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ADP1109 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer ADP1109
Beschreibung Micropower Low Cost Fixed 3.3 V/ 5 V/ 12 V and Adjustable DC-to-DC Converter
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 8 Seiten
ADP1109 Datasheet, Funktion
a
Micropower Low Cost
Fixed 3.3 V, 5 V, 12 V and Adjustable
DC-to-DC Converter
FEATURES
Operates at Supply Voltages 2 V to 12 V
Fixed 3.3 V, 5 V, 12 V and Adjustable Output
Minimum External Components Required
Ground Current: 320 A
Oscillator Frequency: 120 kHz
Logic Shutdown
8-Lead DIP and SO-8 Packages
APPLICATIONS
Cellular Telephones
Single-Cell to 5 V Converters
Laptop and Palmtop Computers
Pagers
Cameras
Battery Backup Supplies
Portable Instruments
Laser Diode Drivers
Hand-Held Inventory Computers
GENERAL DESCRIPTION
The ADP1109 is a versatile step-up switching regulator. The
device requires only minimal external components to operate as
a complete switching regulator.
The ADP1109-5 can deliver 100 mA at 5 V from a 3 V input
and the ADP1109-12 can deliver 60 mA at 12 V from a 5 V
input. The device also features a logic controlled shutdown
capability that, when a logic low is applied, will shut down the
oscillator.
The 120 kHz operating frequency allows for the use of small
surface mount components. The gated oscillator capability
eliminates the need for frequency compensation.
ADP1109
FUNCTIONAL BLOCK DIAGRAMS
VIN SENSE
1.25V
REFERENCE
R2
250k
ADP1109-3.3: R1 = 152k
ADP1109-5: R1 = 83k
ADP1109-12: R1 = 29k
COMPARATOR
+
A1
120kHz
OSCILLATOR
Q1
SW
DRIVER
R1
GND
SHUTDOWN
Fixed Output
VIN FB
1.25V
REFERENCE
COMPARATOR
+
A1
ADP1109
SW
120kHz
OSCILLATOR
Q1
DRIVER
GND
SHUTDOWN
Adjustable Output
TYPICAL APPLICATION
L1
33H
D1
VIN
5V
VIN SW SENSE
ADP1109-12
SHUTDOWN
GND
SHUTDOWN/PROGRAM
+ C1
22F
16V
VOUT
12V
60mA
Flash Memory VPP Generator
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1998






ADP1109 Datasheet, Funktion
ADP1109
As previously mentioned, EL must be greater than PL/fOSC so
that the ADP1109 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.
In practice, the inductor value is easily selected using the equa-
tions above. For example, consider a supply that will generate
12 V at 120 mA from a +5 V source. The inductor power re-
quired is, from Equation 1:
PL = (12 V + 0.5 V – 5 V) × (120 mA) = 900 mW
On each switching cycle, the inductor must supply:
(6)
PL = 900 mW = 7.5 µJ
f OSC 120 kHz
(7)
The required inductor power is fairly low in this example, so
the peak current can also be low. Assuming a peak current of
600 mA as a starting point, Equation 4 can be rearranged to
recommend an inductor value:
L = V IN t = 5 V 5.5 µs = 45.8 µH
IL(MAX ) 600 mA
(8)
Substituting a standard inductor value of 33 µH, with 0.2 dc
resistance, will produce a peak switch current of:
I PEAK
=
5V
1.0
 –1.0 Ω × 5.5 µs
1 e 33 µH

= 768 mA
(9)
Once the peak current is known, the inductor energy can be
calculated from Equation 5:
( ) ( )EL
=
1
2
33 µH
×
2
768 mA = 9.7 µJ
(10)
The inductor energy of 9.7 µJ is greater than the PL/fOSC re-
quirement of 7.5 µJ, so the 33 µH inductor will work in this
application. By substituting other inductor values into the same
equations, the optimum inductor value can be selected. When
selecting an inductor, the peak current must not exceed the
maximum switch current of 1.2 A. If the calculated peak current
is greater than 1.2 A, either the input voltage must be increased
or the load current decreased.
Output Voltage Selection
The output voltage is fed back to the ADP1109 via resistors R1
and R2 (Figure 5). When the voltage at the comparator’s invert-
ing input falls below 1.25 V, the oscillator turns “on” and the
output voltage begins to rise. The output voltage is therefore set
by the formula:
VOUT
= 1.25 V
×
1 +
R2
R1 
(11)
Resistors R1 and R2 are provided internally on fixed-voltage
versions of the ADP1109. In this case, a complete dc-dc con-
verter requires only four external components.
Capacitor Selection
For optimum performance, the ADP1109’s output capacitor
must be carefully selected. Choosing an inappropriate capacitor
can result in low efficiency and/or high output ripple.
Ordinary aluminum electrolytic capacitors are inexpensive, but
often have poor Equivalent Series Resistance (ESR) and Equiva-
lent Series Inductance (ESL). Low ESR aluminum capacitors,
specifically designed for switch mode converter applications, are
also available, and these are a better choice than general purpose
devices. Even better performance can be achieved with tantalum
capacitors, although their cost is higher. Very low values of ESR
can be achieved by using OS-CON capacitors (Sanyo Corpora-
tion, San Diego, CA). These devices are fairly small, available
with tape-and-reel packaging, and have very low ESR.
Diode Selection
In specifying a diode, consideration must be given to speed,
forward voltage drop and reverse leakage current. When the
ADP1109 switch turns off, the diode must turn on rapidly if
high efficiency is to be maintained. Schottky rectifiers, as well as
fast signal diodes such as the 1N4148, are appropriate. The
forward voltage of the diode represents power that is not
delivered to the load, so VF must also be minimized. Again,
Schottky diodes are recommended. Leakage current is especially
important in low current applications, where the leakage can be
a significant percentage of the total quiescent current.
For most circuits, the 1N5818 is a suitable companion to the
ADP1109. This diode has a VF of 0.5 V at 1 A, 4 µA to 10 µA
leakage, and fast turn-on and turn-off times. A surface mount
version, the MBRS130T3, is also available.
For switch currents of 100 mA or less, a Schottky diode such as
the BAT85 provides a VF of 0.8 V at 100 mA and leakage less
than 1 µA. A similar device, the BAT54, is available in an
SOT-23 package. Even lower leakage, in the 1 nA to 5 nA range,
can be obtained with a 1N4148 signal diode.
General purpose rectifiers, such as the 1N4001, are not suitable
for ADP1109 circuits. These devices, which have turn-on times
of 10 µs or more, are far too slow for switching power supply
applications. Using such a diode “just to get started” will result
in wasted time and effort. Even if an ADP1109 circuit appears
to function with a 1N4001, the resulting performance will not
be indicative of the circuit performance when the correct diode
is used.
–6– REV. 0

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