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參數資料
| 型號： | ADP1073AR-5 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 穩壓器 |
| 英文描述： | Circular Connector; No. of Contacts:55; Series:LJT06R; Body Material:Aluminum; Connecting Termination:Crimp; Connector Shell Size:23; Circular Contact Gender:Pin; Circular Shell Style:Straight Plug; Insert Arrangement:23-55 |
| 中文描述： | 0.4 A SWITCHING REGULATOR, 23 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | SOIC-8 |
| 文件頁數： | 10/16頁 |
| 文件大小： | 442K |
| 代理商： | ADP1073AR-5 |
ADP1073
–10–
REV. 0
V
OUT
=
212
mV
×
1
+
R
1
R
2
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| and D1 must be a Schottky diode to prevent
excessive power dissipation in the ADP1073.
Negative-to-Positive Conversion
The circuit of Figure 18 converts a negative input voltage to a
positive output voltage. Operation of this circuit configuration is
similar to the step-up topology of Figure 16, except that the cur-
rent through feedback resistor R1 is level-shifted below ground
by a PNP transistor. The voltage across R1 is (V
OUT
– V
BE(Q1)
).
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 tempera-
ture, which would otherwise be dominated by the –2 mV/
°
C V
BE
contribution of Q1. The output voltage for this circuit is deter-
mined by the formula:
V
OUT
=
212
mV
×
1
+
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
I
LIM
V
IN
SW1
FB
SW2
GND
ADP1073
L1
D2
1N4148
C2
R
LIM
R1
C
L
Q1
2N3906
AO
SET
NC
NC
R2
10k
V
POSITIVE
OUTPUT
NEGATIVE
INPUT
Figure 18. A Negative-to-Positive Converter
Limiting the Switch Current
The ADP1073’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 exceed
1.6 A. The ADP1073 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 Figure 4.
The I
LIM
feature is also valuable for controlling inductor current
when the ADP1073 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 ADP1073’s duty cycle.
When this relationship exists, the inductor current does not go
all the way to zero during 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 19). This increases output ripple and can
require a larger inductor and capacitor. By controlling switch
current with the I
LIM
resistor, output ripple current can be main-
tained at the design values. Figure 20 illustrates the action of the
I
LIM
circuit.
Figure 19. (I
LIM
Operation, R
LIM
= 0
)
Figure 20. (I
LIM
Operation, R
LIM
= 240
)
The internal structure of the I
LIM
circuit is shown in Figure 21.
Q1 is the ADP1073’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
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
internal resistor is used (i.e., the I
LIM
pin is con-
nected directly to V
IN
), the maximum switch current will be
1.5 A. Figure 4 gives R
LIM
values for lower current-limit values.
The 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 ADP1073 will still provide
a properly regulated output voltage.
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