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PDF ADP1108AR-5 Data sheet ( Hoja de datos )

Número de pieza ADP1108AR-5
Descripción Micropower DC-DC Converter Adjustable and Fixed 3.3 V/ 5 V/ 12 V
Fabricantes Analog Devices 
Logotipo Analog Devices Logotipo



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a
Micropower DC-DC Converter
Adjustable and Fixed 3.3 V, 5 V, 12 V
ADP1108
FEATURES
Operates at Supply Voltages From 2.0 V to 30 V
Consumes Only 110 A Supply Current
Step-Up or Step-Down Mode Operation
Minimum External Components Required
Low Battery Detector Comparator On-Chip
User-Adjustable Current Limit
Internal 1 A Power Switch
Fixed or Adjustable Output Voltage Versions
8-Pin DIP or SO-8 Package
APPLICATIONS
Notebook/Palm Top Computers
3 V to 5 V, 5 V to 12 V Converters
9 V to 5 V, 12 V to 5 V Converters
LCD Bias Generators
Peripherals and Add-On Cards
Battery Backup Supplies
Cellular Telephones
Portable Instruments
GENERAL DESCRIPTION
The ADP1108 is a highly versatile micropower switch-mode
dc-dc converter that operates from an input voltage supply as
low as 2.0 V and typically starts up from 1.8 V.
The ADP1108 can be programmed into a step-up or step-down
dc-to-dc converter with only three external components. The
fixed outputs are 3.3 V, 5 V and 12 V. An adjustable version is
also available. In step-up mode, supply voltage range is 2.0 V to
12 V, and 30 V in step-down mode. The ADP1108 can deliver
150 mA at 5 V from a 2 AA cell input and 300 mA at 5 V from
a 9 V input in step-down mode. Switch current limit can be
programmed with a single resistor.
For battery operated and power conscious applications, the
ADP1108 offers a very low power consumption of less than
110 µA.
The auxiliary gain block available in ADP1108 can be used
as a low battery detector, linear post regulator, under voltage
lockout circuit or error amplifier.
FUNCTIONAL BLOCK DIAGRAMS
SET
VIN
1.245V
REFERENCE
A2
GAIN BLOCK/
ERROR AMP
ADP1108
A1 OSCILLATOR
AO
ILIM
SW1
COMPARATOR
DRIVER
GND
FB
SW2
SET
VIN
1.245V
REFERENCE
ADP1108-3.3
ADP1108-5
ADP1108-12
A2
GAIN BLOCK/
ERROR AMP
A1 OSCILLATOR
R1
GND
COMPARATOR
DRIVER
R2
753k
ADP1108-3.3: R1 = 456k
ADP1108-5: R1 = 250k
ADP1108-12: R1 = 87.4k
SENSE
AO
ILIM
SW1
SW2
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: 617/329-4700
World Wide Web Site: http://www.analog.com
Fax: 617/326-8703
© Analog Devices, Inc., 1997

