ADP1108 Analog Devices, ADP1108 Datasheet - Page 7

• Manufacturer Part Number: ADP1108
• Description: Micropower DC-DC Converter Adjustable and Fixed 3.3 V/ 5 V/ 12 V
• Manufacturer: Analog Devices

where: DC = duty cycle (0.7 for the ADP1108)
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
most applications, a V
The inductor value can now be calculated:
where: t
If the input voltage will vary (such as an application that must
operate from a 9 V, 12 V or 15 V source), an R
should be selected from Figure 4. The R
switch current constant as the input voltage rises. Note that
there are separate R
of operation.
For example, assume that +5 V at 250 mA is required from a +9 V
to +18 V source. Deriving the peak current from Equation 6
yields:
The peak current can than be inserted into Equation 7 to cal-
culate the inductor value:
Since 183 H is not a standard value, the next lower standard
value of 150 H would be specified.
To avoid exceeding the maximum switch current when the in-
put voltage is at +18 V, an R
ing Figure 4, a value of 160
500 mA.
Inductor Selection—Positive-to-Negative Converter
The configuration for a positive-to-negative converter using the
ADP1108 is shown in Figure 19. 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 ADP1108 power switch does not saturate in positive-to-
negative mode. The voltage drop across the switch can be mod-
eled 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:
REV. 0
I
P
I
L
PEAK
L
L
V
V
I
V
V
(t )
ON
OUT
| V
V
SW
D
OUT
IN
IN(MIN )
= switch ON time (36 s)
= diode drop (0.5 V for a 1N5818)
OUT
= the minimum input voltage
= output current
V
= voltage drop across the switch
= the output voltage
2 250 mA
R'
L
| V
0.7
I
1 e
L
–V
PEAK
D
LIM
SW
9 – 1.5 – 5
– R't
SW
491 mA
L
values for step-up and step-down modes
–V
I
value of 1.5 V is recommended.
OUT
9 1.5 0.5
OUT
LIM
5 0.5
will limit the switch current to
36 s 183 H
resistor should be specified. Us-
t
ON
SW
IN
is a function of switch
LIM
, L, time and V
491 mA
resistor will keep
LIM
resistor
(Equation 7)
(Equation 8)
(Equation 9)
OUT
. For
–7–
where: R' = 0.65
For example, assume that a –5 V output at 100 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 220 H with 0.3
tance will produce a peak switch current of:
Once the peak current is known, the inductor energy can be cal-
culated from Equation 9:
The inductor energy of 35.5 J is greater than the P
quirement of 28.9 J, so the 220 H inductor will work in
this application.
To avoid exceeding the maximum switch current when the in-
put voltage is at +5.5 V, an R
Referring to Figure 4, a value of 150
application.
Capacitor Selection
For optimum performance, the ADP1108’s output capacitor
must be carefully selected. Choosing an inappropriate capacitor
can result in low efficiency and/or high output ripple.
Ordinary aluminum electrolytic capacitors are inexpensive, but
often have poor Equivalent Series Resistance (ESR) and Equiva-
lent Series Inductance (ESL). Low ESR aluminum capacitors,
specifically designed for switch mode converter applications, are
also available, and these are a better choice than general purpose
devices. Even better performance can be achieved with tantalum
capacitors, although their cost is higher. Very low values of ESR
can be achieved by using OS-CON* capacitors (Sanyo Corpora-
tion, San Diego, CA). These devices are fairly small, available
with tape-and-reel packaging, and have very low ESR.
The effects of capacitor selection on output ripple are demon-
strated in Figures 12, 13, and 14. These figures show the output
of the same ADP1108 converter, which was evaluated with
three different output capacitors. In each case, the peak switch
current is 500 mA and the capacitor value is 100 F. Figure 12
shows a Panasonic HF-series* radial aluminum electrolytic.
When the switch turns off, the output voltage jumps by about
90 mV and then decays as the inductor discharges into the ca-
pacitor. The rise in voltage indicates an ESR of about 0.18 . In
Figure 13, the aluminum electrolytic has been replaced by a
Sprague 593D-series* tantalum device. In this case the output
jumps about 35 mV, which indicates an ESR of 0.07 . Figure
14 shows an OS-CON SA series capacitor in the same circuit,
and ESR is only 0.02 .
*All trademarks are the property of their respective holders.
I
PEAK
V
L
P
= V
L
E
L
0.65
4.5V – 0.75 V
IN
|– 5V| 0.5V
– 0.75 V
1
2
f
+ R
P
OSC
220 H
L
0.3
L
(DC)
550 mW
19 kHz
LIM
1 e
568 mA
resistor should be specified.
100 mA
–0.95
28.9 J
220 H
is appropriate in this
2
36 s
35.5 J
550 mW
ADP1108
568 mA
L
/f
OSC
dc resis-
re-