ADP1173 Analog Devices, ADP1173 Datasheet - Page 7

• Manufacturer Part Number: ADP1173
• Description: Micropower DC-DC Converter
• Manufacturer: Analog Devices

For example, assume that +5 V at 300 mA is required from a
12 V to +24 V input. Deriving the peak current from Equation 6
yields:
The peak current can then be inserted into Equation 7 to calcu-
late the inductor value:
Since 232 H is not a standard value, the next lower standard
value of 220 H would be specified.
To avoid exceeding the maximum switch current when the
input voltage is at +24 V, an R
Using the step-down curve of Figure 5, a value of 180
limit the switch current to 600 mA.
Inductor Selection—Positive-to-Negative Converter
The configuration for a positive-to-negative converter using the
ADP1173 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 ADP1173 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 R' = 0.65
where
For example, assume that a –5 V output at 50 mA is to be
generated from a +4.5 V to +5.5 V source. The power in the
inductor is calculated from Equation 8:
During each switching cycle, the inductor must supply the
following energy:
REV. 0
P
I
L
L
V
(t)
L
= V
| V
V
I
OUT
R'
PEAK
P
L
IN
L
| V
– 0.75 V
1– e
L
| 5V| 0.5V
+ R
2 300 mA
D
_R't
f
12 –1.5 – 5
L
P
OSC
545 mA
0.55
L
L(DC)
I
OUT
275 mW
24 kHz
LIM
12 – 1.5 0.5
23 s 232 H
(50 mA) 275 mW
5 0.5
resistor should be specified.
11.5 J
545 mA
will
(8)
(9)
–7–
Using a standard inductor value of 220 H, with 0.2
resistance, will produce a peak switch current of:
Once the peak current is known, the inductor energy can be
calculated from Equation 5:
The inductor energy of 15.5 J is greater than the P
requirement of 11.5 J, so the 220 H inductor will work in
this application.
The input voltage only varies between 4.5 V and 5.5 V in this
example. Therefore, the peak current will not change enough to
require an R
directly to V
current does not exceed 650 mA.
CAPACITOR SELECTION
For optimum performance, the ADP1173’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
Equivalent Series Inductance (ESL). Low ESR aluminum ca-
pacitors, specifically designed for switch mode converter appli-
cations, 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 Corporation, 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 11, 12, and 13. These figures show the output
of the same ADP1173 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 11
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
capacitor. The rise in voltage indicates an ESR of about
0.18
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 13 shows an OS-CON SA series capacitor in the
same circuit, and ESR is only 0.02 .
*All trademarks are properties of their respective holders.
. In Figure 12, the aluminum electrolytic has been
I
PEAK
LIM
IN
E
. Care should be taken to ensure that the peak
L
resistor and the I
0.65
4.5V – 0.75V
2
1
(220 H ) (375 mA)
0.2
1– e
LIM
–0.85
pin can be connected
220 H
2
23 s
15.5 J
ADP1173
375 mA
L
/f
dc
OSC