ADP1073AN-5 AD [Analog Devices], ADP1073AN-5 Datasheet - Page 7

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

REV. 0
limit feature can be used to limit switch current. Simply select a
resistor (using Figure 4) that will limit the maximum switch
current to the I
V
as V
this data sheet for more information.
Note that the switch current limit feature does not protect the
circuit if the output is shorted to ground. In this case, current is
limited only by the dc resistance of the inductor and the forward
voltage of the diode.
Inductor Selection—Step-Down Converter
The step-down mode of operation is shown in Figure 16. Unlike
the step-up mode, the ADP1073’s power switch does not satu-
rate when operating in the step-down mode. Switch current
should therefore be limited to 600 mA for best performance in
this mode. If the input voltage will vary over a wide range, the
I
The first step in selecting the step-down inductor is to calculate
the peak switch current as follows:
where DC = duty cycle (0.72 for the ADP1073)
As previously mentioned, the switch voltage is higher in step-
down mode than in step-up mode. V
current and is therefore a function of V
For most applications, a V
The inductor value can now be calculated:
where t
If the input voltage will vary (such as an application which must
operate from a battery), an R
from Figure 4. The R
stant as the input voltage rises. Note that there are separate
R
For example, assume that +3.3 V at 150 mA is required from a
9 V battery with a 6 V end-of-life voltage. Deriving the peak
current from Equation 6 yields:
The peak current can than be inserted into Equation 7 to calcu-
late the inductor value:
Since 144 H is not a standard value, the next lower standard
value of 100 H would be specified.
LIM
IN
LIM
V
V
I
V
V
OUT
. This will improve efficiency by producing a constant I
SW
D
OUT
IN
IN
pin can be used to limit the maximum switch current.
values for step-up and step-down modes of operation.
= diode drop (0.5 V for a 1N5818)
increases. See the Limiting the Switch Current section of
= the minimum input voltage
= output current
ON
= voltage drop across the switch
= the output voltage
= switch ON time (38 s)
L
L
I
I
PEAK
PEAK
PEAK
V
6 –1.5 – 3.3
317 mA
IN(MIN )
value calculated for the minimum value of
2 I
2 150 mA
LIM
DC
0.72
I
–V
OUT
resistor will keep switch current con-
PEAK
SW
SW
38 s 144 H
LIM
value of 1.5 V is recommended.
V
–V
IN
V
resistor should be selected
6 – 1.5 0.5
OUT
OUT
3.3 0.5
–V
SW
SW
IN
is a function of switch
t
V
ON
, L, time and V
D
V
D
317 mA
OUT
PEAK
.
(6)
(7)
–7–
To avoid exceeding the maximum switch current when the
input voltage is at +9 V, an R
Inductor Selection—Positive-to-Negative Converter
The configuration for a positive-to-negative converter using the
ADP1073 is shown in Figure 17. As with the step-up converter,
all of the output power for the inverting circuit must be supplied
by the inductor. The required inductor power is derived from
the formula:
The ADP1073 power switch does not saturate in positive-to-
negative mode. The voltage drop across the switch can be
modeled as a 0.75 V base-emitter diode in series with a 0.65
resistor. When the switch turns on, inductor current will rise at
a rate determined by:
where
For example, assume that a –5 V output at 75 mA is to be gen-
erated from a +4.5 V to +5.5 V source. The power in the induc-
tor is calculated from Equation 8:
During each switching cycle, the inductor must supply the fol-
lowing energy:
Using a standard inductor value of 330 H, with 1
tance, will produce a peak switch current of:
Once the peak current is known, the inductor energy can be
calculated from Equation 9:
The inductor energy of 25.5 J is greater than the P
requirement of 21.7 J, so the 330 H inductor will work in this
application.
The input voltage varies between only 4.5 V and 5.5 V in this
example. Therefore, the peak current will not change enough to
require an R
rectly to V
peak current does not exceed 800 mA.
R' = 0.65
V
I
IN
PEAK
L
LIM
. Care should be taken, of course, to ensure that the
= V
P
E
P
L
L
L
resistor and the I
IN
4.5V – 0.75V
1
2
I
| 5V| 0.5V
0.65
– 0.75 V
L
| V
(330 H ) (393 mA)
f
P
(t)
OSC
+ R
OUT
L
V
L(DC)
| V
R'
1
413 mW
L
19 kHz
LIM
D
1– e
1– e
resistor should be specified.
LIM
–R't
(75 mA) 413 mW
I
L
OUT
–1.65
pin can be connected di-
21.7
330 H
2
38 s
J
25.5 J
ADP1073
393 mA
dc resis-
L
/f
OSC
(8)
(9)