ADP1173 Просмотр технического описания (PDF) - Analog Devices
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ADP1173 Datasheet PDF : 16 Pages
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ADP1173
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:
I PEAK
=
2
×
300 mA
0.55
12
5 + 0.5
– 1.5 + 0.5
= 545 mA
The peak current can then be inserted into Equation 7 to calcu-
late the inductor value:
L =12 –1.5 – 5× 23 µs = 232 µH
545 mA
Since 232 µH is not a standard value, the next lower standard
value of 220 µH would be specified.
To avoid exceeding the maximum switch current when the
input voltage is at +24 V, an RLIM resistor should be specified.
Using the step-down curve of Figure 5, a value of 180 Ω will
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:
( ) ( ) PL = |VOUT|+V D × IOUT
(8)
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 Ω
resistor. When the switch turns on, inductor current will rise at
a rate determined by:
IL
(t)
=VL
R'
1–
_ R't
eL
(9)
where R' = 0.65 Ω + RL(DC)
where VL = VIN – 0.75 V
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:
( ) PL = |−5V|+ 0.5V × (50 mA) = 275 mW
During each switching cycle, the inductor must supply the
following energy:
PL
f OSC
= 275 mW
24 kHz
=11.5 µJ
Using a standard inductor value of 220 µH, with 0.2 Ω dc
resistance, will produce a peak switch current of:
I PEAK
=
4.5V – 0.75V
0.65 Ω + 0.2 Ω
1–
e
–0.85 Ω
220
× 23
µH
µs
= 375 mA
Once the peak current is known, the inductor energy can be
calculated from Equation 5:
EL
=
1
(220
2
µH
)
×
(375
mA)2
= 15.5
µJ
The inductor energy of 15.5 µJ is greater than the PL/fOSC
requirement of 11.5 µJ, so the 220 µH 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 RLIM resistor and the ILIM pin can be connected
directly to VIN. Care should be taken to ensure that the peak
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 µF. 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 Ω. In Figure 12, 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 Ω. Figure 13 shows an OS-CON SA series capacitor in the
same circuit, and ESR is only 0.02 Ω.
*All trademarks are properties of their respective holders.
REV. 0
–7–
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