ADP1108 Analog Devices, ADP1108 Datasheet - Page 8

• Manufacturer Part Number: ADP1108
• Description: Micropower DC-DC Converter Adjustable and Fixed 3.3 V/ 5 V/ 12 V
• Manufacturer: Analog Devices

ADP1108
If low output ripple is important, the user should consider using
the ADP3000. This device switches at 400 kHz, which simpli-
fies 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, for-
ward voltage drop and reverse leakage current. When the
ADP1108 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 for-
ward voltage of the diode represents power that is not delivered
to the load, so V
odes are recommended. Leakage current is especially important
in low-current applications, where the leakage can be a signifi-
cant percentage of the total quiescent current.
For most circuits, the 1N5818 is a suitable companion to the
ADP1108. This diode has a V
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 Schottky diode such
as the BAT85 provides a V
Figure 12. Aluminum Electrolytic
100
100
100
0%
Figure 13. Tantalum Electrolytic
0%
90
10
90
10
0%
90
10
Figure 14. OS-CON Capacitor
F
must also be minimized. Again, Schottky di-
50mV
50mV
50mV
5µs
5 s
5 s
F
of 0.8 V at 100 mA and leakage
F
of 0.5 V at 1 A, 4 A to 10 A
C
I
ESR
C
I
ESR
C
I
ESR
SW
SW
SW
OUT
OUT
OUT
= 500mA
= 500mA
= 500mA
=100 F, 16V
=100µF, 6V
=100 F, 16V
0.02
0.07
0.18
–8–
less than 1 A. 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
ADP1108 circuits. These devices, which have turn-on times of
10 s or more, are far too slow for switching power supply applica-
tions. Using such a diode “just to get started” will result in wasted
time and effort. Even if an ADP1108 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 ADP1108 produces an output voltage 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 15 shows an ADP1108 configured for step-up operation.
The collector of the internal power switch is connected to the out-
put side of the inductor, while the emitter is connected to GND.
When the switch turns on, Pin SW1 is pulled near ground. This ac-
tion forces a voltage across L1 equal to V
begins to flow through L1. This current reaches a final value
(ignoring second-order effects) of:
where 36 s is the ADP1108 switch’s “on” time.
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 ADP1108 via resistors R1
and R2. When the voltage at Pin FB falls below 1.245 V, SW1
turns “on” again and the cycle repeats. The output voltage is
therefore set by the formula:
The circuit of Figure 15 shows a direct current path from V
V
is not protected if the output is short circuited to ground.
Circuit Operation, Step-Down (Buck) Mode
The ADP1108’s step-down mode is used to produce an output
voltage lower than the input voltage. For example, the output of
four NiCd cells (+4.8 V) can be converted to a +3 V logic supply.
A typical configuration for step-down operation of the ADP1108 is
shown in Figure 16. In this case, the collector of the internal power
switch is connected to V
OUT
, via the inductor and D1. Therefore, the boost converter
V
IN
Figure 15. Step-Up Mode Operation
GND
I
LIM
5
1
I
ADP1108
PEAK
V
R3
OUT
SW2
V
4
2
IN
IN
V
and the emitter drives the inductor.
1.245 V
SW1
IN
FB
L1
– V
3
8
L
CE (SAT )
D1
1
IN
R2
R1
R1
36 s
V
R2
CE(SAT)
C1
, and current
V
OUT
REV. 0
IN
to