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| 型號(hào)： | ADP3303AR-3.2 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | THREE-TERMINAL POSITIVE FIXED VOLTAGE REGULATORS |
| 中文描述： | 3.2 V FIXED POSITIVE LDO REGULATOR, 0.4 V DROPOUT, PDSO8 |
| 封裝： | SOIC-8 |
| 文件頁(yè)數(shù)： | 7/8頁(yè) |
| 文件大小： | 135K |
| 代理商： | ADP3303AR-3.2 |
ADP3303A
–7–
REV. A
The proper formula to compute R1 and R2 is:
R
k
1 189
.
V
R
k
.
V
SEL
SEL
1
44
2
44
11 189
=
×
=
,
Where
V
SEL
is the desired output voltage.
The output voltage can be selected from 2.2 V to 10 V. R1 is
connected from the OUT pin to the FB pin and R2 is connected
from the FB pin to GND. As an example, the Feedback Resistor
Selection Table shows the feedback resistor values for 3 V and
5 V output voltages.
Table I. Feedback Resistor Selection Table
R1
(1% Resistor)
110 k
187 k
R2
(1% Resistor)
73.2 k
57.6 k
V
OUT
3 V
5 V
OUTPUT CURRENT LIMITING
Short circuit protection is provided by limiting the pass transis-
tors base drive current. Maximum output current is limited to
200 mA.
THERMAL OVERLOAD PROTECTION
The ADP3303A is protected against damage due to excessive
power dissipation by its thermal overload protection circuit,
which limits the die temperature to a maximum of 165
°
C.
Under extreme conditions (i.e., high ambient temperature and
power dissipation), where die temperature starts to rise above
165
°
C, the output current is reduced until the die temperature
has dropped to a safe level. The output current is restored when
the die temperature is reduced.
Current and thermal limit protections are intended to protect
the device against accidental overload conditions. For normal
operation, device power dissipation should be externally limited
so that junction temperatures will not exceed 125
°
C.
CALCULATING JUNCTION TEMPERATURE
Device power dissipation is calculated as follows:
P
D
= (
V
IN
–
V
OUT
)
I
LOAD
+ (
V
IN
)
I
GND
Where
I
LOAD
and
I
GND
are load current and ground current,
V
IN
and
V
OUT
are input and output voltages, respectively.
Assuming
I
LOAD
= 200 mA,
I
GND
= 4 mA,
V
IN
= 5.5 V and
V
OUT
= 3.0 V, device power dissipation is:
P
D
=
(5.5
V –
3.0
V
)
0.2 + 5.5
×
0.004
=
0.522
W
The proprietary thermal coastline TSSOP-14 package of the
ADP3303A, in conjunction with the recommended PCB layout
shown in Figure 21, yields a thermal resistance of 96
°
C/W. As a
result, the die temperature rise for the example circuit is:
T
=
T
J
–
T
A
=
P
D
×
θ
J
A
= 0.522
×
96 = 50.1
°
C
If the maximum ambient temperature is 50
°
C, this yields a
maximum junction temperature of T
JMAX
= 100.1
°
C, which is
below the 125
°
C maximum operating junction temperature
rating.
PRINTED CIRCUIT BOARD LAYOUT CONSIDERATION
The rate at which heat is transferred is directly proportional to
the temperature differential between the die and PC board.
Once heat is transferred to the PC board, it should be dissipated
to the air or other medium.
Surface mount components rely on the conductive traces or
pads to transfer heat away from the device. Appropriate PC
board layout technique should be used to remove heat from
immediate vicinity of the package.
The following general guidelines will be helpful when designing
a board layout:
1. PC board traces with larger cross section areas will remove
more heat. For optimum results, use PC’s with thicker cop-
per and or wider traces.
2. Increase the surface area exposed to open air so heat can be
removed by convection or forced air flow.
3. Do not solder mask or silk screen the heat dissipating traces.
Black anodizing will significantly improve heat reduction by
means of increased radiation.
Figure 22 shows the recommended board layout for the
ADP3303A. Although it is not critical, make sure R1 is con-
nected right at the pin or the point you want to regulate in order
to realize a proper kelvin connection. This will improve overall
precision and stability. The same consideration is valid for the
R2 connection to the ground pin, but a short connection is
strongly suggested. No other components can be connected to
the FB pin except an optional 10 nF–100 nF capacitor (C
NR
) in
parallel to R1 that serves as a noise reduction capacitor.
SHUTDOWN MODE
Applying a TTL high signal to the shutdown pin, or tying it to
the input pin, will turn the output ON. Pulling the shutdown
pin down to 0.3 V or below, or tying it to ground, will turn the
output OFF. In shutdown mode, quiescent current is reduced
to less than 1
μ
A.
INPUT–OUTPUT DROPOUT VOLTAGE AND DROPOUT
DETECTOR
The ADP3303A maintains a regulated output with an input
voltage as low as 150 mV above the nominal output voltage.
Input voltage falling below this level will generate an error signal
indicating that the error amplifier output is reaching its satu-
rated state and will not be able to drive the pass transistor any
harder. Lowering the input voltage any further will result in
output voltage reduction and loss of regulation.
The input voltage threshold which generates the error output
signal depends on the load current. At the rated output current,
it is slightly lower than the nominal output voltage plus the
dropout voltage. However, the threshold is much lower at
lighter loads.
APPLICATION CIRCUITS
Crossover Switch
The circuit in Figure 23 shows that two ADP3303As can be
used to form a mixed supply voltage system. The output
switches between two different levels selected by an external
digital input. Output voltages can be any combination of volt-
ages from the Ordering Guide.
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