTDOWN low will disable the oscillator, and the

ADP1109’s quiescent current will remain 450

µA.

The output voltage of the ADP1109 is set with two external

resistors. Three fixed-voltage models are also available: the

ADP1109-3.3 (+3.3 V), ADP1109-5 (+5 V) and ADP1109-12

(+12 V). The fixed-voltage models are identical to the ADP1109,

except that laser-trimmed voltage-setting resistors are included on

the chip. On the fixed-voltage models of the ADP1109, simply

connect the SENSE pin (Pin 8) directly to the output voltage.

COMPONENT SELECTION

General Notes on Inductor Selection

When the ADP1109 internal power switch turns on, current

begins to flow in the inductor. Energy is stored in the inductor

core while the switch is on, and this stored energy is then trans-

ferred to the load when the switch turns off.

To specify an inductor for the ADP1109, the proper values of

inductance, saturation current and dc resistance must be deter-

mined. This process is not difficult, and specific equations are

provided in this data sheet. In general terms, however, the induc-

tance value must be low enough to store the required amount of

energy (when both input voltage and switch ON time are at a

minimum), but high enough that the inductor will not saturate

ues. The inductor must also store enough energy to supply the

load, without saturating. Finally, the dc resistance of the induc-

tor should be low, so that excessive power will not be wasted by

heating the windings. For most ADP1109 applications, an in-

ductor of 10

µH to 47 µH, with a saturation current rating of

300 mA to 1 A and dc resistance <0.4

Ω is suitable. Ferrite core

inductors that meet these specifications are available in small,

surface-mount packages. Air-core inductors, as well as RF chokes,

are unsuitable because of their low peak current ratings.

The ADP1109 is designed for applications where the input

voltage is fairly stable, such as generating +12 V from a +5 V

logic supply. The ADP1109 does not have an internal switch

current limiting circuit, so the inductor may saturate if the input

voltage is too high. The ADP1111 or ADP3000 should be

considered for battery powered and similar applications where

the input voltage varies.

To minimize Electro-Magnetic Interference (EMI), a toroid or

pot core type inductor is recommended. Rod core inductors are

a lower cost alternative if EMI is not a problem.

Calculating the Inductor Value

Selecting the proper inductor value is a simple, two-step process:

1. Define the operating parameters: minimum input voltage,

maximum input voltage, output voltage and output current.

2. Calculate the inductor value, using the equations in the fol-

lowing section.

Inductor Selection

In a step-up, or boost, converter (Figure 1), the inductor must

store enough power to make up the difference between the input

voltage and the output voltage. The inductor power is calculated

from the equation:

P

+V

()

OUT

()

(1)

Schottky). Energy is stored in the inductor only while the

ADP1109 switch is ON, so the energy stored in the inductor on

each switching cycle must be must be equal to or greater than:

P

L

f

OSC

(2)

in order for the ADP1109 to regulate the output voltage. When the

internal power switch turns ON, current flow in the inductor

increases at the rate of:

R'

1

− e

L









(3)

where L is in Henrys and R' is the sum of the switch equivalent

resistance (typically 0.8

Ω at +25°C) and the dc resistance of

the inductor. In most applications, the voltage drop across the

used:

I

L

t

(4)

Replacing t in the above equation with the ON time of the

ADP1109 (5.5

µs, typical) will define the peak current for a

given inductor value and input voltage. At this point, the induc-

tor energy can be calculated as follows:

1

2

L

(5)