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ADP1073AR 数据表(PDF) 8 Page - Analog Devices

部件名 ADP1073AR
功能描述  Micropower DC.DC Converter Adjustable and Fixed 3.3 V, 5 V, 12 V
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

ADP1073AR 数据表(HTML) 8 Page - Analog Devices

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ADP1073
–8–
REV. 0
Capacitor Selection
For optimum performance, the ADP1073’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
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 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 12, 13 and 14. These figures show the output
of the same ADP1073 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 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 capacitor.
The rise in voltage indicates an ESR of about 0.18
Ω. In
Figure 13, the aluminum electrolytic has been replaced by a
Sprague 593D-series device. In this case the output jumps
about 35 mV, which indicates an ESR of 0.07
Ω. Figure 14
shows an OS-CON SA series capacitor in the same circuit, and
ESR is only 0.02
Ω.
Figure 12. Aluminum Electrolytic
Figure 13. Tantalum Electrolytic
Figure 14. OS-CON Capacitor
If low output ripple is important, the user should consider using
the ADP3000. This device switches at 400 kHz, and the higher
switching frequency simplifies the design of the output filter.
Consult the ADP3000 data sheet for additional details.
All potential current paths must be considered when analyzing
very low power applications, and this includes capacitor leakage
current. OS-CON capacitors have leakage in the 5
µA to 10 µA
range, which will reduce efficiency when the load is also in the
microampere range. Tantalum capacitors, with typical leakage
in the 1
µA to 5 µA range, are recommended for very low power
applications.
Diode Selection
In specifying a diode, consideration must be given to speed,
forward voltage drop and reverse leakage current. When the
ADP1073 switch turns off, the diode must turn on rapidly if
high efficiency is to be maintained. Schottky rectifiers, as well as
fast signal diodes such as the 1N4148, are appropriate. The
forward voltage of the diode represents power that is not deliv-
ered to the load, so VF must also be minimized. Again, Schottky
diodes are recommended. Leakage current is especially impor-
tant in low current applications, where the leakage can be a
significant percentage of the total quiescent current.
For most circuits, the 1N5818 is a suitable companion to the
ADP1073. This diode has a VF of 0.5 V at 1 A, 4
µA to 10 µA
leakage and fast turn-on and turn-off times. A surface mount
version, the MBRS130T3, is also available. For applications
where the ADP1073 is “off” most of the time, such as when the
load is intermittent, a silicon diode may provide higher overall
efficiency due to lower leakage. For example, the 1N4933 has a
1 A capability, but with a leakage current of less than 1
µA. The
higher forward voltage of the 1N4933 reduces efficiency when
the ADP1073 delivers power, but the lower leakage may outweigh
the reduction in efficiency.
For switch currents of 100 mA or less, a Schottky diode such as
the BAT85 provides a VF of 0.8 V at 100 mA and leakage less
than 1
µA. A similar device, the BAT54, is available in an SOT-23
package. Even lower leakage, in the 1 nA to 5 nA range, can be
obtained with a 1N4148 signal diode.
General purpose rectifiers, such as the 1N4001, are not suitable
for ADP1073 circuits. These devices, which have turn-on times
of 10
µs or more, are too slow for switching power supply appli-
cations. Using such a diode “just to get started” will result in
wasted time and effort. Even if an ADP1073 circuit appears to
function with a 1N4001, the resulting performance will not be
indicative of the circuit performance when the correct diode is
used.


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