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.
a
Micropower DC-DC Converter
Adjustable and Fixed 3.3 V, 5 V, 12 V
ADP1108
FUNCTIONAL BLOCK DIAGRAMS
SET
V
IN
GAIN BLOCK/
ERROR AMP
COMPARATOR
SW2
FBGND
SW1
AO
I
LIM
OSCILLATOR
DRIVER
A1
A2
1.245V
REFERENCE
ADP1108
SET
VIN
GAIN BLOCK/
ERROR AMP
COMPARATOR SW2
SENSEGND
SW1
AO
ILIM
OSCILLATOR
DRIVER
A1
A2
1.245V
REFERENCE
ADP1108-3.3
ADP1108-5
ADP1108-12
R2
753kΩ
R1
ADP1108-3.3: R1 = 456kΩ
ADP1108-5: R1 = 250kΩ
ADP1108-12: R1 = 87.4kΩ
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.
FEATURES
Operates at Supply Voltages From 2.0 V to 30 V
Consumes Only 110 mA 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
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
REV. 0
–2–
ADP1108–SPECIFICATIONS
Parameter Symbol Conditions Min Typ Max Units
QUIESCENT CURRENT I
Q
Switch Off 90 150 µA
QUIESCENT CURRENT, I
Q
No Load, T
A
= +25°C
BOOST MODE CONFIGURATION ADP1108-3.3 90 µA
ADP1108-5 90 µA
ADP1108-12 90 µA
INPUT VOLTAGE V
IN
Step-Up Mode 2.0 12.6 V
Step-Down Mode 30 V
COMPARATOR TRIP POINT VOLTAGE ADP1108
1
1.20 1.245 1.30 V
OUTPUT SENSE VOLTAGE V
OUT
ADP1108-3.3
2
3.13 3.3 3.46 V
ADP1108-5
2
4.75 5.00 5.25 V
ADP1108-12
2
11.4 12.0 12.6 V
COMPARATOR HYSTERESIS ADP1108 5 12 mV
OUTPUT HYSTERESIS ADP1108-3.3 13 30 mV
ADP1108-5 20 55 mV
ADP1108-12 50 100 mV
OSCILLATOR FREQUENCY 14 19 25 kHz
DUTY CYCLE Full Load 55 70 78 %
SWITCH ON TIME t
ON
I
LIM
Tied to V
IN
25 36 48 µs
FEEDBACK PIN BIAS CURRENT VFB = 0 V 25 200 nA
SET PIN BIAS CURRENT V
SET
= V
REF
60 130 nA
GAIN BLOCK OUTPUT LOW V
OL
I
SINK
= 100 µA, V
SET
= 1.00 V 0.15 0.4 V
REFERENCE LINE REGULATION 2.0 V ≤ V
IN
≤ 5 V 0.2 0.4 %/V
5 V ≤ V
IN
≤ 30 V 0.02 0.075 %/V
SW
SAT
VOLTAGE, STEP-UP MODE V
SAT
V
IN
= 3.0 V, I
SW
= 650 mA 0.5 0.75 V
V
IN
= 5.0 V, I
SW
= 1 A, 0.8 1.00 V
T
A
= +25°CV
SW
SAT
VOLTAGE,
SAT
V
IN
= 12 V, I
SW
= 650 mA,
T
A
= +25°C 1.1 1.5 V
STEP-DOWN MODE 1.7 V
GAIN BLOCK GAIN A
V
R
L
= 100K
3
400 1000 V/V
CURRENT LIMIT 220 Ω from I
LIM
to V
IN
,
T
A
= +25°C 500 mA
CURRENT LIMIT TEMPERATURE
COEFFICIENT –0.3 %/°C
SWITCH OFF LEAKAGE CURRENT Measured at SW1 Pin,
T
A
= +25°C110µA
MAXIMUM EXCURSION BELOW GND V
SW2
1
SW1
≤ 10 µA, Switch Off
T
A
= +25°C –400 –350 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.
(08C to +708C, VIN = 3.0 V unless otherwise noted)
ADP1108
–3–
REV. 0
ABSOLUTE MAXIMUM RATINGS*
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.