1 page




ADP1108AR-5 pdf
0.58
0.53
0.48
0.43
VCE (SAT) @ ISW = 0.65A
0.38
0.33
0.28
–40
0 25 70
TEMPERATURE – °C
85
Figure 10. Switch Saturation Voltage In
Step-Up Mode vs. Temperature
ADP1108
1.2
1.15
1.1
1.05
1
0.95
0.9
–40
VCE (SAT) @ ISW = 0.65A
0 25 70
TEMPERATURE – °C
85
Figure 11. Switch Saturation Voltage In
Step-Down Mode vs. Temperature
THEORY OF OPERATION
The ADP1108 is a flexible, low power Switch Mode Power
Supply (SMPS) controller. The regulated output voltage can be
greater than the input voltage (boost or step-up mode) or less
than the input (buck or step-down mode). This device uses a
gated-oscillator technique to provide very high performance
with low quiescent current.
A functional block diagram of the ADP1108 is shown on
the front page. The internal 1.245 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 1.245 V, the 19 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
exceeds 1.245 V, the oscillator is shut off. While the oscillator is
off, the ADP1108 quiescent current is only 110 µA. The
comparator includes a small amount of hysteresis, which
ensures loop stability without requiring external components
for frequency compensation.
The maximum current in the internal power switch can be set
by connecting a resistor between VIN and the ILIM pin. When
the maximum current is exceeded, the switch is turned OFF.
The current limit circuitry has a time delay of about 2 µs. If
an external resistor is not used, connect ILIM to VIN. Further
information on ILIM is included in the Limiting the Switch
Current section of this data sheet.
The ADP1108 internal oscillator provides 36 µs ON and 17 µs
OFF times, which is ideal for applications where the ratio
between VIN and VOUT is roughly a factor of three (such as
generating +5 V from a +2 V input). The 36 µs/17 µs ratio
permits continuous mode operation in such cases, which
increases the available output power.
An uncommitted gain block on the ADP1108 can be connected
as a low-battery detector. The inverting input of the gain block
is internally connected to the 1.245 V reference. The noninverting
input is available at the SET pin. A resistor divider, connected
between VIN and GND with the junction connected to the SET
pin, causes the AO output to go LOW when the low battery set
point is exceeded. The AO output is an open collector NPN
transistor that can sink 100 µA.
The ADP1108 provides external connections for both the collector
and emitter of its internal power switch, which permits both
step-up and step-down modes of operation. For the step-up mode,
the emitter (Pin SW2) is connected to GND and the collector
(Pin SW1) drives the inductor. For step-down mode, the emitter
drives the inductor while the collector is connected to VIN.
The output voltage of the ADP1108 is set with two external
resistors. Three fixed-voltage models are also available: ADP1108-
3.3 (+3.3 V), ADP1108-5 (+5 V) and ADP1108-12 (+12 V). The
fixed-voltage models are identical to the ADP1108, except that
laser-trimmed voltage-setting resistors are included on the chip.
On the fixed-voltage models of the ADP1108, simply connect
the feedback pin (Pin 8) directly to the output voltage.
COMPONENT SELECTION
General Notes on Inductor Selection
When the ADP1108 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
transferred to the load when the switch turns off. Both the
collector and the emitter of the switch transistor are accessible
on the ADP1108, so the output voltage can be higher, lower, or
of opposite polarity than the input voltage.
To specify an inductor for the ADP1108, the proper values of
inductance, saturation current, and dc resistance must be
determined. This process is not difficult, and specific equations
for each circuit configuration are provided in this data sheet. In
general terms, however, the inductance 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 VIN and
switch ON time are at their maximum values. The inductor
must also store enough energy to supply the load, without
saturating. Finally, the dc resistance of the inductor should be
low, so that excessive power will not be wasted by heating the
windings. For most ADP1108 applications, an inductor of
47 µH to 330 µ, with a saturation current rating of 300 mA to
1 A and dc resistance < 0.4 is suitable. Ferrite core inductors
that meet these specifications are available in small, surface-
mount packages.
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.
REV. 0
–5–

5 Page





ADP1108AR-5 arduino
ADP1108
PROGRAMMING THE GAIN BLOCK
The gain block of the ADP1108 can be used as a low battery de-
tector, error amplifier or linear post regulator. The gain block
consists of an op amp with PNP inputs and an open-collector
NPN output. The inverting input is internally connected to the
ADP1108’s 1.245 V reference, while the noninverting input is
available at the SET Pin. The NPN output transistor will sink
about 300 µA.
Figure 24a shows the gain block configured as a low-battery
monitor. Resistors R1 and R2 should be set to high values to re-
duce quiescent current, but not so high that bias current in the
SET input causes large errors. A value of 33 kfor R2 is a good
compromise.
The value for R1 is then calculated from the formula:
R1 = V LOBATT – 1.245 V
1.245 V
 R2 
where VLOBATT is the desired low battery trip point. Since the
gain block output is an open-collector NPN, a pull-up resistor
should be connected to the positive logic power supply.
VBAT
R1 ADP1108
1.245V
SET REF
7
R2
33k
GND
5
+5V
2
VIN 47k
AO
6
TO
PROCESSOR
R1 = VLB –1.245V
37.7µA
VLB = BATTERY TRIP POINT
Figure 24a. Setting the Low Battery Detector Trip Point
The circuit of Figure 24a may produce multiple pulses when ap-
proaching the trip point, due to noise coupled into the SET in-
put. To prevent multiple interrupts to the digital logic, hysteresis
can be added to the circuit (Figure 24b). Resistor R3, with a
value of 1 Mto 10 M, provides the hysteresis. The addition
of R3 will change the trip point slightly, so the new value for R1
will be:
R1 =
V LOBATT – 1.245 V

1.245 V
R2

V

L – 1.245 V
RL + R3

where VL is the logic power supply voltage, RL is the pull-up
resistor and R3 creates the hysteresis.
VBAT
R1
R2
33k
ADP1108
1.245V
SET REF
7
GND
5
+5V
2
VIN 47k
AO
6
TO
PROCESSOR
R3
1.6M
Figure 24b. Adding Hysteresis to the Low Battery Detector
VIN
6.5V TO 20V
C2
0.22
ZETEX
ZTX749
100100
L1*
100H
D1
1N5818
5VOUT
200mA AT 6.5V
500mA AT 8V
C1
220
21
3
VIN ILIM
SW1
SENSE 8
ADP1108-5
GND SW2
54
*L1 = COILTRONICS CTX100-4
Figure 25. 6.5 V to 5 V Step-Down Converter
VIN
6.5V TO 20V
+
C2
220
21
3
VIN ILIM SW1
ADP1108-5
GND
5
SENSE 8
SW2
4
L1*
300µH
MBRS130T3
*L1 = COILTRONICS CTX300-4
+
330µF
–5V OUTPUT
150mA
Figure 26. Positive to –5 V Converter
REV. 0
–11–

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