a
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
Micropower DC-DC Converter
Adjustable and Fixed 3.3 V, 5 V, 12 V
ADP1108
FUNCTIONAL BLOCK DIAGRAMS
SET
ADP1108
A2
V
IN
GAIN BLOCK/
ERROR AMP
1.245V
REFERENCE
I
LIM
SW1
A1
OSCILLATOR
DRIVER
SW2
AO
COMPARATOR
GND
FB
SET
ADP1108-3.3
ADP1108-5
ADP1108-12
V
IN
A2
GAIN BLOCK/
ERROR AMP
1.245V
REFERENCE
A1
OSCILLATOR
DRIVER
SW2
AO
I
LIM
SW1
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.
R1
COMPARATOR
R2
753kΩ
ADP1108-3.3: R1 = 456kΩ
ADP1108-5: R1 = 250kΩ
ADP1108-12: R1 = 87.4kΩ
SENSE
GND
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
ADP1108–SPECIFICATIONS
(0 C to +70 C, V = 3.0 V unless otherwise noted)
IN
Parameter
QUIESCENT CURRENT
QUIESCENT CURRENT,
BOOST MODE CONFIGURATION
Symbol
I
Q
I
Q
Conditions
Switch Off
No Load, T
A
= +25°C
ADP1108-3.3
ADP1108-5
ADP1108-12
Step-Up Mode
Step-Down Mode
ADP1108
1
Min
Typ
90
90
90
90
Max
150
Units
µA
µA
µA
µA
INPUT VOLTAGE
COMPARATOR TRIP POINT VOLTAGE
OUTPUT SENSE VOLTAGE
V
IN
2.0
1.20
3.13
4.75
11.4
1.245
3.3
5.00
12.0
5
13
20
50
14
19
70
36
25
60
0.15
0.2
0.02
0.5
0.8
12.6
30
1.30
3.46
5.25
12.6
12
30
55
100
25
78
48
200
130
0.4
0.4
0.075
0.75
1.00
V
V
V
V
V
V
mV
mV
mV
mV
kHz
%
µs
nA
nA
V
%/V
%/V
V
V
V
V
V
V/V
mA
%/°C
V
OUT
ADP1108-3.3
2
ADP1108-5
2
ADP1108-12
2
ADP1108
ADP1108-3.3
ADP1108-5
ADP1108-12
COMPARATOR HYSTERESIS
OUTPUT HYSTERESIS
OSCILLATOR FREQUENCY
DUTY CYCLE
SWITCH ON TIME
FEEDBACK PIN BIAS CURRENT
SET PIN BIAS CURRENT
GAIN BLOCK OUTPUT LOW
REFERENCE LINE REGULATION
SW
SAT
VOLTAGE, STEP-UP MODE
V
SAT
V
OL
t
ON
Full Load
I
LIM
Tied to V
IN
VFB = 0 V
V
SET
= V
REF
I
SINK
= 100
µA,
V
SET
= 1.00 V
2.0 V
≤
V
IN
≤
5 V
5 V
≤
V
IN
≤
30 V
V
IN
= 3.0 V, I
SW
= 650 mA
V
IN
= 5.0 V, I
SW
= 1 A,
T
A
= +25°C
V
IN
= 12 V, I
SW
= 650 mA,
T
A
= +25°C
A
V
R
L
= 100K
3
220
Ω
from I
LIM
to V
IN
,
T
A
= +25°C
55
25
SW
SAT
VOLTAGE,
SAT
STEP-DOWN MODE
GAIN BLOCK GAIN
CURRENT LIMIT
CURRENT LIMIT TEMPERATURE
COEFFICIENT
SWITCH OFF LEAKAGE CURRENT
MAXIMUM EXCURSION BELOW GND
V
SW2
1.1
400
1000
500
–0.3
1.5
1.7
Measured at SW1 Pin,
T
A
= +25°C
1
SW1
≤
10
µA,
Switch Off
T
A
= +25°C
1
–400
10
–350
µA
mV
NOTES
1
This specification guarantees that both the high and low trip points of the comparator fall within the 1.20 V to 1.30 V range.
2
The output voltage waveform will exhibit a sawtooth shape due to the comparator hysteresis. The output voltage on the fixed output versions will always be within the
specified range.
3
100 kΩ resistor connected between a 5 V source and the AO pin.
All limits at temperature extremes are guaranteed via correlation using standard Quality Control methods.
Specifications subject to change without notice.
–2–
REV. 0
ADP1108
ABSOLUTE MAXIMUM RATINGS*
PIN CONFIGURATIONS
8-Lead Plastic DIP
(N-8)
8-Lead SOIC
(SO-8)
Supply Voltage (V
IN
) . . . . . . . . . . . . . . . . . . . . . . . . . . . +36 V
SW1 Pin Voltage (V
SW1
) . . . . . . . . . . . . . . . . . . . . . . . . +50 V
SW2 Pin Voltage (V
SW2
) . . . . . . . . . . . . . . . . . . –0.5 V to V
IN
Feedback Pin Voltage (ADP1108) . . . . . . . . . . . . . . . . +5.5 V
Sense Pin Voltage (ADP1108, 3.3, 5, 12) . . . . . . . . . . . +36 V
Maximum Power Dissipation . . . . . . . . . . . . . . . . . . 500 mW
Maximum Switch Current . . . . . . . . . . . . . . . . . . . . . . . .1.5 A
Operating Temperature Range . . . . . . . . . . . . . 0°C to 170°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . +300°C
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those listed in the operational
sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
I
LIM
1
V
IN
2
8 FB (SENSE)*
I
LIM
1
V
IN
2
8 FB (SENSE)*
7 SET
TOP VIEW
SW1 3 (Not to Scale) 6 AO
SW2 4
5 GND
ADP1108
7 SET
TOP VIEW
SW1 3 (Not to Scale) 6 AO
SW2 4
5 GND
ADP1108
* FIXED VERSIONS
* FIXED VERSIONS
PIN FUNCTION DESCRIPTIONS
ORDERING GUIDE
Mnemonic
Package*
N-8
SO-8
N-8
SO-8
N-8
SO-8
N-8
SO-8
I
LIM
Function
For normal conditions this pin is connected to
V
IN
. When lower current is required, a resistor
should be connected between I
LIM
and V
IN
.