ORDERING GUIDE
Model Output Voltage Package*
ADP1108AN ADJ N-8
ADP1108AR ADJ SO-8
ADP1108AN-3.3 3.3 V N-8
ADP1108AR-3.3 3.3 V SO-8
ADP1108AN-5 5 V N-8
ADP1108AR-5 5 V SO-8
ADP1108AN-12 12 V N-8
ADP1108AR-12 12 V SO-8
*N = Plastic DIP, SO = Small Outline Package.
PIN CONFIGURATIONS
8-Lead Plastic DIP 8-Lead SOIC
(N-8) (SO-8)
1
2
3
4
8
7
6
5
TOP VIEW
(Not to Scale)
ADP1108
I
LIM
V
IN
SW1
SW2
FB (SENSE)*
SET
AO
GND
* FIXED VERSIONS
1
2
3
4
8
7
6
5
TOP VIEW
(Not to Scale)
ADP1108
I
LIM
V
IN
SW1
SW2
FB (SENSE)*
SET
AO
GND
* FIXED VERSIONS
PIN FUNCTION DESCRIPTIONS
Mnemonic Function
I
LIM
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.
V
IN
Input Voltage.
SW1 Collector of Power Transistor. For step-down
configuration, connect to V
IN
. For step-up
configuration, connect to an inductor/diode.
SW2 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.
GND Ground.
AO Auxiliary Gain (GB) Output. The open
collector can sink 100 µA.
SET Gain Amplifier Input. The amplifier has
positive input connected to SET pin and
negative input connected to 1.245 V reference.
FB/SENSE 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.
WARNING!
ESD SENSITIVE DEVICE
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.
ADP1108
–4– REV. 0
–Typical Performance Characteristics
SWITCH CURRENT – Amps
1.2
0.6
0
0.1 0.2 1.20.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
1
0.8
0.4
0.2
V
IN
= 2.0V
V
IN
= 3.0V
V
IN
= 5.0V
V
CC
(SAT)
– Volts
Figure 1. Saturation Voltage vs. I
SWITCH
Current in Step-Up Mode
R
LIM
– Ω
SWITCH CURRENT – mA
1000
900
1100
10 1k
800
700
600
500
400
300
200
100
V
IN
= 12V WITH L = 250µH @ V
OUT
= 5V
V
IN
= 24V WITH L = 500µH @ V
OUT
= 5V
100
Figure 4. Maximum Switch Current
vs. R
LIM
In Step-Down Mode
TEMPERATURE – °C
OSCILLATOR FREQUENCY – kHz
22
21
13–40 0 85
25 70
17
16
15
14
20
19
18
Figure 7. Oscillator Frequency vs.
Temperature
SWITCH CURRENT – Amps
SWITCH ON VOLTAGE – Volts
1.6
0.6
00.15 0.75
0.25 0.35 0.45 0.55 0.65
1.4
0.8
0.4
0.2
1.2
1 V
CE (SAT)
0.05
Figure 2. Switch ON Voltage vs.
Switch Current In Step-Down Mode
SWITCH CURRENT – mA
SUPPLY CURRENT – mA
100
00 100 900
200 300 400 600 700 800500
90
50
30
20
10
80
70
40
60
V
IN
= 2V
V
IN
= 5V
Figure 5. Supply Current vs. Switch
Current
TEMPERATURE – °C
67
66
57
–40 0 85
25 70
61
60
59
58
65
64
63
62
DUTY CYCLE – %
Figure 8. Duty Cycle vs. Temperature
R
LIM
– Ω
SWITCH CURRENT – mA
1100
1000
10010 100 1k
900
800
700
600
500
400
300
200
2V < V
IN
< 5V
Figure 3. Maximum Switch Current
vs. R
LIM
In Step-Up Mode
TEMPERATURE – °C
QUIESCENT CURRENT – µA
120
110
40
–40 0 85
25 70
80
70
60
50
100
90
Figure 6. Quiescent Current vs.
