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ADP1110 Datasheet(PDF) 6 Page - Analog Devices

Part No. ADP1110
Description  Micropower, Step-Up/Step-Down Switching Regulator; Adjustable and Fixed 3.3 V, 5 V, 12 V
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADP1110 Datasheet(HTML) 6 Page - Analog Devices

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ADP1110
–6–
REV. 0
THEORY OF OPERATION
The ADP1110 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 ADP1110 is shown on the
first page. The internal 220 mV 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
220 mV, the 70 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
220 mV, the oscillator is shut off. While the oscillator is off, the
ADP1110 quiescent current is only 300
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 800 ns. If an
external resistor is not used, connect ILIM to VIN. Further informa-
tion on ILIM is included in the “Applications” section of this data
sheet.
The ADP1110 internal oscillator provides 10
µs ON and 5 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 single 1.5 V cell). Wider range conversions, as well as
step-down converters, can also be accomplished with a slight
loss in the maximum output power that can be obtained.
An uncommitted gain block on the ADP1110 can be connected
as a low–battery detector. The inverting input of the gain block
is internally connected to the 220 mV 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 300
µA.
The ADP1110 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 ADP1110 is set with two external
resistors. Three fixed-voltage models are also available:
ADP1110–3.3 (+3.3 V), ADP1110–5 (+5 V) and ADP1110-12
(+12 V). The fixed-voltage models are identical to the
ADP1110 except that laser-trimmed voltage-setting resistors are
included on the chip. Only three external components are
required to form a +3.3 V, +5 V or +12 V converter. On the
fixed-voltage models of the ADP1110, simply connect the
SENSE pin (Pin 8) directly to the output voltage.
COMPONENT SELECTION
General Notes on Inductor Selection
When the ADP1110 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. Because both
the collector and the emitter of the switch transistor are
accessible on the ADP1110, the output voltage can be higher,
lower, or of opposite polarity than the input voltage.
To specify an inductor for the ADP1110, 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 ADP1110 applications, an inductor of
15
µH to 100 µH with a saturation current rating of 300 mA to
1A and dc resistance <0.4
Ω 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
400
0
070
25
250
200
150
50
350
300
BIAS CURRENT
100
Figure 14. Set Pin Bias Current vs. Temperature
TEMPERATURE – C
220
219
211
070
25
215
214
213
212
217
216
218
REFERENCE VOLTAGE
Figure 15. Reference Voltage vs. Temperature


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