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參數資料
| 型號： | ADP1173AR-5 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 穩壓器 |
| 英文描述： | RECTIFIER SCHOTTKY SINGLE 5A 40V 175A-ifsm 0.55V-vf 0.5mA-ir TO220AC 50/TUBE |
| 中文描述： | 1.5 A SWITCHING REGULATOR, 32 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | PLASTIC, SO-8 |
| 文件頁數： | 5/16頁 |
| 文件大小： | 478K |
| 代理商： | ADP1173AR-5 |
ADP1173
–5–
REV. 0
APPLICATIONS
Theory of Operation
The ADP1173 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 ADP1173 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 24 kHz oscillator turns on. A driver amplifier pro-
vides 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 ADP1173 quiescent current is only 110
μ
A. The com-
parator includes a small amount of hysteresis, which ensures
loop stability without requiring external components for fre-
quency 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
μ
s. 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 ADP1173 internal oscillator provides 23
μ
s ON and 19
μ
s
OFF times, which is ideal for applications where the ratio
between V
IN
and V
OUT
is roughly a factor of two (such as
converting +3 V to + 5 V). However, wider range conversions
(such as generating +12 V from a +5 V supply) can easily be
accomplished.
An uncommitted gain block on the ADP1173 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 noninvert-
ing input is available at the SET pin. A resistor divider, con-
nected 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 which can sink 100
μ
A.
The ADP1173 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 ADP1173 is set with two external
resistors. Three fixed-voltage models are also available:
ADP1173-3.3 (+3.3 V), ADP1173-5 (+5 V) and ADP1173-12
(+12 V). The fixed-voltage models are identical to the ADP1173,
except that laser-trimmed voltage-setting resistors are included
on the chip. On the fixed-voltage models of the ADP1173,
simply connect the feedback pin (Pin 8) directly to the output
voltage.
COMPONENT SELECTION
General Notes on Inductor Selection
When the ADP1173 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 ADP1173, so the output voltage can be higher, lower or
of opposite polarity than the input voltage.
To specify an inductor for the ADP1173, 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 ADP1173 applications, an inductor of
47
μ
H to 470
μ
H, with a saturation current rating of 300 mA to
1 A and dc resistance <1
is suitable. Ferrite core inductors
which 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.
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
following sections.
Inductor Selection—Step-Up Converter
In a step-up, or boost, converter (Figure 14), the inductor must
store enough power to make up the difference between the
input voltage and the output voltage. The power that must be
stored is calculated from the equation:
P
L
=
V
OUT
+
V
D
–
V
IN
(
MIN
)
where
V
D
is the diode forward voltage (
≈
0.5 V for a 1N5818
Schottky). Energy is only stored in the inductor while the
ADP1173 switch is ON, so the energy stored in the inductor on
each switching cycle must be must be equal to or greater than:
P
L
f
OSC
in order for the ADP1173 to regulate the output voltage.
(
)
×
I
OUT
(
)
(1)
(2)
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