ADP1108

–9–

REV. 0

current to flow in L1. This current reaches a final value of:

L

×36 µs

where 36

µs is the ADP1108 switch’s “on” time.

5

1

8

4

2

FB

SW2

GND

ADP1108

L1

D1

1N5818

C2

100

Ω

R1

R2

3

C1

Figure 16. Step-Down Mode Operation

When the switch turns off, the magnetic field collapses. The po-

larity across the inductor changes and the switch side of the in-

ductor is driven below ground. Schottky diode D1 then turns on

and current flows into the load. Notice that the Absolute Maxi-

mum Rating for the ADP1108’s SW2 pin is 0.5 V below ground.

To avoid exceeding this limit, D1 must be a Schottky diode. If a

silicon diode is used for D1, Pin SW2 can go to

0.8 V, which

will cause potentially damaging power dissipation within the

ADP1108.

The output voltage of the buck regulator is fed back to the

ADP1108’s FB pin by resistors R1 and R2. When the voltage at

Pin FB falls below 1.245 V, the internal power switch turns

“on” again and the cycle repeats. The output voltage is set by

the formula:

V

1

+ R1

R2









When operating the ADP1108 in step-down mode, the output

voltage is impressed across the internal power switch’s emitter-

base junction while the switch is off. To protect the switch, the

output voltage should be limited to 6.2 V or less. If a higher out-

put voltage is required, a Schottky diode should be placed in se-

ries with SW2, as shown in Figure 17.

5

1

8

4

2

FB

SW2

GND

ADP1108

L1

D1

1N5818

C2

100

Ω

R1

R2

3

C1

D2

If the input voltage to the ADP1108 varies over a wide range, a

current limiting resistor at Pin 1 may be required. If a particular

circuit requires high peak inductor current with minimum input

supply voltage, then the peak current may exceed the switch

maximum rating and/or saturate the inductor when the supply

voltage is at the maximum value. See the Limiting the Switch

Current section of this data sheet for specific recommendations.

Increasing Output Current in the Step-Down Regulator

Unlike the boost configuration, the ADP1108’s internal power

switch is not saturated when operating in step-down mode. A

conservative value for the voltage across the switch in step-down

mode is 1.5 V. This results in high power dissipation within the

ADP1108 when high peak current is required. To increase the

output current, an external PNP switch can be added (Figure

18). In this circuit, the ADP1108 provides base drive to Q1

through R3 while R4 ensures that Q1 turns off rapidly. The

ADP1108’s internal current limiting function will not work in

this circuit, R5 is provided for this purpose. With the value

shown, R5 limits current to 2 A. In addition to reducing power

dissipation on the ADP1108, this circuit also reduces the switch

voltage. When selecting an inductor value for the circuit of Fig-

ure 18, the switch voltage can be calculated from the formula:

V

L1*

100 H

200mA AT 6.5V

500mA AT 8V

R4

100

Ω

R5

0.22

Ω

Q1

ZETEX

ZTX749

*L1 = COILTRONICS CTX100-4

R2

100

Ω

5

8

2

SW1

SW2

GND

ADP1108-5

D1

1N5818

C2

6.5V TO 20V

3

C1

R3

220

Ω

1

4

SENSE

Figure 18. High Current Step-Down Operation

Positive-to-Negative Conversion

The ADP1108 can convert a positive input voltage to a negative

output voltage, as shown in Figure 19. This circuit is essentially

identical to the step-down application of Figure 16, except that

the “output” side of the inductor is connected to power ground.

When the ADP1108’s internal power switch turns off, current

potential. The ADP1108 will continue to turn the switch on un-

til its FB pin is 1.245 V above its GND pin, so the output volt-

age is determined by the formula:

V

1

+ R1

R2









5

1

8

4

2

SW1

FB

SW2

GND

ADP1108

L1

D1

1N5818

C2

R3

R1

R2

3

Figure 19. A Positive-to-Negative Converter

The design criteria for the step-down application also apply to

the positive-to-negative converter. The output voltage should be

limited to |6.2 V|, unless a diode is inserted in series with the

SW2 pin (see Figure 17). Also, D1 must again be a Schottky di-

ode to prevent excessive power dissipation in the ADP1108.