Limiting the switch current to 400 mA is
achieved by connecting a 220
Ω
resistor.
Input Voltage.
Collector of Power Transistor. For step-down
configuration, connect to V
IN
. For step-up
configuration, connect to an inductor/diode.
Emitter of Power Transistor. For step-down
configuration, connect to inductor/diode. For
step-up configuration, connect to ground. Do
not allow this pin to go more than a diode
drop below ground.
Ground.
Auxiliary Gain (GB) Output. The open
collector can sink 100
µA.
Gain Amplifier Input. The amplifier has
positive input connected to SET pin and
negative input connected to 1.245 V reference.
On the ADP1108 (adjustable) version this pin
is connected to the comparator input. On the
ADP1108-3.3, ADP1108-5 and ADP1108-12,
the pin goes directly to the internal application
resistor that set output voltage.
Model
ADP1108AN
ADP1108AR
ADP1108AN-3.3
ADP1108AR-3.3
ADP1108AN-5
ADP1108AR-5
ADP1108AN-12
ADP1108AR-12
Output Voltage
ADJ
ADJ
3.3 V
3.3 V
5V
5V
12 V
12 V
V
IN
SW1
*N = Plastic DIP, SO = Small Outline Package.
SW2
GND
AO
SET
FB/SENSE
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the ADP1108 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
REV. 0
–3–
ADP1108–Typical Performance Characteristics
1.2
1.6
SWITCH ON VOLTAGE – Volts
1100
1000
SWITCH CURRENT – mA
1
1.4
1.2
1
V
CE (SAT)
0.8
0.6
0.4
0.2
V
IN
= 3.0V
V
IN
= 2.0V
V
IN
= 5.0V
900
800
700
600
500
400
300
200
2V < V
IN
< 5V
V
CC (SAT)
– Volts
0.8
0.6
0.4
0.2
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
SWITCH CURRENT – Amps
0
0.05
0.15 0.25 0.35 0.45 0.55 0.65
SWITCH CURRENT – Amps
0.75
100
10
100
R
LIM
–
Ω
1k
Figure 1. Saturation Voltage vs. I
SWITCH
Current in Step-Up Mode
Figure 2. Switch ON Voltage vs.
Switch Current In Step-Down Mode
Figure 3. Maximum Switch Current
vs. R
LIM
In Step-Up Mode
1100
1000
V
IN
= 24V WITH L = 500µH @ V
OUT
= 5V
100
90
120
110
QUIESCENT CURRENT – µA
SWITCH CURRENT – mA
SUPPLY CURRENT – mA
900
800
700
600
500
400
300
200
100
10
100
R
LIM
–
Ω
1k
V
IN
= 12V WITH L = 250µH @ V
OUT
= 5V
80
70
60
50
40
30
20
10
0
0
100 200 300 400 500 600 700 800 900
SWITCH CURRENT – mA
V
IN
= 2V
V
IN
= 5V
100
90
80
70
60
50
40
–40
0
25
70
TEMPERATURE –
°C
85
Figure 4. Maximum Switch Current
vs. R
LIM
In Step-Down Mode
Figure 5. Supply Current vs. Switch
Current
Figure 6. Quiescent Current vs.
Temperature
22
OSCILLATOR FREQUENCY – kHz
67
66
65
35
34.5
34
21
20
19
18
17
16
15
14
13
–40
0
25
70
TEMPERATURE –
°C
85
SWITCH ON TIME – µs
0
25
70
TEMPERATURE –
°C
85
DUTY CYCLE – %
64
63
62
61
60
59
58
57
–40
33.5
33
32.5
32
31.5
31
30.5
30
–40
0
25
70
TEMPERATURE –
°C
85
Figure 7. Oscillator Frequency vs.
Temperature
Figure 8. Duty Cycle vs. Temperature
Figure 9. Switch ON Time vs.
Temperature
–4–
REV. 0
ADP1108
0.58
1.2
0.53
V
CE (SAT)
– Volts
1.15
V
CE (SAT)
@ I
SW
= 0.65A
V
CE (SAT)
– Volts
85
0.48
V
CE (SAT)
@ I
SW
= 0.65A
0.43
1.1
1.05
0.38
1
0.33
0.95
0.28
–40
0
25
70
TEMPERATURE –
°C
0.9
–40
0
25
70
TEMPERATURE –
°C
85
Figure 10. Switch Saturation Voltage In
Step-Up Mode vs. Temperature
THEORY OF OPERATION
Figure 11. Switch Saturation Voltage In
Step-Down Mode vs. Temperature
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 V
IN
and the I
LIM
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 I
LIM
to V
IN
. Further
information on I
LIM
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 V
IN
and V
OUT
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 V
IN
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 V
IN
.
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 V
IN
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.
–5–
REV. 0
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