ADP1073AN-5 AD [Analog Devices], ADP1073AN-5 Datasheet - Page 10

• Manufacturer Part Number: ADP1073AN-5
• Description: Micropower DC.DC Converter Adjustable and Fixed 3.3 V, 5 V, 12 V
• Manufacturer: AD [Analog Devices]

ADP1073
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
However, diode D2 level-shifts the base of Q1 about 0.6 V below
ground, thereby cancelling the V
also reduces the circuit’s output voltage sensitivity to tempera-
ture, which would otherwise be dominated by the –2 mV/ C V
contribution of Q1. The output voltage for this circuit is deter-
mined by the formula:
Unlike the positive step-up converter, the negative-to-positive
converter’s output voltage can be either higher or lower than the
input voltage.
Limiting the Switch Current
The ADP1073’s R
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
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
switch current is shown in Figure 4.
The I
when the ADP1073 goes into continuous conduction mode. This
occurs in the step-up mode when the following condition is met:
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
NEGATIVE
IN
INPUT
LIM
increases. The relationship between R
Figure 18. A Negative-to-Positive Converter
C2
feature is also valuable for controlling inductor current
IN
rises to 4 V, however, the switch current will exceed
V
V
V
R
OUT
OUT
OUT
V
LIM
IN
I
AO SET
NC
LIM
LIM
–V
V
212 mV
212 mV
ADP1073
DIODE
pin permits the switch current to be lim-
NC
SW
V
IN
GND
L1
1– DC
SW1
SW2
FB
1
1
BE
1
1N5818
R2
R2
R1
R1
of Q1. The addition of D2
2N3906
D1
R2
R1
Q1
LIM
1N4148
10k
OUT
D2
and maximum
– V
C
BE(Q1)
L
POSITIVE
OUTPUT
).
BE
–10–
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
tained at the design values. Figure 20 illustrates the action of the
I
The internal structure of the I
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
internal 80
resistors parallel the base-emitter junction of the oscillator-
disable transistor, Q3. When the voltage across R1 and R
exceeds 0.6 V, Q3 turns on and terminates the output pulse. If
only the 80
nected directly to V
1.5 A. Figure 4 gives R
The delay through the current limiting circuit is approximately
2 s. If the switch ON time is reduced to less than 5 s, accu-
racy of the current trip point is reduced. Attempting to program
a switch ON time of 2 s or less will produce spurious responses
in the switch ON time. However, the ADP1073 will still provide
a properly regulated output voltage.
LIM
circuit.
Figure 20. (I
Figure 19. (I
resistor and through the R
internal resistor is used (i.e., the I
Q2
LIM
is 0.5% of I
IN
resistor, output ripple current can be main-
), the maximum switch current will be
LIM
LIM
LIM
values for lower current-limit values.
Operation, R
Operation, R
Q1
LIM
. Current flows to Q2 through an
circuit is shown in Figure 21.
LIM
LIM
LIM
resistor. These two
= 240 )
= 0 )
LIM
pin is con-
REV. 0
LIM