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參數資料
| 型號： | ADP1108AR-5 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 穩壓器 |
| 英文描述： | Circular Connector; No. of Contacts:128; Series:LJT06R; Body Material:Aluminum; Connecting Termination:Crimp; Connector Shell Size:25; Circular Contact Gender:Pin; Circular Shell Style:Straight Plug; Insert Arrangement:25-35 |
| 中文描述： | 1.5 A SWITCHING REGULATOR, 25 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | SOIC-8 |
| 文件頁數： | 10/12頁 |
| 文件大?。?/td> | 234K |
| 代理商： | ADP1108AR-5 |
ADP1108
–10–
REV. 0
Negative-to-Positive Conversion
T he 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. T he 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. T he addition of D2 also reduces
the circuit’s output voltage sensitivity to temperature, which would
otherwise be dominated by the
2
2 mV/
8
C V
BE
contribution of Q1.
T he output voltage for this circuit is determined by the formula:
V
OUT
=
1.245
V
×
R
1
R
2
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
SW2
4
GND
ADP1108
L1
D2
1N4148
C2
R
LIM
R1
3
C
L
Q1
2N3906
6
AO
SET
7
NC
NC
5
R2
10k
POSITIVE
OUTPUT
NEGATIVE
INPUT
Figure 20. A Negative-to-Positive Converter
Limiting the Switch Current
T he ADP1108’s R
LIM
pin permits the switch current to be lim-
ited with a single resistor. T his current limiting action occurs on
a pulse by pulse basis. T his 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. T he 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. T he relationship between R
LIM
and maximum switch current is shown in Figures 3 and 4.
T he I
LIM
feature is also valuable for controlling inductor current
when the ADP1108 goes into continuous-conduction mode. T his
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).
T his 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
= 0
V
V
IN
= 2.23V
Figure 21. (I
LIM
Operation, R
LIM
= 0
)
0%
100
90
10
50μs
200mA
V
OUT
= + 5V
I
L
= 100mA
L = 120μH
R
LIM
= 120
V
V
IN
= 2.23V
Figure 22. (I
LIM
Operation, R
LIM
= 120
)
T he 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. T he 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
resistor and through the R
LIM
resistor. T hese 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
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
(EXTERNAL)
DRIVER
I
LIM
Q1
ADP1108
Q2
Q3
R1
80
(INTERNAL)
Figure 23. ADP1108 Current Limit Operation
T he delay through the current limiting circuit is approximately
2
μ
s. If the switch ON time is reduced to less than 5
μ
s, accu-
racy of the current trip-point is reduced. Attempting to program
a switch ON time of 2
μ
s or less will produce spurious responses
in the switch ON time. However, the ADP1108 will still provide
a properly regulated output voltage.
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| ADP1109AAN-12 | Circular Connector; MIL SPEC:MIL-C-5015; Body Material:Metal; Series:GT; No. of Contacts:7; Connector Shell Size:16S; Connecting Termination:Solder; Circular Shell Style:Square Flange Receptacle; Body Style:Straight |
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| ADP1109AAR-12 | Circular Connector; MIL SPEC:MIL-C-5015; Body Material:Metal; Series:GT; No. of Contacts:85; Connector Shell Size:40; Connecting Termination:Solder; Circular Shell Style:Square Flange Receptacle; Body Style:Straight |
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相關代理商/技術參數 |
參數描述 |
|---|---|
| ADP1109 | 制造商:AD 制造商全稱:Analog Devices 功能描述:Micropower Low Cost Fixed 3.3 V, 5 V, 12 V and Adjustable DC-to-DC Converter |
| ADP1109A | 制造商:AD 制造商全稱:Analog Devices 功能描述:Micropower Low Cost Fixed 3.3 V, 5 V, 12 V and Adjustable DC-to-DC Converter |
| ADP1109AAN | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| ADP1109AAN-12 | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| ADP1109AAN-3.3 | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
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