Temperature
TEMPERATURE – °C
SWITCH ON TIME – µs
35
34.5
30–40 0 85
25 70
32
31.5
31
30.5
34
33.5
33
32.5
Figure 9. Switch ON Time vs.
Temperature
ADP1108
–5–
REV. 0
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 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 µH, with a saturation current rating of 300 mA to
1 A and dc resistance < 0.4 V 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.
TEMPERATURE – °C
V
CE (SAT)
– Volts
0.58
0.53
0.28
–40 0 85
25 70
0.48
0.43
0.38
0.33
V
CE (SAT)
@ I
SW
= 0.65A
Figure 10. Switch Saturation Voltage In
Step-Up Mode vs. Temperature
TEMPERATURE – °C
V
CE (SAT)
– Volts
1.2
1.15
0.9
–40 0 85
25 70
1.1
1.05
1
0.95
V
CE (SAT)
@ I
SW
= 0.65A
Figure 11. Switch Saturation Voltage In
Step-Down Mode vs. Temperature
ADP1108
–6– REV. 0
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.
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:
P
L
=V
OUT
+V
D
−V
IN MIN
()
()
×I
OUT
()
(Equation 1)
where V
D
is the diode forward voltage (≈ 0.5 V for a 1N5818
Schottky). Energy is only stored in the inductor while the
ADP1108 switch is ON, so the energy stored in the inductor on
each switching cycle must be equal to or greater than:
PL
fOSC
(Equation 2)
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:
I
L
(t)=V
IN
R'1−e
–R't
L
(Equation 3)
where L is in henrys and R
9
is the sum of the switch equivalent
resistance (typically 0.8 Ω at +25°C) and the dc resistance of
the inductor. If the voltage drop across the switch is small
compared to V
IN
, a simpler equation can be used:
I
L
(t)=V
IN
Lt
(Equation 4)
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:
E
L
=1
2L×I
2
PEAK
(Equation 5)
As previously mentioned, E
L
must be greater than P
L
/f
OSC
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:
P
L
=12V+0.5V–2V
()
×30 mA
()
=315 mW
On each switching cycle, the inductor must supply:
P
L
f
OSC
=315mW
19kHz =16.6µJ
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:
L=V
IN
I
L(MAX)
t=2V
500 mA 36 µs=144 µH
Substituting a standard inductor value of 100 µH with 0.2 Ω dc
resistance, will produce a peak switch current of:
I
PEAK
=2V
1.0 Ω1–e
–1.0 Ω×36 µs
100 µH
=605 mA
Once the peak current is known, the inductor energy can be
calculated from Equation 5:
E
L
=1
2100 µH×605mA
()
2
=18.3µJ
The inductor energy of 18.3 µJ is greater than the P
L
/f
OSC
requirement of 16.6 µJ, so the 100 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 1.5 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 V
IN
is at its minimum, the 1.5 A limit may be exceeded at the
maximum value of V
IN
. 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
current to the I
PEAK
value calculated for the minimum value of
V
IN
. This will improve efficiency by producing a constant
I
PEAK
as V
IN
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
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 I
LIM
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
=2I
OUT
DC V
OUT
+V
D
V
IN
–V
SW
+V
D
(Equation 6)
ADP1108
–7–
REV. 0
where: DC = duty cycle (0.7 for the ADP1108)
V
SW
= voltage drop across the switch
V
D
= diode drop (0.5 V for a 1N5818)
I
OUT
= output current
V
OUT
= the output voltage
V
IN
= the minimum input voltage
As previously mentioned, the switch voltage is higher in step-
down mode than in step-up mode. V
SW
is a function of switch
current and is therefore a function of V
IN
, L, time and V
OUT
. For
most applications, a V
SW
value of 1.5 V is recommended.
The inductor value can now be calculated:
L=V
IN(MIN)
–V
SW
–V
OUT
I
PEAK
×t
ON
(Equation 7)
where: t
ON
= switch ON time (36 µs)
If the input voltage will vary (such as an application that must
operate from a 9 V, 12 V or 15 V source), an R
LIM
resistor
should be selected from Figure 4. The R
LIM
resistor will keep
switch current constant as the input voltage rises. Note that
there are separate R
LIM
values for step-up and step-down modes
of operation.
