ADP1108

–8–

REV. 0

10

0%

5 s

50mV

100

90

ESR

0.18

Figure 12. Aluminum Electrolytic

10

0%

5µs

50mV

100

90

ESR

0.07

Figure 13. Tantalum Electrolytic

10

0%

5 s

50mV

100

90

ESR

0.02

Figure 14. OS-CON Capacitor

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

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

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 Schottky diode such

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

ADP1108 circuits. These devices, which have turn-on times of

10

µs 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-

begins to flow through L1. This current reaches a final value

(ignoring second-order effects) of:

I

PEAK

≅

V

L

× 36 µs

where 36

µs is the ADP1108 switch’s “on” time.

3

5

1

8

4

2

SW1

FB

SW2

GND

ADP1108

L1

D1

C1

R3

R1

R2

Figure 15. 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 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:

V

× 1 + R1

R2









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