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ADP1108AN Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer ADP1108AN
Beschreibung Micropower DC-DC Converter Adjustable and Fixed 3.3 V/ 5 V/ 12 V
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 12 Seiten
ADP1108AN Datasheet, Funktion
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






ADP1108AN Datasheet, Funktion
ADP1108
Calculating the Inductor Value
Selecting the proper inductor value is a simple three-step
process:
1. Define the operating parameters: minimum input voltage,
maximum input voltage, output voltage and output current.
2. Select the appropriate conversion topology (step-up, step-
down or inverting).
3. Calculate the inductor value, using the equations in the fol-
lowing sections.
On each switching cycle, the inductor must supply:
PL
f OSC
=
315 mW
19 kHz
=16.6 µ J
The required inductor power is fairly low in this example, so the
peak current can also be low. Assuming a peak current of
500 mA as a starting point, Equation 4 can be rearranged to
recommend an inductor value:
Inductor Selection—Step-Up Converter
In a step-up or boost converter (Figure 15), 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:
L
=
V IN
IL(MAX )
t
=
2V
500 mA
36
µs
= 144
µH
Substituting a standard inductor value of 100 µH with 0.2 dc
resistance, will produce a peak switch current of:
( ) ( )PL = VOUT +V D V IN(MIN ) × IOUT
(Equation 1)
where VD is the diode forward voltage (0.5 V for a 1N5818
Schottky). Energy is only stored in the inductor while the
I PEAK
=
2V
1.0
1 –
e
–1.0 Ω×36
100 µH
µs

= 605 mA
Once the peak current is known, the inductor energy can be
ADP1108 switch is ON, so the energy stored in the inductor on calculated from Equation 5:
each switching cycle must be equal to or greater than:
PL
( )f OSC
(Equation 2)
EL
=
1
2
100
µH
×
605 mA
2
=18.3 µ J
in order for the ADP1108 to regulate the output voltage.
When the internal power switch turns ON, current flow in the
inductor increases at the rate of:
The inductor energy of 18.3 µJ is greater than the PL/fOSC
requirement of 16.6 µJ, so the 100 µH inductor will work in this
application. By substituting other inductor values into the same
IL
(t)
=
V IN
R'
1
R 't
eL

(Equation 3)
where L is in henrys and RЈ is the sum of the switch equivalent
resistance (typically 0.8 at +25°C) and the dc resistance of
the inductor. If the voltage drop across the switch is small
compared to VIN, a simpler equation can be used:
I
L
(t
)
=
V IN
L
t
(Equation 4)
equations, the optimum inductor value can be selected. When
selecting an inductor, the peak current must not exceed the
maximum switch current of 1.5 A. If the calculated peak current
is greater than 1.5 A, either the ADP3000 should be considered
or an external power transistor can be used.
The peak current must be evaluated for both minimum and
maximum values of input voltage. If the switch current is high
when VIN is at its minimum, the 1.5 A limit may be exceeded at the
maximum value of VIN. In this case, the current limit feature of
the ADP1108 can be used to limit switch current. Simply select
a resistor (using Figure 3) that will limit the maximum switch
Replacing t in the above equation with the ON time of the
ADP1108 (36 µs, typical) will define the peak current for a
given inductor value and input voltage. At this point, the
inductor energy can be calculated as follows:
current to the IPEAK value calculated for the minimum value of
VIN. This will improve efficiency by producing a constant
IPEAK as VIN increases. See the Limiting the Switch Current
section of this data sheet for more information.
Note that the switch current limit feature does not protect the
EL
=
1L
2
×
I2
PEAK
(Equation 5)
As previously mentioned, EL must be greater than PL/fOSC so the
ADP1108 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 satura-
tion 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 equations
above. For example, consider a supply that will generate 12 V
at 30 mA from a 3 V battery, assuming a 2 V end-of-life voltage.
The inductor power required is from Equation 1:
PL = (12 V + 0.5V – 2V ) × (30 mA) = 315 mW
circuit if the output is shorted to ground. In this case, current is
limited only by the dc resistance of the inductor and the forward
voltage of the diode.
Inductor Selection—Step-Down Converter
The step-down mode of operation is shown in Figure 16. Unlike
the step-up mode, the ADP1108’s power switch does not
saturate when operating in the step-down mode. Therefore,
switch current should be limited to 650 mA in this mode. If the
input voltage will vary over a wide range, the ILIM pin can be
used to limit the maximum switch current. Higher switch current
is possible by adding an external switching transistor, as shown
in Figure 18.
The first step in selecting the step-down inductor is to calculate
the peak switch current as follows:
I PEAK
=
2
IOUT
DC

V
VOUT + V D
IN V SW + V
D

(Equation 6)
–6– REV. 0

6 Page









ADP1108AN pdf, datenblatt
ADP1108
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Lead Plastic DIP
(N-8)
0.430 (10.92)
0.348 (8.84)
85
0.280 (7.11)
0.240 (6.10)
14
0.325 (8.25)
PIN 1
0.060 (1.52)
0.300 (7.62)
0.210 (5.33)
0.015 (0.38)
MAX
0.130
0.160 (4.06)
0.115 (2.93)
(3.30)
MIN
0.022 (0.558) 0.100 0.070 (1.77)
0.014 (0.356) (2.54) 0.045 (1.15)
SEATING
PLANE
BSC
0.195 (4.95)
0.115 (2.93)
0.015 (0.381)
0.008 (0.204)
8-Lead SOIC
(SO-8)
0.1968 (5.00)
0.1890 (4.80)
8
0.1574 (4.00)
0.1497 (3.80) 1
5
0.2440 (6.20)
4 0.2284 (5.80)
PIN 1
0.0098 (0.25)
0.0040 (0.10)
0.0688 (1.75)
0.0532 (1.35)
0.0196
0.0099
(0.50)
(0.25)
x
45°
SEATING
PLANE
0.0500 0.0192 (0.49)
(1.27) 0.0138 (0.35)
BSC
0.0098 (0.25)
0.0075 (0.19)
8°
0° 0.0500 (1.27)
0.0160 (0.41)
–12–
REV. 0

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