For example, assume that +5 V at 250 mA is required from a +9 V
to +18 V source. Deriving the peak current from Equation 6
yields:
I
PEAK
=2×250 mA
0.7 5+0.5
9−1.5 +0.5
=491mA
The peak current can than be inserted into Equation 7 to cal-
culate the inductor value:
L=9–1.5–5
491mA ×36 µs=183 µH
Since 183 µH is not a standard value, the next lower standard
value of 150 µH would be specified.
To avoid exceeding the maximum switch current when the in-
put voltage is at +18 V, an R
LIM
resistor should be specified. Us-
ing Figure 4, a value of 160 Ω will limit the switch current to
500 mA.
Inductor Selection—Positive-to-Negative Converter
The configuration for a positive-to-negative converter using the
ADP1108 is shown in Figure 19. As with the step-up converter,
all of the output power for the inverting circuit must be supplied
by the inductor. The required inductor power is derived from
the formula:
P
L
|V
OUT
|+V
D
()
×I
OUT
()
(Equation 8)
The ADP1108 power switch does not saturate in positive-to-
negative mode. The voltage drop across the switch can be mod-
eled as a 0.75 V base-emitter diode in series with a 0.65 Ω
resistor. When the switch turns on, inductor current will rise at
a rate determined by:
I
L
(t)=V
L
R'1−e
–R't
L
(Equation 9)
where: R' = 0.65 Ω + R
L
(DC)
V
L
= V
IN
– 0.75 V
For example, assume that a –5 V output at 100 mA is to be gen-
erated from a +4.5 V to +5.5 V source. The power in the induc-
tor is calculated from Equation 8:
P
L
=|–5V|+0.5V
()
×100 mA
()
=550 mW
During each switching cycle, the inductor must supply the fol-
lowing energy:
P
L
f
OSC
=550mW
19kHz =28.9µJ
Using a standard inductor value of 220 µH with 0.3 Ω dc resis-
tance will produce a peak switch current of:
I
PEAK
=4.5V–0.75V
0.65 Ω+0.3 Ω1−e
–0.95 Ω×36 µs
220 µH
=568 mA
Once the peak current is known, the inductor energy can be cal-
culated from Equation 9:
E
L
=1
2220 µH×568mA
()
2
=35.5µJ
The inductor energy of 35.5 µJ is greater than the P
L
/f
OSC
re-
quirement of 28.9 µJ, so the 220 µH inductor will work in
this application.
To avoid exceeding the maximum switch current when the in-
put voltage is at +5.5 V, an R
LIM
resistor should be specified.
Referring to Figure 4, a value of 150 V is appropriate in this
application.
Capacitor Selection
For optimum performance, the ADP1108’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.
The effects of capacitor selection on output ripple are demon-
strated in Figures 12, 13, and 14. These figures show the output
of the same ADP1108 converter, which was evaluated with
three different output capacitors. In each case, the peak switch
current is 500 mA and the capacitor value is 100 µF. Figure 12
shows a Panasonic HF-series* radial aluminum electrolytic.
When the switch turns off, the output voltage jumps by about
90 mV and then decays as the inductor discharges into the ca-
pacitor. The rise in voltage indicates an ESR of about 0.18 V. In
Figure 13, the aluminum electrolytic has been replaced by a
Sprague 593D-series* tantalum device. In this case the output
jumps about 35 mV, which indicates an ESR of 0.07 V. Figure
14 shows an OS-CON SA series capacitor in the same circuit,
and ESR is only 0.02 V.
*All trademarks are the property of their respective holders.
