ADP1109A Analog Devices, ADP1109A Datasheet - Page 6

• Manufacturer Part Number: ADP1109A
• Description: Micropower Low Cost Fixed 3.3 V/ 5 V/ 12 V and Adjustable DC-to-DC Converter
• Manufacturer: Analog Devices

ADP1109A
APPLICATION INFORMATION
THEORY OF OPERATION
The ADP1109A is a flexible, low power switch-mode power
supply (SMPS) controller for step-up dc/dc converter applica-
tions. 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 360 A.
A functional block diagram of the ADP1109A is shown on the
front page. 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 pro-
vides base drive to the internal power switch, and the switching
action raises the output voltage. When the voltage at the FB pin
exceeds 1.25 V, the oscillator is shut off. While the oscillator is
off, the ADP1109A quiescent current is only 460 A. The com-
parator includes a small amount of hysteresis, which ensures
loop stability without requiring external components for fre-
quency compensation.
A shutdown feature permits the oscillator to be shut off. Hold-
ing SHUTDOWN low will disable the oscillator, and the
ADP1109A’s quiescent current will remain 460 A.
The output voltage of the ADP1109A is set with two external
resistors. Three fixed-voltage models are also available: the
ADP1109A-3.3 (+3.3 V), ADP1109A-5 (+5 V) and ADP1109A-12
(+12 V). The fixed-voltage models are identical to the ADP1109A,
except that laser-trimmed voltage-setting resistors are included on
the chip. On the fixed-voltage models of the ADP1109A, simply
connect the SENSE pin (Pin 8) directly to the output voltage.
COMPONENT SELECTION
General Notes on Inductor Selection
When the ADP1109A 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 ADP1109A, 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
when both V
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 ADP1109A applications, an
inductor of 10 H to 47 H, with a saturation current rating of
300 mA to 1 A and dc resistance <0.4
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 ADP1109A is designed for applications where the input
voltage is fairly stable, such as generating +12 V from a +5 V
logic supply. The ADP1109A 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
IN
and switch ON time are at their maximum val-
is suitable. Ferrite core
–6–
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,
2. Calculate the inductor value, using the equations in the fol-
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:
where V
Schottky). Energy is only stored in the inductor while the
ADP1109A switch is ON, so the energy stored in the inductor
on each switching cycle must be must be equal to or greater
than:
in order for the ADP1109A to regulate the output voltage. When
the internal power switch turns ON, current flow in the inductor
increases at the rate of:
where L is in Henrys and R' is the sum of the switch equivalent
resistance (typically 0.8
the inductor. In most applications, the voltage drop across the
switch is small compared to V
used:
Replacing t in the above equation with the ON time of the
ADP1109A (5.5 s, 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:
As previously mentioned, E
that the ADP1109A can deliver the necessary power to the load.
For best efficiency, peak current should be limited to 1 A or
less. Higher switch currents will reduce efficiency because of
increased saturation voltage in the switch. High peak current
also increases output ripple. As a general rule, keep peak current
as low as possible to minimize losses in the switch, inductor and
diode.
maximum input voltage, output voltage and output current.
lowing section.
P
I
I
E
f
L
L
P
OSC
L
L
L
D
t
t
is the diode forward voltage ( 0.5 V for a 1N5818
1
2
V
L I
OUT
V
V
R'
L
IN
IN
2
t
peak
1 e
V
D
R't
L
V
IN MIN
at +25 C) and the dc resistance of
L
must be greater than P
IN
so a simpler equation can be
I
OUT
L
/f
OSC
REV. 0
so
(1)
(2)
(3)
(4)
(5)