ADP1073 データシートの表示（PDF） - Analog Devices
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ADP1073 Datasheet PDF : 16 Pages
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ADP1073
V OUT
= 212 mV
×
1+
R1
R2
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 (VOUT – VBE(Q1)).
However, diode D2 level-shifts the base of Q1 about 0.6 V below
ground, thereby cancelling the VBE 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 VBE
contribution of Q1. The output voltage for this circuit is deter-
mined by the formula:
V OUT
= 212 mV
×
1+
R1
R2
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
L1
1N5818
RLIM
R1
ILIM VIN
Q1
SW1 2N3906
C2
ADP1073 FB
AO SET GND SW2
R2
POSITIVE
OUTPUT
CL
D2
1N4148
10k⍀
NEGATIVE
INPUT
NC NC
Figure 18. A Negative-to-Positive Converter
Limiting the Switch Current
The ADP1073’s RLIM 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 VIN 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 VIN increases. The relationship between RLIM and maximum
switch current is shown in Figure 4.
The ILIM 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:
VOUT +V DIODE < 1
V IN –V SW 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 ILIM resistor, output ripple current can be main-
tained at the design values. Figure 20 illustrates the action of the
ILIM circuit.
Figure 19. (ILIM Operation, RLIM = 0 Ω)
Figure 20. (ILIM Operation, RLIM = 240 Ω)
The internal structure of the ILIM 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 IQ2 is 0.5% of IQ1. Current flows to Q2 through an
internal 80 Ω resistor and through the RLIM resistor. These two
resistors parallel the base-emitter junction of the oscillator-
disable transistor, Q3. When the voltage across R1 and RLIM
exceeds 0.6 V, Q3 turns on and terminates the output pulse. If
only the 80 Ω internal resistor is used (i.e., the ILIM pin is con-
nected directly to VIN), the maximum switch current will be
1.5 A. Figure 4 gives RLIM values for lower current-limit values.
The delay through the current limiting circuit is approximately
2 µs. If the switch ON time is reduced to less than 5 µs, accu-
racy of the current trip point is reduced. Attempting to program
a switch ON time of 2 µs or less will produce spurious responses
in the switch ON time. However, the ADP1073 will still provide
a properly regulated output voltage.
–10–
REV. 0
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