ADP1111
–9–
REV. 0
Figure 15. Aluminum Electrolytic
Figure 16. Tantalum Electrolytic
Figure 17. OS-CON Capacitor
If low output ripple is important, the user should consider the
ADP3000. Because this device switches at 400 kHz, lower peak
current can be used. Also, the higher switching frequency
simplifies the design of the output filter. Consult the ADP3000
data sheet for additional details.
DIODE SELECTION
In specifying a diode, consideration must be given to speed,
forward voltage drop and reverse leakage current. When the
ADP1111 switch turns off, the diode must turn on rapidly if
high efficiency is to be maintained. Shottky rectifiers, as well as
fast signal diodes such as the 1N4148, are appropriate. The
forward voltage of the diode represents power that is not
delivered to the load, so V
F
must also be minimized. Again,
Schottky diodes are recommended. Leakage current is especially
important 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
ADP1111. This diode has a V
F
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 switch currents of 100 mA or less, a Shottky diode such as
the BAT85 provides a V
F
of 0.8 V at 100 mA and leakage less
than 1 μA. 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 ADP1111 circuits. These devices, which have turn-on times
of 10 μs or more, are far 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 ADP1111 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 ADP1111 produces an output voltage that is
higher than the input voltage. For example, +12 V can be gener-
ated from a +5 V logic power supply or +5 V can be derived
from two alkaline cells (+3 V).
Figure 18 shows an ADP1111 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
IN
– V
CE(SAT)
, and current begins to flow through L1. This
current reaches a final value (ignoring second-order effects) of:
I
PEAK
≅
V
IN
− V
CE (SAT )
L
•7μs
where 7
μ
s is the ADP1111 switch’s “on” time.
I
LIM
V
IN
SW1
FB
GND SW2
ADP1111
5 4
+
V
IN
L1
D1
1N5818
C1
R2
R1
V
OUT
R3
(OPTIONAL)
1
2
3
8
Figure 18. Step-Up Mode Operation
When the switch turns off, the magnetic field collapses. The
polarity across the inductor changes, current begins to flow
through D1 into the load, and the output voltage is driven above
the input voltage.
The output voltage is fed back to the ADP1111 via resistors R1
and R2. When the voltage at pin FB falls below 1.25 V, SW1
turns “on” again, and the cycle repeats. The output voltage is
therefore set by the formula:
V
OUT
= 1. 25 V • 1 +
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
The circuit of Figure 18 shows a direct current path from V
IN
to
V
OUT
, via the inductor and D1. Therefore, the boost converter
is not protected if the output is short circuited to ground.
REV. A