ADP1108
–8– REV. 0
10
0%
5
m
s
50mV
100
90
C
OUT
=100mF, 16V
I
SW
= 500mA
ESR z 0.18V
Figure 12. Aluminum Electrolytic
10
0%
5µs
50mV
100
90
C
OUT
=100µF, 6V
I
SW
= 500mA
ESR z 0.07V
Figure 13. Tantalum Electrolytic
10
0%
5
m
s
50mV
100
90
C
OUT
=100mF, 16V
I
SW
= 500mA
ESR z 0.02V
Figure 14. OS-CON Capacitor
If low output ripple is important, the user should consider using
the ADP3000. This device switches at 400 kHz, which simpli-
fies the design of the output filter. Consult the ADP3000 data
sheet for additional details.
DIODE SELECTION
In specifying a diode, consideration must be given to speed, for-
ward voltage drop and reverse leakage current. When the
ADP1108 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 for-
ward voltage of the diode represents power that is not delivered
to the load, so V
F
must also be minimized. Again, Schottky di-
odes are recommended. Leakage current is especially important
in low-current applications, where the leakage can be a signifi-
cant percentage of the total quiescent current.
For most circuits, the 1N5818 is a suitable companion to the
ADP1108. This diode has a V
F
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 V
F
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
ADP1108 circuits. These devices, which have turn-on times of
10 µs or more, are far too slow for switching power supply applica-
tions. Using such a diode “just to get started” will result in wasted
time and effort. Even if an ADP1108 circuit appears to function
with a 1N4001, the resulting performance will not be indicative of
the circuit performance when the correct diode is used.
Circuit Operation, Step-Up (Boost) Mode
In boost mode, the ADP1108 produces an output voltage higher
than the input voltage. For example, +12 V can be generated
from a +5 V logic power supply or +5 V can be derived from
two alkaline cells (+3 V).
Figure 15 shows an ADP1108 configured for step-up operation.
The collector of the internal power switch is connected to the out-
put side of the inductor, while the emitter is connected to GND.
When the switch turns on, Pin SW1 is pulled near ground. This ac-
tion forces a voltage across L1 equal to V
IN
2V
CE(SAT)
, and current
begins to flow through L1. This current reaches a final value
(ignoring second-order effects) of:
I
PEAK
≅V
IN
–V
CE(SAT)
L×36 µs
where 36 µs is the ADP1108 switch’s “on” time.
3
5
1
8
4
2
I
LIM
V
IN
SW1
FB
SW2
GND
ADP1108
L1 D1 V
OUT
C1
R3
R1
R2
V
IN
Figure 15. Step-Up Mode Operation
When the switch turns off, the magnetic field collapses. The
polarity across the inductor changes, current begins to flow
through D1 into the load and the output voltage is driven above
the input voltage.
The output voltage is fed back to the ADP1108 via resistors R1
and R2. When the voltage at Pin FB falls below 1.245 V, SW1
turns “on” again and the cycle repeats. The output voltage is
therefore set by the formula:
V
OUT
=1.245V×1+R1
R2
The circuit of Figure 15 shows a direct current path from V
IN
to
V
OUT
, via the inductor and D1. Therefore, the boost converter
is not protected if the output is short circuited to ground.
Circuit Operation, Step-Down (Buck) Mode
The ADP1108’s step-down mode is used to produce an output
voltage lower than the input voltage. For example, the output of
four NiCd cells (+4.8 V) can be converted to a +3 V logic supply.
A typical configuration for step-down operation of the ADP1108 is
shown in Figure 16. In this case, the collector of the internal power
switch is connected to V
IN
and the emitter drives the inductor.
ADP1108
–9–
REV. 0
When the switch turns on, SW2 is pulled up toward V
IN
. This
forces a voltage across L1 equal to (V
IN
2V
CE
) 2 V
OUT
, and causes
current to flow in L1. This current reaches a final value of:
I
PEAK
≅V
IN
–V
CE
–V
OUT
L×36µs
where 36 µs is the ADP1108 switch’s “on” time.
5
1
8
4
2
I
LIM
V
IN
SW1 FB
SW2
GND
ADP1108
L1
D1
1N5818
V
OUT
C2
R
LIM
100Ω
R1
R2
V
IN
3
C1
Figure 16. Step-Down Mode Operation
When the switch turns off, the magnetic field collapses. The po-
larity across the inductor changes and the switch side of the in-
ductor is driven below ground. Schottky diode D1 then turns on
and current flows into the load. Notice that the Absolute Maxi-
mum Rating for the ADP1108’s SW2 pin is 0.5 V below ground.
