ADP1173 Analog Devices, ADP1173 Datasheet - Page 8

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

ADP1173
If low output ripple is important, the user should consider 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 A to 10 A
range, which will reduce efficiency when the load is also in the
microampere range. Tantalum capacitors, with typical leakage
in the 1 A to 5 A range, are recommended for very low power
applications.
Figure 11. Aluminum Electrolytic
Figure 12. Tantalum Electrolytic
Figure 13. OS-CON Capacitor
–8–
DIODE SELECTION
In specifying a diode, consideration must be given to speed,
forward voltage drop and reverse leakage current. When the
ADP1173 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 V
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
ADP1173. This diode has a V
leakage, and fast turn-on and turn-off times. A surface mount
version, the MBRS130T3, is also available. For applications
where the ADP1173 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 A. The
higher forward voltage of the 1N4933 reduces efficiency when
the ADP1173 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 V
than 1 A. A similar device, the BAT54, is available in a SOT23
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 ADP1173 circuits. These devices, which have turn-on times
of 10 s or more, are too slow for switching power supply
applications. Using such a diode “just to get started” will result
in wasted time and effort. Even if an ADP1173 circuit appears
to function with a 1N4001, the resulting performance will not
be indicative of the circuit performance when the correct diode
is used.
CIRCUIT OPERATION, STEP-UP (BOOST) MODE
In boost mode, the ADP1173 produces an output voltage that
is higher than the input voltage. For example, +12 V can be
generated from a +5 V logic power supply or +5 V can be
derived from two alkaline cells (+3 V).
Figure 16 shows an ADP1173 configured for step-up operation.
The collector of the internal power switch is connected to the
output side of the inductor, while the emitter is connected to
GND. When the switch turns on, pin SW1 is pulled near ground.
This action forces a voltage across L1 equal to V
and current begins to flow through L1. This current reaches a
final value (ignoring second-order effects) of:
where 23 s is the ADP1173 switch’s “on” time.
I
PEAK
F
must also be minimized. Again, Schottky
F
V
of 0.8 V at 100 mA and leakage less
IN
–V
F
of 0.5 V at 1 A, 4 A to 10 A
L
CE(SAT )
23 s
IN
–V
CE(SAT),
REV. 0