To avoid exceeding this limit, D1 must be a Schottky diode. If a
silicon diode is used for D1, Pin SW2 can go to 20.8 V, which
will cause potentially damaging power dissipation within the
ADP1108.
The output voltage of the buck regulator is fed back to the
ADP1108’s FB pin by resistors R1 and R2. When the voltage at
Pin FB falls below 1.245 V, the internal power switch turns
“on” again and the cycle repeats. The output voltage is set by
the formula:
V
OUT
=1.245V×1+R1
R2
When operating the ADP1108 in step-down mode, the output
voltage is impressed across the internal power switch’s emitter-
base junction while the switch is off. To protect the switch, the
output voltage should be limited to 6.2 V or less. If a higher out-
put voltage is required, a Schottky diode should be placed in se-
ries with SW2, as shown in Figure 17.
5
1
8
4
2
I
LIM
V
IN
SW1 FB
SW2
GND
ADP1108
L1
D1
1N5818
V
OUT
C2
R
LIM
100Ω
R1
R2
V
IN
3
C1
D2
Figure 17. Step-Down Model, V
OUT
> 6.2 V
If the input voltage to the ADP1108 varies over a wide range, a
current limiting resistor at Pin 1 may be required. If a particular
circuit requires high peak inductor current with minimum input
supply voltage, then the peak current may exceed the switch
maximum rating and/or saturate the inductor when the supply
voltage is at the maximum value. See the Limiting the Switch
Current section of this data sheet for specific recommendations.
Increasing Output Current in the Step-Down Regulator
Unlike the boost configuration, the ADP1108’s internal power
switch is not saturated when operating in step-down mode. A
conservative value for the voltage across the switch in step-down
mode is 1.5 V. This results in high power dissipation within the
ADP1108 when high peak current is required. To increase the
output current, an external PNP switch can be added (Figure
18). In this circuit, the ADP1108 provides base drive to Q1
through R3 while R4 ensures that Q1 turns off rapidly. The
ADP1108’s internal current limiting function will not work in
this circuit, R5 is provided for this purpose. With the value
shown, R5 limits current to 2 A. In addition to reducing power
dissipation on the ADP1108, this circuit also reduces the switch
voltage. When selecting an inductor value for the circuit of Fig-
ure 18, the switch voltage can be calculated from the formula:
V
SW
=V
R5
+V
Q1(SAT )
≅0.6V+0.4V≅1V
L1*
100mH5V
OUT
200mA AT 6.5V
500mA AT 8V
R4
100Ω
R5
0.22Ω
Q1
ZETEX
ZTX749
*L1 = COILTRONICS CTX100-4
R2
100Ω
5
8
2
I
LIM
V
IN
SW1
SW2GND
ADP1108-5
D1
1N5818
C2
V
IN
6.5V TO 20V
3
C1
R3
220Ω
1
4
SENSE
Figure 18. High Current Step-Down Operation
Positive-to-Negative Conversion
The ADP1108 can convert a positive input voltage to a negative
output voltage, as shown in Figure 19. This circuit is essentially
identical to the step-down application of Figure 16, except that
the “output” side of the inductor is connected to power ground.
When the ADP1108’s internal power switch turns off, current
flowing in the inductor forces the output (2V
OUT
) to a negative
potential. The ADP1108 will continue to turn the switch on un-
til its FB pin is 1.245 V above its GND pin, so the output volt-
age is determined by the formula:
V
OUT
=1.245V×1+R1
R2
5
1
8
4
2
I
LIM
V
IN
SW1 FB
SW2
GND
ADP1108
L1
D1
1N5818
–V
OUT
C2
R3
R1
R2
V
IN
3
C
L
Figure 19. A Positive-to-Negative Converter
The design criteria for the step-down application also apply to
the positive-to-negative converter. The output voltage should be
limited to |6.2 V|, unless a diode is inserted in series with the
SW2 pin (see Figure 17). Also, D1 must again be a Schottky di-
ode to prevent excessive power dissipation in the ADP1108.
ADP1108
–10– REV. 0
Negative-to-Positive Conversion
The circuit of Figure 20 converts a negative input voltage to a posi-
tive output voltage. Operation of this circuit configuration is similar
to the step-up topology of Figure 15, except that the current
through feedback resistor R1 is level-shifted below ground by a
PNP transistor. The voltage across R1 is (V
OUT
–V
BEQ1
). However,
diode D2 level-shifts the base of Q1 about 0.6 V below ground,
thereby cancelling the V
BE
of Q1. The addition of D2 also reduces
the circuit’s output voltage sensitivity to temperature, which would
otherwise be dominated by the 22 mV/8C V
BE
contribution of Q1.
The output voltage for this circuit is determined by the formula:
V
OUT
=1.245V×R1
R2
Unlike the positive step-up converter, the negative-to-positive
converter’s output voltage can be either higher or lower than the
input voltage.
D1
1N5818
1
8
2
I
LIM
V
IN
SW1
FB
SW2GND
ADP1108
L1
D2
1N4148
C2
R
LIM
R1
3
C
L
Q1
2N3906
6
AO SET
7
NC NC
54
R2 10kΩ
POSITIVE
OUTPUT
NEGATIVE
INPUT
Figure 20. A Negative-to-Positive Converter
Limiting the Switch Current
The ADP1108’s R
LIM
pin permits the switch current to be lim-
ited with a single resistor. This current limiting action occurs on
a pulse by pulse basis. This feature allows the input voltage to
vary over a wide range, without saturating the inductor or ex-
ceeding the maximum switch rating. For example, a particular
design may require peak switch current of 800 mA with a 2.0 V
input. If V
IN
rises to 4 V, however, the switch current will ex-
ceed 1.6 A. The ADP1108 limits switch current to 1.5 A and
thereby protects the switch, but the output ripple will increase.
Selecting the proper resistor will limit the switch current to
800 mA, even if V
IN
increases. The relationship between R
LIM
and maximum switch current is shown in Figures 3 and 4.
The I
LIM
feature is also valuable for controlling inductor current
when the ADP1108 goes into continuous-conduction mode. This
occurs in the step-up mode when the following condition is met:
V
OUT
+V
DIODE
V
IN
–V
SW
<1
1–DC
where DC is the ADP1108’s duty cycle. When this relationship
exists, the inductor current does not go all the way to zero dur-
ing the time that the switch is OFF. When the switch turns on
for the next cycle, the inductor current begins to ramp up from
the residual level. If the switch ON time remains constant, the
inductor current will increase to a high level (see Figure 21).
This increases output ripple and can require a larger inductor
and capacitor. By controlling switch current with the I
LIM
resis-
tor, output ripple current can be maintained at the design val-
ues. Figure 22 illustrates the action of the I
LIM
circuit.
0%
100
90
10
50µs
200mA
V
OUT
= + 5V
I
L
= 100mA L = 120µH
R
LIM
= 0V
V
IN
= 2.23V
Figure 21. (I
LIM
Operation, R
LIM
= 0
Ω
)
0%
100
90
10
50µs
200mA
V
OUT
= + 5V
I
L
= 100mA
L = 120µH
R
LIM
= 120V
V
IN
= 2.23V
Figure 22. (I
LIM
Operation, R
LIM
= 120
Ω
)
The internal structure of the I
LIM
circuit is shown in Figure 23.
Q1 is the ADP1108’s internal power switch, which is paralleled
by sense transistor Q2. The relative sizes of Q1 and Q2 are
scaled so that I
Q2
is 0.5% of I
Q1
. Current flows to Q2 through an
internal 80 Ω resistor and through the R
LIM
resistor. These two
resistors parallel the base-emitter junction of the oscillator-
disable transistor, Q3. When the voltage across R1 and R
LIM
exceeds 0.6 V, Q3 turns on and terminates the output pulse. If
only the 80 Ω internal resistor is used (i.e. the I
LIM
pin is connected
directly to V
IN
), the maximum switch current will be 1.5 A.
OSCILLATOR
V
IN
SW2
SW1
R
LIM
(EXTERNAL)
DRIVER
I
LIM
Q1
ADP1108
Q2
Q3
R1
80Ω
(INTERNAL)
Figure 23. ADP1108 Current Limit Operation
The delay through the current limiting circuit is approximately
2 µs. If the switch ON time is reduced to less than 5 µs, accu-
racy of the current trip-point is reduced. Attempting to program
a switch ON time of 2 µs or less will produce spurious responses
in the switch ON time. However, the ADP1108 will still provide
a properly regulated output voltage.
ADP1108
–11–
REV. 0
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 kV for R2 is a good
compromise.
The value for R1 is then calculated from the formula:
R1=V
LOBATT
–1.245V
1.245V
R2
where V
LOBATT
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.
2
V
IN
+5V
GND
ADP1108
R1
AO
SET
5
R2
33kΩ
47kΩ
TO
PROCESSOR
6
1.245V
REF
7
V
BAT
V
LB
–1.245V
37.7µA
V
LB
= BATTERY TRIP POINT
R1 =
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 MV to 10 MV, provides the hysteresis. The addition
of R3 will change the trip point slightly, so the new value for R1
will be:
R1=V
LOBATT
–1.245V
1.245V
R2
–V
L
–1.245V
R
L
+R3
where V
L
is the logic power supply voltage, R
L
is the pull-up
resistor and R3 creates the hysteresis.
2
V
IN
+5V
GND
ADP1108
R1
AO
SET
5
R2
33kΩ
47kΩ
TO
PROCESSOR
6
1.245V
REF
7
V
BAT
R3
1.6MΩ
Figure 24b. Adding Hysteresis to the Low Battery Detector
L1*
100mH5VOUT
200mA AT 6.5V
500mA AT 8V
100Ω
0.22ΩZETEX
ZTX749
*L1 = COILTRONICS CTX100-4
100Ω
5
8
2ILIM
VIN SW1
SW2GND
ADP1108-5
D1
1N5818
C2
VIN
6.5V TO 20V
3
C1
220Ω
1
4
SENSE
Figure 25. 6.5 V to 5 V Step-Down Converter
I
LIM
V
IN
SW1
ADP1108-5
L1*
300µH
220Ω
SENSE
SW2
GND
4
5
1
23
8
*L1 = COILTRONICS CTX300-4
–5V OUTPUT
150mA
+C2
V
IN
6.5V TO 20V
MBRS130T3 +330µF
Figure 26. Positive to –5 V Converter
ADP1108
–12– REV. 0
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Lead Plastic DIP
(N-8)
8
14
5
0.430 (10.92)
0.348 (8.84)
0.280 (7.11)
0.240 (6.10)
PIN 1
SEATING
PLANE
0.022 (0.558)
0.014 (0.356)
0.060 (1.52)
0.015 (0.38)
0.210 (5.33)
MAX 0.130
(3.30)
MIN
0.070 (1.77)
0.045 (1.15)
0.100
(2.54)
BSC
0.160 (4.06)
0.115 (2.93)
0.325 (8.25)
0.300 (7.62)
0.015 (0.381)
0.008 (0.204)
0.195 (4.95)
0.115 (2.93)
8-Lead SOIC
(SO-8)
0.1968 (5.00)
0.1890 (4.80)
85
41
0.2440 (6.20)
0.2284 (5.80)
PIN 1
0.1574 (4.00)
0.1497 (3.80)
0.0688 (1.75)
0.0532 (1.35)
SEATING
PLANE
0.0098 (0.25)
0.0040 (0.10)
0.0192 (0.49)
0.0138 (0.35)
0.0500
(1.27)
BSC 0.0098 (0.25)
0.0075 (0.19) 0.0500 (1.27)
0.0160 (0.41)
8°
0°
0.0196 (0.50)
0.0099 (0.25) x 45°
C2992–12–2/97
PRINTED IN U.S.A.
